Bio


Professor Santiago's research focuses on the study of microscale transport phenomena including electrokinetic flow, electrohydrodynamic instabilities, and general convective-diffusion-electromigration processes. His research includes the optimization and development of novel micro- and nano-devices for pumping liquids, on-chip electrophoresis, sample preconcentration methods, and miniature fuel cells. The applications of this work include microfabricated bioanalytical systems for genetic analysis, drug discovery, bioweapon detection, drug delivery, and power generation.

Academic Appointments


Honors & Awards


  • Fellow, American Society of Mechanical Engineering (2012)
  • Post-Doctoral Fellowship, Ford Foundation (1997)
  • Frederick Emmons Terman Fellow (Faculty) Award, Stanford University (1998)
  • Collegiate Inventors Award, National Inventors Hall of Fame (2001)
  • National Science Foundation Early Career Development (CAREER) Award, NSF (2003)
  • Presidential Early Career Award for Scientist and Engineers, PECASE (2004)
  • Outstanding Achievement in Academia Award, GEM Consortium (2006)
  • Outstanding Alumnus Award, Mechanical Engineering Department of the University of Florida (2008)
  • Fellow, American Physical Society (2010)

Professional Education


  • PhD, University of Illinois at Urbana-Champaign (1995)
  • MS, University of Illinois at Urbana-Champaign (1992)

2013-14 Courses


Journal Articles


  • Impedance-based study of capacitive porous carbon electrodes with hierarchical and bimodal porosity JOURNAL OF POWER SOURCES Suss, M. E., Baumann, T. F., Worsley, M. A., Rose, K. A., Jaramillo, T. F., Stadermann, M., Santiago, J. G. 2013; 241: 266-273
  • Temperature effects on electrophoresis. Analytical chemistry Rogacs, A., Santiago, J. G. 2013; 85 (10): 5103-5113

    Abstract

    We present a model capturing the important contributors to the effects of temperature on the observable electrophoretic mobilities of small ions, and on solution conductivity and pH. Our temperature model includes relations for temperature-dependent viscosity, ionic strength corrections, degree of ionization (pK), and ion solvation effects on mobility. We incorporate thermophysical data for water viscosity, temperature-dependence of the Onsager-Fuoss model for finite ionic strength effects on mobility, temperature-dependence of the extended Debye-Huckel theory for correction of ionic activity, the Clarke-Glew approach and tabulated thermodynamic quantities of ionization reaction for acid dissociation constants as a function of temperature, and species-specific, empirically evaluated correction terms for temperature-dependence of Stokes' radii. We incorporated our model into a MATLAB-based simulation tool we named Simulation of Temperature Effects on ElectroPhoresis (STEEP). We validated our model using conductivity and pH measurements across a temperature variation of 25-70 °C for a set of electrolytes routinely used in electrophoresis. The model accurately captures electrolyte solution pH and conductivity, including important effects not captured by simple Walden-type relations.

    View details for DOI 10.1021/ac400447k

    View details for PubMedID 23627294

  • Isotachophoresis with ionic spacer and two-stage separation for high sensitivity DNA hybridization assay. Analyst Eid, C., Garcia-Schwarz, G., Santiago, J. G. 2013; 138 (11): 3117-3120

    Abstract

    We present an on-chip electrophoretic assay for rapid and high sensitivity nucleic acid (NA) detection. The assay uses isotachophoresis (ITP) to enhance NA hybridization and an ionic spacer molecule to subsequently separate reaction products. In the first stage, the probe and target focus and mix rapidly in free solution under ITP. The reaction mixture then enters a region containing a sieving matrix, which allows the spacer ion to overtake and separate the slower probe-target complex from free, unhybridized probes. This results in the formation of two focused ITP peaks corresponding to probe and probe-target complex signals. For a 149 nt DNA target, we achieve a 220 fM limit of detection (LOD) within 10 min, with a 3.5 decade dynamic range. This LOD constitutes a 12× improvement over previous ITP-based hybridization assays. The technique offers an alternative to traditional DNA hybridization assays, and can be multiplexed and extended to detect other biomolecules.

    View details for DOI 10.1039/c3an00374d

    View details for PubMedID 23615527

  • Two- and three-dimensional modeling and optimization applied to the design of a fast hydrodynamic focusing microfluidic mixer for protein folding PHYSICS OF FLUIDS Ivorra, B., Redondo, J. L., Santiago, J. G., Ortigosa, P. M., Ramos, A. M. 2013; 25 (3)

    View details for DOI 10.1063/1.4793612

    View details for Web of Science ID 000316951900006

  • Particle Tracking and Multispectral Collocation Method for Cytometry-Like and Particle-to-Particle Binding Assays Particle Tracking and Multispectral Collocation Method for Particle-to-Particle Binding Assays, Analytical Chemistry Rogacs, A., Santiago, J., G. 2013; 1 (86): 608-614
  • Integration of rapid DNA hybridization and capillary zone electrophoresis using bidirectional isotachophoresis ANALYST Bahga, S. S., Han, C. M., Santiago, J. G. 2013; 138 (1): 87-90

    Abstract

    We present a method for rapid, sequence-specific detection of multiple DNA fragments by integrating isotachophoresis (ITP) based DNA hybridization and capillary zone electrophoresis (CZE) using bidirectional ITP. Our method leverages the high preconcentration ability of ITP to accelerate slow, second-order DNA hybridization kinetics, and the high resolving power of CZE to separate and identify reaction products. We demonstrate the speed and sensitivity of our assay by detecting 5 pM, 39 nt ssDNA target within 3 min, using a molecular beacon probe. We also demonstrate the feasibility of our assay for multiplexed detection of multiple-length ssDNA targets by simultaneously detecting 39 and 90 nt ssDNA targets.

    View details for DOI 10.1039/c2an36249j

    View details for Web of Science ID 000311823200007

    View details for PubMedID 23103998

  • Unraveling the potential and pore-size dependent capacitance of slit-shaped graphitic carbon pores in aqueous electrolytes PHYSICAL CHEMISTRY CHEMICAL PHYSICS Kalluri, R. K., Biener, M. M., Suss, M. E., Merrill, M. D., Stadermann, M., Santiago, J. G., Baumann, T. F., Biener, J., Striolo, A. 2013; 15 (7): 2309-2320

    Abstract

    Understanding and leveraging physicochemical processes at the pore scale are believed to be essential to future performance improvements of supercapacitors and capacitive desalination (CD) cells. Here, we report on a combination of electrochemical experiments and fully atomistic simulations to study the effect of pore size and surface charge density on the capacitance of graphitic nanoporous carbon electrodes. Specifically, we used cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) to study the effect of potential and pore size on the capacitance of nanoporous carbon foams. Molecular dynamics simulations were performed to study the pore-size dependent accumulation of aqueous electrolytes in slit-shaped graphitic carbon pores of different widths (0.65 to 1.6 nm). Experimentally, we observe a pronounced increase of the capacitance of sub-nm pores as the applied potential window gets wider, from a few F g(-1) for narrow potential ranges (-0.3 to 0.3 V vs. Ag/AgCl) to ~40 F g(-1) for wider potential windows (-0.9 V to 0.9 V vs. Ag/AgCl). By contrast, the capacitance of wider pores does not depend significantly on the applied potential window. Molecular dynamics simulations confirm that the penetration of ions into pores becomes more difficult with decreasing pore width and increasing strength of the hydration shell. Consistent with our experimental results, we observe a pore- and ion-size dependent threshold-like charging behavior when the pore width becomes comparable to the size of the hydrated ion (0.65 nm pores for Na(+) and 0.79 nm pores for Cl(-) ions). The observed pore-size and potential dependent accumulation of ions in slit-shaped carbon pores can be explained by the hydration structure of the ions entering the charged pores. The results are discussed in view of their effect on energy-storage and desalination efficiency.

    View details for DOI 10.1039/c2cp43361c

    View details for Web of Science ID 000313891400008

    View details for PubMedID 23295944

  • Coupling isotachophoresis and capillary electrophoresis: a review and comparison of methods ANALYST Bahga, S. S., Santiago, J. G. 2013; 138 (3): 735-754

    Abstract

    We present a comprehensive review and comparison of the methodologies for increasing sensitivity and resolution of capillary electrophoresis (CE) using online transient isotachophoresis (tITP). We categorize the diverse set of coupled tITP and CE (tITP-CE) methods based on their fundamental principles for disrupting isotachophoretic preconcentration and triggering electrophoretic separation. Based on this classification, we discuss important features, advantages, limitations, and optimization principles of various tITP-CE methods. We substantiate our discussion with original simulations, instructive examples, and published experimental results.

    View details for DOI 10.1039/c2an36150g

    View details for Web of Science ID 000312944400002

    View details for PubMedID 23232502

  • A method for non-invasive full-field imaging and quantification of chemical species LAB ON A CHIP Shkolnikov, V., Santiago, J. G. 2013; 13 (8): 1632-1643

    Abstract

    We present a novel method for full-field scalar visualization and quantification of species concentration fields. We term this method species-altered fluorescence imaging (SAFI). The method employs electrically neutral fluorescent dyes whose quantum yields are selectively quenched or enhanced by species of interest. SAFI enables simultaneous imaging of material interfaces and provides non-invasive, scalar-field quantitation of two-dimensional species concentration fields. We describe criteria for choosing SAFI dyes and tabulate 35 promising SAFI dyes and their relevant properties. Next, we describe species concentration quantification with SAFI via Stern-Volmer quenching and discuss the sensitivity and resolution of our method. We demonstrate this method with two dyes, 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) and 10-(3-sulfopropyl)acridinium betaine (SAB). We demonstrate our method in full-field visualization of several challenging electrokinetic flows: isotachophoresis (ITP) in both cationic and anionic modes, and in a convective electrokinetic instability (EKI) flow. Through these experiments we collectively quantify ion concentration shock velocities, simultaneously measure concentrations of five species, and quantify the development of an unsteady, chaotic, 2D flow.

    View details for DOI 10.1039/c3lc41293h

    View details for Web of Science ID 000316275200026

    View details for PubMedID 23463253

  • Integrated Printed Circuit Board Device for Cell Lysis and Nucleic Acid Extraction ANALYTICAL CHEMISTRY Marshall, L. A., Wu, L. L., Babikian, S., Bachman, M., Santiago, J. G. 2012; 84 (21): 9640-9645

    Abstract

    Preparation of raw, untreated biological samples remains a major challenge in microfluidics. We present a novel microfluidic device based on the integration of printed circuit boards and an isotachophoresis assay for sample preparation of nucleic acids from biological samples. The device has integrated resistive heaters and temperature sensors as well as a 70 ?m × 300 ?m × 3.7 cm microfluidic channel connecting two 15 ?L reservoirs. We demonstrated this device by extracting pathogenic nucleic acids from 1 ?L dispensed volume of whole blood spiked with Plasmodium falciparum. We dispensed whole blood directly onto an on-chip reservoir, and the system's integrated heaters simultaneously lysed and mixed the sample. We used isotachophoresis to extract the nucleic acids into a secondary buffer via isotachophoresis. We analyzed the convective mixing action with micro particle image velocimetry (micro-PIV) and verified the purity and amount of extracted nucleic acids using off-chip quantitative polymerase chain reaction (PCR). We achieved a clinically relevant limit of detection of 500 parasites per microliter. The system has no moving parts, and the process is potentially compatible with a wide range of on-chip hybridization or amplification assays.

    View details for DOI 10.1021/ac302622v

    View details for Web of Science ID 000310664600107

    View details for PubMedID 23046297

  • Effect of PVP on the electroosmotic mobility of wet-etched glass microchannels ELECTROPHORESIS Milanova, D., Chambers, R. D., Bahga, S. S., Santiago, J. G. 2012; 33 (21): 3259-3262

    Abstract

    We present an experimental study on the effect of polymer PVP on EOF mobility of microchannels wet etched into optical white soda lime glass, also known as Crown glass. We performed experiments to evaluate the effect of PVP concentration and pH on EOF mobility. We used on-chip capillary zone electrophoresis and a neutral fluorescent dye as a passive marker to quantify the electroosmotic flow. We performed experiments under controlled conditions by varying pH from 5.2 and 10.3 and concentration of PVP from 0 to 2.0% w/w at constant ionic strength (30 mM). Our experiments show that PVP at concentrations of 1.0% or above very effectively suppress EOF at low pH (6.6). At high pH of 10.3, PVP has a much weaker suppressing effect on EOF and increasing its concentration above about 0.5% showed negligible effect on EOF mobility. Finally, we briefly discuss the effects of pH on using PVP as an adsorbed coating. Our experiments provide useful guidelines on choosing correct pH and concentration of PVP for effective EOF suppression in glass channels.

    View details for DOI 10.1002/elps.201200336

    View details for Web of Science ID 000310476600019

    View details for PubMedID 23065690

  • Capacitive desalination with flow-through electrodes ENERGY & ENVIRONMENTAL SCIENCE Suss, M. E., Baumann, T. F., Bourcier, W. L., Spadaccini, C. M., Rose, K. A., Santiago, J. G., Stadermann, M. 2012; 5 (11): 9511-9519

    View details for DOI 10.1039/c2ee21498a

    View details for Web of Science ID 000310006200035

  • Robust and high-resolution simulations of nonlinear electrokinetic processes in variable cross-section channels ELECTROPHORESIS Bahga, S. S., Bercovici, M., Santiago, J. G. 2012; 33 (19-20): 3036-3051

    Abstract

    We present a model and an associated numerical scheme to simulate complex electrokinetic processes in channels with nonuniform cross-sectional area. We develop a quasi-1D model based on local cross-sectional area averaging of the equations describing unsteady, multispecies, electromigration-diffusion transport. Our approach uses techniques of lubrication theory to approximate electrokinetic flows in channels with arbitrary variations in cross-section; and we include chemical equilibrium calculations for weak electrolytes, Taylor-Aris type dispersion due of nonuniform bulk flow, and the effects of ionic strength on species mobility and on acid-base equilibrium constants. To solve the quasi-1D governing equations, we provide a dissipative finite volume scheme that adds numerical dissipation at selective locations to ensure both unconditional stability and high accuracy. We couple the numerical scheme with a novel adaptive grid refinement algorithm that further improves the accuracy of simulations by minimizing numerical dissipation. We benchmark our numerical scheme with existing numerical schemes by simulating nonlinear electrokinetic problems, including ITP and electromigration dispersion in CZE. Simulation results show that our approach yields fast, stable, and high-resolution solutions using an order of magnitude less grid points compared to the existing dissipative schemes. To highlight our model's capabilities, we demonstrate simulations that predict increase in detection sensitivity of ITP in converging cross-sectional area channels. We also show that our simulations of ITP in variable cross-sectional area channels have very good quantitative agreement with published experimental data.

    View details for DOI 10.1002/elps.201200264

    View details for Web of Science ID 000310289600013

    View details for PubMedID 22996734

  • Electric fields yield chaos in microflows PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Posner, J. D., Perez, C. L., Santiago, J. G. 2012; 109 (36): 14353-14356

    Abstract

    We present an investigation of chaotic dynamics of a low Reynolds number electrokinetic flow. Electrokinetic flows arise due to couplings of electric fields and electric double layers. In these flows, applied (steady) electric fields can couple with ionic conductivity gradients outside electric double layers to produce flow instabilities. The threshold of these instabilities is controlled by an electric Rayleigh number, Ra(e). As Ra(e) increases monotonically, we show here flow dynamics can transition from steady state to a time-dependent periodic state and then to an aperiodic, chaotic state. Interestingly, further monotonic increase of Ra(e) shows a transition back to a well-ordered state, followed by a second transition to a chaotic state. Temporal power spectra and time-delay phase maps of low dimensional attractors graphically depict the sequence between periodic and chaotic states. To our knowledge, this is a unique report of a low Reynolds number flow with such a sequence of periodic-to-aperiodic transitions. Also unique is a report of strange attractors triggered and sustained through electric fluid body forces.

    View details for DOI 10.1073/pnas.1204920109

    View details for Web of Science ID 000308912600021

    View details for PubMedID 22908251

  • Integration of On-Chip Isotachophoresis and Functionalized Hydrogels for Enhanced-Sensitivity Nucleic Acid Detection ANALYTICAL CHEMISTRY Garcia-Schwarz, G., Santiago, J. G. 2012; 84 (15): 6366-6369

    Abstract

    We introduce an on-chip electrokinetic assay to perform high-sensitivity nucleic acid (NA) detection. This assay integrates electrokinetic sample focusing using isotachophoresis (ITP) with a background signal-removal strategy that employs photopatterened, DNA-functionalized hydrogels. In this multistage assay, ITP first enhances hybridization kinetics between target NAs and end-labeled complementary reporters. After enhanced hybridization, migration through a DNA-functionalized hydrogel region removes excess reporters through affinity interactions. We demonstrate our assay on microRNAs, an important class of low-abundance biomarkers. The assay exhibits 4 orders of magnitude dynamic range, near 1 pM detection limits starting from less than 100 fg of microRNA, and high selectivity for mature microRNA sequences, all within a 10 min run time. This new microfluidic framework provides a unique quantitative assay for NA detection.

    View details for DOI 10.1021/ac301586q

    View details for Web of Science ID 000307159200019

    View details for PubMedID 22803507

  • Bacterial RNA Extraction and Purification from Whole Human Blood Using Isotachophoresis ANALYTICAL CHEMISTRY Rogacs, A., Qu, Y., Santiago, J. G. 2012; 84 (14): 5858-5863

    Abstract

    We demonstrate a novel assay for physicochemical extraction and isotachophoresis-based purification of 16S rRNA from whole human blood infected with Pseudomonas putida . This on-chip assay is unique in that the extraction can be automated using isotachophoresis in a simple device with no moving parts, it protects RNA from degradation when isolating from ribonuclease-rich matrices (such as blood), and produces a purified total nucleic acid sample that is compatible with enzymatic amplification assays. We show that the purified RNA is compatible with reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and demonstrate a clinically relevant sensitivity of 0.03 bacteria per nanoliter using RT-qPCR.

    View details for DOI 10.1021/ac301021d

    View details for Web of Science ID 000306441200006

    View details for PubMedID 22816776

  • Rapid hybridization of nucleic acids using isotachophoresis PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Bercovici, M., Han, C. M., Liao, J. C., Santiago, J. G. 2012; 109 (28): 11127-11132

    Abstract

    We use isotachophoresis (ITP) to control and increase the rate of nucleic acid hybridization reactions in free solution. We present a new physical model, validation experiments, and demonstrations of this assay. We studied the coupled physicochemical processes of preconcentration, mixing, and chemical reaction kinetics under ITP. Our experimentally validated model enables a closed form solution for ITP-aided reaction kinetics, and reveals a new characteristic time scale which correctly predicts order 10,000-fold speed-up of chemical reaction rate for order 100 pM reactants, and greater enhancement at lower concentrations. At 500 pM concentration, we measured a reaction time which is 14,000-fold lower than that predicted for standard second-order hybridization. The model and method are generally applicable to acceleration of reactions involving nucleic acids, and may be applicable to a wide range of reactions involving ionic reactants.

    View details for DOI 10.1073/pnas.1205004109

    View details for Web of Science ID 000306642100027

    View details for PubMedID 22733732

  • Concentration cascade of leading electrolyte using bidirectional isotachophoresis ELECTROPHORESIS Bahga, S. S., Santiago, J. G. 2012; 33 (6): 1048-1059

    Abstract

    We present a novel method of creating concentration cascade of leading electrolyte (LE) in isotachophoresis (ITP) by using bidirectional ITP. ITP establishes ion-concentration shock waves between high-mobility LE and low-mobility trailing electrolyte (TE) ions. In bidirectional ITP, we set up simultaneous shock waves between anions and cations such that these waves approach each other and interact. The shock interaction causes a sudden decrease in LE concentration ahead of the focused anions and a corresponding decrease in analyte zone concentrations. This readjustment of analyte zone concentrations is accompanied by a corresponding increase in their zone lengths, in accordance to conservation laws. The method generates in situ gradient in the LE concentration, and therefore can be achieved in a single, straight channel simply by establishing the initial electrolyte chemistry. We have developed an analytical model useful in designing the process for maximum sensitivity and estimating increase in sample zone length due to shock interaction. We also illustrate the technique and evaluate its effectiveness in increasing detection sensitivity using transient simulations of species transport equations. We validated the theoretical predictions using experimental visualizations of bidirectional ITP zones for various electrolyte chemistries. Lastly, we use our technique to demonstrate a factor of 20 increase in the sensitivity of ITP-based detection of 2,4,6-trichlorophenol.

    View details for DOI 10.1002/elps.201100510

    View details for Web of Science ID 000303155700020

    View details for PubMedID 22528425

  • An Integrated Printed Circuit Board Device for Cell Lysis and Nucleic Acid Extraction Analytical Chemistry Marshall, L., A., Li, L., Babikain, S., Bachman, M., Santiago, J., G. 2012; 21 (84): 9640-9645
  • Desalination and hydrogen, chlorine, and sodium hydroxide production via electrophoretic ion exchange and precipitation PHYSICAL CHEMISTRY CHEMICAL PHYSICS Shkolnikov, V., Bahga, S. S., Santiago, J. G. 2012; 14 (32): 11534-11545

    Abstract

    We demonstrate and analyze a novel desalination method which works by electrophoretically replacing sodium and chloride in feed salt water with a pair of ions, calcium and carbonate, that react and precipitate out. The resulting calcium carbonate precipitate is benign to health, and can be filtered or settled out, yielding low ionic strength product water. The ion exchange and precipitation employs self-sharpening interfaces induced by movement of multiple ions in an electric field to prevent contamination of the product water. Simultaneously, the electrolysis associated with the electromigration produces hydrogen gas, chlorine gas, and sodium hydroxide. We conducted an experimental study of this method's basic efficacy to desalinate salt water from 100 to 600 mol m(-3) sodium chloride. We also present physicochemical models of the process, and analyze replacement reagents consumption, permeate recovery ratio, and energy consumption. We hypothesize that the precipitate can be recycled back to replacement reagents using the well-known, commercially implemented Solvay process. We show that the method's permeate recovery ratio is 58% to 46%, which is on par with that of reverse osmosis. We show that the method's energy consumption requirement over and above that necessary to generate electrolysis is 3 to 10 W h l(-1), which is on par with the energy consumed by state-of-the-art desalination methods. Furthermore, the method operates at ambient temperature and pressure, and uses no specialized membranes. The process may be feasible as a part of a desalination-co-generation facility: generating fresh water, hydrogen and chlorine gas, and sodium hydroxide.

    View details for DOI 10.1039/c2cp42121f

    View details for Web of Science ID 000306708300034

    View details for PubMedID 22806549

  • On-chip isotachophoresis for separation of ions and purification of nucleic acids. Journal of visualized experiments : JoVE Garcia-Schwarz, G., Rogacs, A., Bahga, S. S., Santiago, J. G. 2012: e3890-?

    Abstract

    Electrokinetic techniques are a staple of microscale applications because of their unique ability to perform a variety of fluidic and electrophoretic processes in simple, compact systems with no moving parts. Isotachophoresis (ITP) is a simple and very robust electrokinetic technique that can achieve million-fold preconcentration and efficient separation and extraction based on ionic mobility. For example, we have demonstrated the application of ITP to separation and sensitive detection of unlabeled ionic molecules (e.g. toxins, DNA, rRNA, miRNA) with little or no sample preparation and to extraction and purification of nucleic acids from complex matrices including cell culture, urine, and blood. ITP achieves focusing and separation using an applied electric field and two buffers within a fluidic channel system. For anionic analytes, the leading electrolyte (LE) buffer is chosen such that its anions have higher effective electrophoretic mobility than the anions of the trailing electrolyte (TE) buffer (Effective mobility describes the observable drift velocity of an ion and takes into account the ionization state of the ion, as described in detail by Persat et al.). After establishing an interface between the TE and LE, an electric field is applied such that LE ions move away from the region occupied by TE ions. Sample ions of intermediate effective mobility race ahead of TE ions but cannot overtake LE ions, and so they focus at the LE-TE interface (hereafter called the "ITP interface"). Further, the TE and LE form regions of respectively low and high conductivity, which establish a steep electric field gradient at the ITP interface. This field gradient preconcentrates sample species as they focus. Proper choice of TE and LE results in focusing and purification of target species from other non-focused species and, eventually, separation and segregation of sample species. We here review the physical principles underlying ITP and discuss two standard modes of operation: "peak" and "plateau" modes. In peak mode, relatively dilute sample ions focus together within overlapping narrow peaks at the ITP interface. In plateau mode, more abundant sample ions reach a steady-state concentration and segregate into adjoining plateau-like zones ordered by their effective mobility. Peak and plateau modes arise out of the same underlying physics, but represent distinct regimes differentiated by the initial analyte concentration and/or the amount of time allotted for sample accumulation. We first describe in detail a model peak mode experiment and then demonstrate a peak mode assay for the extraction of nucleic acids from E. coli cell culture. We conclude by presenting a plateau mode assay, where we use a non-focusing tracer (NFT) species to visualize the separation and perform quantitation of amino acids.

    View details for DOI 10.3791/3890

    View details for PubMedID 22415002

  • Extraction of DNA from Malaria-Infected Erythrocytes Using Isotachophoresis ANALYTICAL CHEMISTRY Marshall, L. A., Han, C. M., Santiago, J. G. 2011; 83 (24): 9715-9718

    Abstract

    We demonstrate a technique for purification of nucleic acids from malaria parasites infecting human erythrocytes using isotachophoresis (ITP). We release nucleic acids from malaria-infected erythrocytes by lysing with heat and proteinase K for 10 min and immediately, thereafter, load sample onto a capillary device. We study the effect of temperature on lysis efficiency. We also implement pressure-driven counterflow during ITP extraction to extend focusing time and increase nucleic acid yield. We show that the purified genomic DNA samples are compatible with polymerase chain reaction (PCR) and demonstrate a clinically relevant limit of detection of 0.5 parasites per nanoliter using quantitative PCR.

    View details for DOI 10.1021/ac202567j

    View details for Web of Science ID 000297946900077

    View details for PubMedID 22074444

  • Electrophoretic mobility measurements of fluorescent dyes using on-chip capillary electrophoresis ELECTROPHORESIS Milanova, D., Chambers, R. D., Bahga, S. S., Santiago, J. G. 2011; 32 (22): 3286-3294

    Abstract

    We present an experimental study of the effect of pH, ionic strength, and concentrations of the electroosmotic flow (EOF)-suppressing polymer polyvinylpyrrolidone (PVP) on the electrophoretic mobilities of commonly used fluorescent dyes (fluorescein, Rhodamine 6G, and Alexa Fluor 488). We performed on-chip capillary zone electrophoresis experiments to directly quantify the effective electrophoretic mobility. We use Rhodamine B as a fluorescent neutral marker (to quantify EOF) and CCD detection. We also report relevant acid dissociation constants and analyte diffusivities based on our absolute estimate (as per Nernst-Einstein diffusion). We perform well-controlled experiments in a pH range of 3-11 and ionic strengths ranging from 30 to 90? mM. We account for the influence of ionic strength on the electrophoretic transport of sample analytes through the Onsager and Fuoss theory extended for finite radii ions to obtain the absolute mobility of the fluorophores. Lastly, we briefly explore the effect of PVP on adsorption-desorption dynamics of all three analytes, with particular attention to cationic R6G.

    View details for DOI 10.1002/elps.201100210

    View details for Web of Science ID 000298098700026

    View details for PubMedID 22102501

  • Coupled Isotachophoretic Preconcentration and Electrophoretic Separation Using Bidirectional Isotachophoresis ANALYTICAL CHEMISTRY Bahga, S. S., Chambers, R. D., Santiago, J. G. 2011; 83 (16): 6154-6162

    Abstract

    We present a novel technique for coupling isotachophoretic preconcentration and electrophoretic separation using bidirectional isotachophoresis (ITP). Bidirectional ITP simultaneously sets up sharp ITP interfaces between relatively high- and low-mobility cations and high- and low-mobility anions. These two interfaces can migrate toward each other and be described as ion concentration shock waves. We here demonstrate a bidirectional ITP process in which we use the interaction of these anionic and cationic ITP shock waves to trigger a transformation from ITP preconcentration to electrophoretic separation. We use anionic ITP to focus anionic sample species prior to shock interaction. The interaction of the counter-propagating anionic and cationic ITP shocks then changes the local pH (and ionic strength) of the focused analyte zones. Under this new condition, the analytes no longer focus and begin to separate electrophoretically. The method provides faster and much less dispersive transition from ITP preconcentration to electrophoretic separation compared with traditional (unidirectional) transient ITP. It eliminates the need for intermediate steps between focusing and separation, such as manual buffer exchanges. We illustrate the technique with numerical simulations of species transport equations. We have validated our simulations with experimental visualization of bidirectional ITP zones. We then show the effectiveness of the technique by coupling ITP preconcentration and high-resolution separation of a 1 kbp DNA ladder via shock interaction in bidirectional ITP.

    View details for DOI 10.1021/ac200268f

    View details for Web of Science ID 000293758800007

    View details for PubMedID 21728346

  • Sample dispersion in isotachophoresis JOURNAL OF FLUID MECHANICS Garcia-Schwarz, G., Bercovici, M., Marshall, L. A., Santiago, J. G. 2011; 679: 455-475
  • Rapid Detection of Urinary Tract Infections Using Isotachophoresis and Molecular Beacons ANALYTICAL CHEMISTRY Bercovici, M., Kaigala, G. V., Mach, K. E., Han, C. M., Liao, J. C., Santiago, J. G. 2011; 83 (11): 4110-4117

    Abstract

    We present a novel assay for rapid detection and identification of bacterial urinary tract infections using isotachophoresis (ITP) and molecular beacons. We applied on-chip ITP to extract and focus 16S rRNA directly from bacterial lysate and used molecular beacons to achieve detection of bacteria specific sequences. We demonstrated detection of E. coli in bacteria cultures as well as in patient urine samples in the clinically relevant range 1E6-1E8 cfu/mL. For bacterial cultures we further demonstrate quantification in this range. The assay requires minimal sample preparation (a single centrifugation and dilution), and can be completed, from beginning of lysing to detection, in under 15 min. We believe that the principles presented here can be used for design of other rapid diagnostics or detection methods for pathogenic diseases.

    View details for DOI 10.1021/ac200253x

    View details for Web of Science ID 000290978500022

    View details for PubMedID 21545089

  • MicroRNA Profiling by Simultaneous Selective Isotachophoresis and Hybridization with Molecular Beacons ANALYTICAL CHEMISTRY Persat, A., Santiago, J. G. 2011; 83 (6): 2310-2316

    Abstract

    We present and demonstrate a novel assay for the detection and quantification of microRNA (miRNA) that leverages isotachophoresis (ITP) and molecular beacon (MB) hybridization. We use ITP to selectively preconcentrate miRNA from total RNA. We simultaneously focus MBs and use the ITP zone as a 10 pL reactor with active mixing where MBs fluoresce upon hybridization to target miRNA. To increase both sensitivity and selectivity, we leverage a multistage ITP strategy composed of three discrete regions of different concentrations of denaturant, sieving matrix, and magnesium chloride. We show that ITP hybridization is specific and selective to the miRNA target. We demonstrate ITP hybridization of miRNA in a biologically relevant case by detecting and quantifying miR-122 in human kidney and liver. ITP hybridization is a cheap, simple, high-speed, and amplification-free miRNA profiling method which requires small amounts (order 100 ng) of sample. The technique therefore represents an attractive alternative to PCR or Northern blot for miRNAs.

    View details for DOI 10.1021/ac103225c

    View details for Web of Science ID 000288182900060

    View details for PubMedID 21329391

  • High-sensitivity detection using isotachophoresis with variable cross-section geometry ELECTROPHORESIS Bahga, S. S., Kaigala, G. V., Bercovici, M., Santiago, J. G. 2011; 32 (5): 563-572

    Abstract

    We present a theoretical and experimental study on increasing the sensitivity of ITP assays by varying channel cross-section. We present a simple, unsteady, diffusion-free model for plateau mode ITP in channels with axially varying cross-section. Our model takes into account detailed chemical equilibrium calculations and handles arbitrary variations in channel cross-section. We have validated our model with numerical simulations of a more comprehensive model of ITP. We show that using strongly convergent channels can lead to a large increase in sensitivity and simultaneous reduction in assay time, compared to uniform cross-section channels. We have validated our theoretical predictions with detailed experiments by varying channel geometry and analyte concentrations. We show the effectiveness of using strongly convergent channels by demonstrating indirect fluorescence detection with a sensitivity of 100 nM. We also present simple analytical relations for dependence of zone length and assay time on geometric parameters of strongly convergent channels. Our theoretical analysis and experimental validations provide useful guidelines on optimizing chip geometry for maximum sensitivity under constraints of required assay time, chip area and power supply.

    View details for DOI 10.1002/elps.201000338

    View details for Web of Science ID 000288094500010

    View details for PubMedID 21308693

  • Electroosmotic pump performance is affected by concentration polarizations of both electrodes and pump SENSORS AND ACTUATORS A-PHYSICAL Suss, M. E., Mani, A., Zangle, T. A., Santiago, J. G. 2011; 165 (2): 310-315

    Abstract

    Current methods of optimizing electroosmotic (EO) pump performance include reducing pore diameter and reducing ionic strength of the pumped electrolyte. However, these approaches each increase the fraction of total ionic current carried by diffuse electric double layer (EDL) counterions. When this fraction becomes significant, concentration polarization (CP) effects become important, and traditional EO pump models are no longer valid. We here report on the first simultaneous concentration field measurements, pH visualizations, flow rate, and voltage measurements on such systems. Together, these measurements elucidate key parameters affecting EO pump performance in the CP dominated regime. Concentration field visualizations show propagating CP enrichment and depletion fronts sourced by our pump substrate and traveling at order mm/min velocities through millimeter-scale channels connected serially to our pump. The observed propagation in millimeter-scale channels is not explained by current propagating CP models. Additionally, visualizations show that CP fronts are sourced by and propagate from the electrodes of our system, and then interact with the EO pump-generated CP zones. With pH visualizations, we directly detect that electrolyte properties vary sharply across the anode enrichment front interface. Our observations lead us to hypothesize possible mechanisms for the propagation of both pump- and electrode-sourced CP zones. Lastly, our experiments show the dynamics associated with the interaction of electrode and membrane CP fronts, and we describe the effect of these phenomena on EO pump flow rates and applied voltages under galvanostatic conditions.

    View details for DOI 10.1016/j.sna.2010.10.002

    View details for Web of Science ID 000288108500025

    View details for PubMedID 21516230

  • High sensitivity detection using isotachophoresis with variable cross-section geometry Electrophoresis Bahga, S, S., Kaigala, G., V., Bercovici, M., Santiago, J., G. 2011; 32: 311-314
  • Quantification of Global MicroRNA Abundance by Selective Isotachophoresis ANALYTICAL CHEMISTRY Persat, A., Chivukula, R. R., Mendell, J. T., Santiago, J. G. 2010; 82 (23): 9631-9635

    Abstract

    We here present and demonstrate a novel technique based on isotachophoresis (ITP) for the quantification of global microRNA (miRNA) abundance in total RNA. We leverage the selectivity of ITP to concentrate miRNA and exclude longer RNA molecules from the focused zone. We designed a novel ITP strategy where we initially establish three contiguous zones of sieving polymer, electrolyte, and denaturant concentrations. This allows for successive preconcentration, selection, and detection of miRNA. We optimized chemistry in each zone for high sensitivity and exquisite selectivity for miRNA. This technique allows for the measurement of the total miRNA content in a sample and its comparison between different cell types and tissues. We demonstrated and validated the efficacy of this technique by comparing global miRNA abundance in subconfluent and confluent cell cultures.

    View details for DOI 10.1021/ac102496m

    View details for Web of Science ID 000284668600009

    View details for PubMedID 21062022

  • Design and fabrication of porous polymer wick structures SENSORS AND ACTUATORS B-CHEMICAL Shkolnikov, V., Strickland, D. G., Fenning, D. P., Santiago, J. G. 2010; 150 (2): 556-563
  • A two-liquid electroosmotic pump using low applied voltage and power SENSORS AND ACTUATORS A-PHYSICAL Litster, S., Suss, M. E., Santiago, J. G. 2010; 163 (1): 311-314
  • Active water management at the cathode of a planar air-breathing polymer electrolyte membrane fuel cell using an electroosmotic pump JOURNAL OF POWER SOURCES Fabian, T., O'Hayre, R., Litster, S., Prinz, F. B., Santiago, J. G. 2010; 195 (11): 3640-3644
  • Passive water management at the cathode of a planar air-breathing proton exchange membrane fuel cell JOURNAL OF POWER SOURCES Fabian, T., O'Hayre, R., Litster, S., Prinz, F. B., Santiago, J. G. 2010; 195 (10): 3201-3206
  • A self-priming, roller-free, miniature, peristaltic pump operable with a single, reciprocating actuator SENSORS AND ACTUATORS A-PHYSICAL Shkolnikov, V., Ramunas, J., Santiago, J. G. 2010; 160 (1-2): 141-146
  • Effects of Constant Voltage on Time Evolution of Propagating Concentration Polarization ANALYTICAL CHEMISTRY Zangle, T. A., Mani, A., Santiago, J. G. 2010; 82 (8): 3114-3117

    Abstract

    We extend the analytical theory of propagating concentration polarization (CP) to describe and compare the effects of constant-voltage versus constant-current conditions on the transient development of CP enrichment and depletion zones. We support our analysis with computational and experimental results. We find that at constant voltage, enrichment and depletion regions spread as t(1/2) as opposed to the previously observed t(1) scaling for constant current conditions. At low, constant voltages, the growth and propagation of CP zones can easily be misinterpreted as nonpropagating behavior.

    View details for DOI 10.1021/ac100432q

    View details for Web of Science ID 000276557600004

    View details for PubMedID 20349992

  • In situ-polymerized wicks for passive water management in proton exchange membrane fuel cells JOURNAL OF POWER SOURCES Strickland, D. G., Santiago, J. G. 2010; 195 (6): 1667-1675
  • Ionic strength effects on electrophoretic focusing and separations ELECTROPHORESIS Bahga, S. S., Bercovici, M., Santiago, J. G. 2010; 31 (5): 910-919

    Abstract

    We present a numerical and experimental study of the effects of ionic strength on electrophoretic focusing and separations. We review the development of ionic strength models for electrophoretic mobility and chemical activity and highlight their differences in the context of electrophoretic separation and focusing simulations. We couple a fast numerical solver for electrophoretic transport with the Onsager-Fuoss model for actual ionic mobility and the extended Debye-Huckle theory for correction of ionic activity. Model predictions for fluorescein mobility as a function of ionic strength and pH compare well with data from CZE experiments. Simulation predictions of preconcentration factors in peak mode ITP also compare well with the published experimental data. We performed ITP experiments to study the effect of ionic strength on the simultaneous focusing and separation. Our comparisons of the latter data with simulation results at 10 and 250 mM ionic strength show the model is able to capture the observed qualitative differences in ITP analyte zone shape and order. Finally, we present simulations of CZE experiments where changes in the ionic strength result in significant change in selectivity and order of analyte peaks. Our simulations of ionic strength effects in capillary electrophoresis compare well with the published experimental data.

    View details for DOI 10.1002/elps.200900560

    View details for Web of Science ID 000275696400016

    View details for PubMedID 20191554

  • Fluorescent Carrier Ampholytes Assay for Portable, Label-Free Detection of Chemical Toxins in Tap Water ANALYTICAL CHEMISTRY Bercovici, M., Kaigala, G. V., Backhouse, C. J., Santiago, J. G. 2010; 82 (5): 1858-1866

    Abstract

    We present a novel method for fluorescence-based indirect detection of analytes and demonstrate its use for label-free detection of chemical toxins in a hand-held device. We fluorescently label a mixture of low-concentration carrier ampholytes and introduce it into an isotachophoresis (ITP) separation. The carrier ampholytes provide a large number of fluorescent species with a wide range of closely spaced effective electrophoretic mobilities. Analytes focus under ITP and displace subsets of these carrier ampholytes. The analytes are detected indirectly and quantified by analyzing the gaps in the fluorescent ampholyte signal. The large number (on the order of 1000) of carrier ampholytes enables detection of a wide range of analytes, requiring little a priori knowledge of their electrophoretic properties. We discuss the principles of the technique and demonstrate its use in the detection of various analytes using a standard microscope system. We then present the integration of the technique into a self-contained hand-held device and demonstrate detection of chemical toxins (2-nitrophenol and 2,4,6-trichlorophenol) in tap water, with no sample preparation steps.

    View details for DOI 10.1021/ac902526g

    View details for Web of Science ID 000274841300044

    View details for PubMedID 20141152

  • Method for Analyte Identification Using Isotachophoresis and a Fluorescent Carrier Ampholyte Assay ANALYTICAL CHEMISTRY Bercovici, M., Kaigala, G. V., Santiago, J. G. 2010; 82 (5): 2134-2138

    Abstract

    We present a novel method for identification of unlabeled analytes using fluorescent carrier ampholytes and isotachophoresis (ITP). The method is based on previous work where we showed that the ITP displacement of carrier ampholytes can be used for detection of unlabeled (nonfluorescent) analytes. We here propose a signal analysis method based on integration of the associated fluorescent signal. We define a normalized signal integral which is equivalent to an accurate measure of the amount of carrier ampholytes which are focused between the leading electrolyte and the analyte. We show that this parameter can be related directly to analyte effective mobility. Using several well characterized analytes, we construct calibration curves relating effective mobility and carrier ampholyte displacement at two different leading electrolyte (LE) buffers. On the basis of these calibration curves, we demonstrate the extraction of fully ionized mobility and dissociation constant of 2-nitrophenol and 2,4,6-trichlorophenol from ITP experiments with fluorescent carrier ampholytes. This extraction is based on no a priori assumptions or knowledge of these two toxic chemicals. This technique allows simultaneous identification of multiple analytes by their physiochemical properties in a few minutes and with no sample preparation.

    View details for DOI 10.1021/ac9025658

    View details for Web of Science ID 000274841300080

    View details for PubMedID 20141174

  • Compact adaptive-grid scheme for high numerical resolution simulations of isotachophoresis JOURNAL OF CHROMATOGRAPHY A Bercovici, M., Lele, S. K., Santiago, J. G. 2010; 1217 (4): 588-599

    Abstract

    In a previous publication we demonstrated a fast simulation tool for solution of electrophoretic focusing and separation. We here describe the novel mathematical model and numerical algorithms used to create this code. These include the representation of advection-diffusion equations on an adaptive grid, high-resolution discretization of the equations (sixth order compact), a new variational-based approach for controlling the motion of grid points, and new boundary conditions which enable solution in a moving frame of reference. We discuss the advantages of combining a high-resolution discretization with an adaptive grid in accurately resolving sharp interfaces in isotachophoresis, and provide verification against known analytical solutions and comparison with prevailing exiting numerical algorithms.

    View details for DOI 10.1016/j.chroma.2009.11.072

    View details for Web of Science ID 000274479200020

    View details for PubMedID 20022605

  • Evidence shows concentration polarization and its propagation can be key factors determining electroosmotic pump performance SENSORS AND ACTUATORS B-CHEMICAL Strickland, D. G., Suss, M. E., Zangle, T. A., Santiago, J. G. 2010; 143 (2): 795-798
  • Miniaturized system for isotachophoresis assays LAB ON A CHIP Kaigala, G. V., Bercovici, M., Behnam, M., Elliott, D., Santiago, J. G., Backhouse, C. J. 2010; 10 (17): 2242-2250

    Abstract

    We present an inexpensive hand-held device (240 g) that implements microchip isotachophoresis (ITP) with laser induced fluorescence (LIF) detection. This self-contained instrument integrates the functionality required for high voltage generation onto a microelectronic chip, includes LIF detection and is powered by a universal serial bus (USB) link connected to a laptop computer. Using this device we demonstrate focusing and detection of a fluorescent species with a limit of detection of 100 pM. We show that the response of the detector is linear with the initial analyte concentration, making this device suitable for quantitative analysis. We also demonstrate the use of our simulation tools for design and prediction of ITP assays, and validate these results with a demonstration of multiplexed indirect detection of (unlabeled) analytes performed using the device. We find good agreement between simulations and experimental results. Using a label-free isotachaphoresis assay implemented in the hand-held device we detect two explosives and an endocrine disruptor spiked in river water, with no prior sample processing.

    View details for DOI 10.1039/c004120c

    View details for Web of Science ID 000280812600010

    View details for PubMedID 20571691

  • Theory and experiments of concentration polarization and ion focusing at microchannel and nanochannel interfaces CHEMICAL SOCIETY REVIEWS Zangle, T. A., Mani, A., Santiago, J. G. 2010; 39 (3): 1014-1035

    Abstract

    In this tutorial review aimed at researchers using nanofluidic devices, we summarize the current state of theoretical and experimental approaches to describing concentration polarization (CP) in hybrid microfluidic-nanofluidic systems. We also analyze experimental results for these systems and place them in the context of recent theoretical developments. We then extend the theory to explain the behavior of both positively and negatively charged, low-concentration, analyte species in systems with CP. We conclude by discussing several applications of CP in microfluidics.

    View details for DOI 10.1039/b902074h

    View details for Web of Science ID 000274920300011

    View details for PubMedID 20179822

  • Purification of Nucleic Acids from Whole Blood Using Isotachophoresis ANALYTICAL CHEMISTRY Persat, A., Marshall, L. A., Santiago, J. G. 2009; 81 (22): 9507-9511

    Abstract

    We present and demonstrate a novel technique for the purification of nucleic acids from biological samples using isotachophoresis (ITP). We demonstrate a simple and rapid method to achieve ITP-based extraction, preconcentration, and purification of DNA from nanoliter volumes of whole blood. We show that ITP purification yields genomic DNA samples which can be quantitated with fluorescence measurements and are immediately compatible with polymerase chain reaction (PCR) (e.g., a PCR-friendly solution free of significant inhibitors). We hypothesize ITP purification is applicable to processing of a wide range of complex biological samples.

    View details for DOI 10.1021/ac901965v

    View details for Web of Science ID 000271662400039

    View details for PubMedID 19831356

  • Engineering model for coupling wicks and electroosmotic pumps with proton exchange membrane fuel cells for active water management ELECTROCHIMICA ACTA Litster, S., Buie, C. R., Santiago, J. G. 2009; 54 (26): 6223-6233
  • Two-phase hydrodynamics in a miniature direct methanol fuel cell INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER Buie, C. R., Santiago, J. G. 2009; 52 (21-22): 5158-5166
  • Electrokinetic control of sample splitting at a channel bifurcation using isotachophoresis NEW JOURNAL OF PHYSICS Persat, A., Santiago, J. G. 2009; 11
  • Effects of carbon dioxide on peak mode isotachophoresis: Simultaneous preconcentration and separation LAB ON A CHIP Khurana, T. K., Santiago, J. G. 2009; 9 (10): 1377-1384

    Abstract

    We present a method that achieves simultaneous preconcentration and separation of analytes using peak-mode isotachophoresis with a single step injection in simple, off-the-shelf microchannels or capillaries. We leverage ions resulting from dissolved atmospheric carbon dioxide to weakly disrupt isotachophoretic preconcentration and induce separation of analyte species. We experimentally study the region between the leading and trailing electrolytes, and individually identify the carbonate and carbamate zones that result from the hydration and carbamation reaction of dissolved atmospheric carbon dioxide, respectively. The width of these zones and the gradient regions between them grow with time and create an electric field gradient that causes analytes to separate. Using this assay, we achieve focusing and separation of a 25 bp DNA ladder in a straight, 34 microm wide microchannel in a single loading step. As a demonstration of the fractionation capabilities of the assay, we show simultaneous preconcentration and separation of a DNA ladder from two proteins, GFP and allophycocyanin.

    View details for DOI 10.1039/b815460k

    View details for Web of Science ID 000268227400010

    View details for PubMedID 19417904

  • Imaging and Quantification of Isotachophoresis Zones Using Nonfocusing Fluorescent Tracers ANALYTICAL CHEMISTRY Chambers, R. D., Santiago, J. G. 2009; 81 (8): 3022-3028

    Abstract

    We present a novel method for visualizing isotachophoresis (ITP) zones. We introduce negligibly small concentrations of a fluorophore that is not focused by isotachophoresis. This nonfocusing tracer (NFT) migrates through multiple isotachophoresis zones. As it enters each zone, the NFT concentration adapts to the local electric field in each zone. ITP zones can then be visualized with a point detector or camera. The method can be used to detect, identify, and quantify unknown analyte zones and can visualize complex and even transient electrophoresis processes. This visualization technique is particularly suited to microfluidic and laboratory-on-a-chip applications, as typical fluorescence microscopes and charge-coupled device (CCD) cameras can provide high-resolution spatiotemporal data. We present a theoretical description, a methodology for identifying analytes, and experimental validation. We also visualize and analyze a complex, transient DNA ITP preconcentration and separation.

    View details for DOI 10.1021/ac802698a

    View details for Web of Science ID 000265158800025

    View details for PubMedID 19290665

  • On the Propagation of Concentration Polarization from Microchannel-Nanochannel Interfaces Part II: Numerical and Experimental Study LANGMUIR Zangle, T. A., Mani, A., Santiago, J. G. 2009; 25 (6): 3909-3916

    Abstract

    We present results of a combined computational and experimental study of the propagation of concentration polarization (CP) zones in a microchannel-nanochannel system. Our computational model considers the combined effects of bulk flow, electromigration, and diffusion and accurately captures the dynamics of CP. Using wall charge inside the nanochannel as a single fitting parameter, we predict experimentally observed enrichment and depletion shock velocities. Our model can also be used to compute the existence of CP with propagating enrichment and depletion shocks on the basis of measured ion mobility and wall properties. We present experiments where the background electrolyte consists of only a fluorescent ion and its counterion. These results are used to validate the computational model and to confirm predicted trends from an analytical model presented in the first of this two-paper series. We show experimentally that the enrichment region concentration is effectively independent of the applied current, while the enrichment and depletion shock velocities increase in proportion to current density.

    View details for DOI 10.1021/1a803318e

    View details for Web of Science ID 000264145000085

    View details for PubMedID 19275188

  • On the Propagation of Concentration Polarization from Microchannel-Nanochannel Interfaces Part I: Analytical Model and Characteristic Analysis LANGMUIR Mani, A., Zangle, T. A., Santiago, J. G. 2009; 25 (6): 3898-3908

    Abstract

    We develop two models to describe ion transport in variable-height micro- and nanochannels. For the first model, we obtain a one-dimensional (unsteady) partial differential equation governing flow and charge transport through a shallow and wide electrokinetic channel. In this model, the effects of electric double layer (EDL) on axial transport are taken into account using exact solutions of the Poisson-Boltzmann equation. The second simpler model, which is approachable analytically, assumes that the EDLs are confined to near-wall regions. Using a characteristics analysis, we show that the latter model captures concentration polarization (CP) effects and provides useful insight into its dynamics. Two distinct CP regimes are identified: CP with propagation in which enrichment and depletion shocks propagate outward, and CP without propagation where polarization effects stay local to micro- nanochannel interfaces. The existence of each regime is found to depend on a nanochannel Dukhin number and mobility of the co-ion nondimensionalized by electroosmotic mobility. Interestingly, microchannel dimensions and axial diffusion are found to play an insignificant role in determining whether CP propagates. The steady state condition of propagating CP is shown to be controlled by channel heights, surface chemistry, and co-ion mobility instead of the reservoir condition. Both models are validated against experimental results in Part II of this two-paper series.

    View details for DOI 10.1021/1a803317p

    View details for Web of Science ID 000264145000084

    View details for PubMedID 19275187

  • Electrokinetics in nanochannels. Part II. Mobility dependence on ion density and ionic current measurements (vol 325, pg 539, 2008) JOURNAL OF COLLOID AND INTERFACE SCIENCE Baldessari, F., Santiago, J. G. 2009; 331 (2): 550-550
  • Electrokinetics in nanochannels. Part I. Electric double layer overlap and channel-to-well equilibrium (vol 325, pg 526, 2008) JOURNAL OF COLLOID AND INTERFACE SCIENCE Baldessari, F., Santiago, J. G. 2009; 331 (2): 549-549
  • Dry gas operation of proton exchange membrane fuel cells with parallel channels: Non-porous versus porous plates JOURNAL OF POWER SOURCES Litster, S., Santiago, J. G. 2009; 188 (1): 82-88
  • Open source simulation tool for electrophoretic stacking, focusing, and separation JOURNAL OF CHROMATOGRAPHY A Bercovici, M., Lele, S. K., Santiago, J. G. 2009; 1216 (6): 1008-1018

    Abstract

    We present the development, formulation, and performance of a new simulation tool for electrophoretic preconcentration and separation processes such as capillary electrophoresis, isotachophoresis, and field amplified sample stacking. The code solves the one-dimensional transient advection-diffusion equations for multiple multivalent weak electrolytes (including ampholytes) and includes a model for pressure-driven flow and Taylor-Aris dispersion. The code uses a new approach for the discretization of the equations, consisting of a high resolution compact scheme which is combined with an adaptive grid algorithm. We show that this combination allows for accurate resolution of sharp concentration gradients at high electric fields, while at the same time significantly reducing the computational time. We demonstrate smooth, stable, and accurate solutions at current densities as high as 5000A/m(2) using only 300 grid points, and a 75-fold reduction in computational time compared with equivalent uniform grid techniques. The code is available as an open source for free at http://microfluidics.stanford.edu.

    View details for DOI 10.1016/j.chroma.2008.12.022

    View details for Web of Science ID 000263087700017

    View details for PubMedID 19124132

  • Corrigendum to ‘Electrokinetics in Nanochannels. Part I: Electric Double Layer Overlap and Channel-to-Well Equilibrium Journal of Colloid and Interface Science Baldessari, F., Santiago, J., G. 2009; 2 (331): 549-549
  • Special issue on fundamental principles and techniques in microfluidics LAB ON A CHIP Santiago, J. G., Chen, C. 2009; 9 (17): 2423-2424

    View details for DOI 10.1039/b913538n

    View details for Web of Science ID 000268975200001

    View details for PubMedID 19680567

  • Basic principles of electrolyte chemistry for microfluidic electrokinetics. Part II: Coupling between ion mobility, electrolysis, and acid-base equilibria LAB ON A CHIP Persat, A., Suss, M. E., Santiago, J. G. 2009; 9 (17): 2454-2469

    Abstract

    We present elements of electrolyte dynamics and electrochemistry relevant to microfluidic electrokinetics experiments. In Part I of this two-paper series, we presented a review and introduction to the fundamentals of acid-base chemistry. Here, we first summarize the coupling between acid-base equilibrium chemistry and electrophoretic mobilities of electrolytes, at both infinite and finite dilution. We then discuss the effects of electrode reactions on microfluidic electrokinetic experiments and derive a model for pH changes in microchip reservoirs during typical direct-current electrokinetic experiments. We present a model for the potential drop in typical microchip electrophoresis device. The latter includes finite element simulation to estimate the relative effects of channel and reservoir dimensions. Finally, we summarize effects of electrode and electrolyte characteristics on potential drop in microfluidic devices. As a whole, the discussions highlight the importance of the coupling between electromigration and electrophoresis, acid-base equilibria, and electrochemical reactions.

    View details for DOI 10.1039/b906468k

    View details for Web of Science ID 000268975200004

    View details for PubMedID 19680571

  • Hydrodynamic interactions in metal rodlike-particle suspensions due to induced charge electroosmosis PHYSICAL REVIEW E Rose, K. A., Hoffman, B., Saintillan, D., Shaqfeh, E. S., Santiago, J. G. 2009; 79 (1)

    Abstract

    We present a theoretical and experimental study of the role of hydrodynamic interactions on the motion and dispersion of metal rodlike particles in the presence of an externally applied electric field. In these systems, the electric field polarizes the particles and induces an electroosmotic flow relative to the surface of each particle. The simulations include the effect of the gravitational body force, buoyancy, far-field hydrodynamic interactions, and near-field lubrication forces. The particles in the simulations and experiments were observed to experience repeated pairing interactions in which they come together axially with their ends approaching each other, slide past one another until their centers approach, and then push apart. These interactions were confirmed in measurements of particle orientations and velocities, pair distribution functions, and net dispersion of the suspension. For large electric fields, the pair distribution functions show accumulation and depletion regions consistent with many pairing events. For particle concentrations of 10;{8}particles/mL and higher, dispersion within the suspension dramatically increases with increased field strength.

    View details for DOI 10.1103/PhysRevE.79.011402

    View details for Web of Science ID 000262976600043

    View details for PubMedID 19257030

  • Rapid and selective extraction, isolation, preconcentration, and quantitation of small RNAs from cell lysate using on-chip isotachophoresis LAB ON A CHIP Schoch, R. B., Ronaghi, M., Santiago, J. G. 2009; 9 (15): 2145-2152

    Abstract

    We present a technique which enables the separation of small RNAs-such as microRNAs (miRNAs), short interfering RNAs (siRNAs), and Piwi-interacting RNAs (piRNAs)-from >or=66 nucleotide RNAs and other biomolecules contained in a cell lysate. In particular, the method achieves separation of small RNAs from precursor miRNAs (pre-miRNAs) in less than 3 min. We use on-chip isotachophoresis (ITP) for the simultaneous extraction, isolation, preconcentration and quantitation of small RNAs (approximately 22 nucleotides) and employ the high-efficiency sieving matrix Pluronic F-127; a thermo-responsive triblock copolymer which allows convenient microchannel loading at low temperature. We present the isolation of small RNAs from the lysate of 293A human kidney cells, and quantitate the number of short RNA molecules per cell to be 2.9x10(7). We estimate this quantity is an aggregate of roughly 500 types of short RNA molecules per 293A cell. Currently, the minimal cell number for small RNA extraction and detection with our method is approximately 900 (from a 5 microL sample volume), and we believe that small RNA analysis from tens of cells is realizable. Techniques for rapid and sensitive extraction and isolation of small RNAs from cell lysate are much-needed to further uncover their full range and functionality, including RNA interference studies.

    View details for DOI 10.1039/b903542g

    View details for Web of Science ID 000268033900005

    View details for PubMedID 19606290

  • Basic principles of electrolyte chemistry for microfluidic electrokinetics. Part I: Acid-base equilibria and pH buffers LAB ON A CHIP Persat, A., Chambers, R. D., Santiago, J. G. 2009; 9 (17): 2437-2453

    Abstract

    We review fundamental and applied acid-base equilibrium chemistry useful to microfluidic electrokinetics. We present elements of acid-base equilibrium reactions and derive rules for pH calculation for simple buffers. We also present a general formulation to calculate pH of more complex, arbitrary mixtures of electrolytes, and discuss the effects of ionic strength and temperature on pH calculation. More practically, we offer advice on buffer preparation and on buffer reporting. We also discuss "real world" buffers and likely contamination sources. In particular, we discuss the effects of atmospheric carbon dioxide on buffer systems, namely, the increase in ionic strength and acidification of typical electrokinetic device buffers. In Part II of this two-paper series, we discuss the coupling of acid-base equilibria with electrolyte dynamics and electrochemistry in typical microfluidic electrokinetic systems.

    View details for DOI 10.1039/b906465f

    View details for Web of Science ID 000268975200003

    View details for PubMedID 19680570

  • Corrigendum to ‘Electrokinetics in Nanochannels: Part II: Mobility Dependence on Ion Density and Ionic Current Measurements Journal of Colloid and Interface Science Baldessari, F., Santiago, J., G. 2009; 2 (331): 550-550
  • Electrokinetics in nanochannels - Part I. Electric double layer overlap and channel-to-well equilibrium JOURNAL OF COLLOID AND INTERFACE SCIENCE Baldessari, F. 2008; 325 (2): 526-538

    Abstract

    In this paper a new model is described for calculating the electric potential field in a long, thin nanochannel with overlapped electric double layers. Electrolyte concentration in the nanochannel is predicted self-consistently via equilibrium between ionic solution in the wells and within the nanochannel. Differently than published models that require detailed iterative numerical solutions of coupled differential equations, the framework presented here is self-consistent and predictions are obtained solving a simple one-dimensional integral. The derivation clearly shows that the electric potential field depends on three new parameters: the ratio of ion density in the channel to ion density in the wells; the ratio of free-charge density to bulk ion density within the channel; and a modified Debye-Hückel thickness, which is the relevant scale for shielding of surface net charge. For completeness, three wall-surface boundary conditions are analyzed: specified zeta-potential; specified surface net charge density; and charge regulation. Predictions of experimentally observable quantities based on the model proposed here, such as depth-averaged electroosmotic flow and net ionic current, are significantly different than results from previous overlapped electric double layer models. In this first paper of a series of two, predictions are presented where channel depth is varied at constant well concentration. Results show that under conditions of electric double layer overlap, electroosmosis contributes only a small fraction of the net ionic current, and that most of the measurable current is due to ionic conduction in conditions of increased counterion density in the nanochannel. In the second of this two-paper series, predictions are presented where well-concentration is varied and the channel depth is held constant, and the model described here is employed to study the dependence of ion mobility on ionic strength, and compare predictions to measurements of ionic current as a function of channel depth and ion density.

    View details for DOI 10.1016/j.jcis.2008.06.007

    View details for Web of Science ID 000258553900033

    View details for PubMedID 18639883

  • Electrokinetics in nanochannels - Part II. Mobility dependence on ion density and ionic current measurements JOURNAL OF COLLOID AND INTERFACE SCIENCE Baldessari, F. 2008; 325 (2): 539-546

    Abstract

    In the first of this two-paper series, a new model was developed for calculating the electric potential field in a long, thin nanochannel with overlapped electric double layers. The model takes into account the dependence of ion mobility on local ion densities and pH. This model is used here to study and demonstrate the effect of ion density and pH on ionic current measurements. A comparison is shown of predictions based on each of three boundary conditions, as studied in Part I. The model developed in Part I is validated by comparing simulations with measurements of ionic current as a function of sodium borate concentration. Results show that predictions based on extended Debye-Hückel theory for ion mobility significantly improve the accuracy of simulations, but that these do not predict exact scaling behavior. A simple bulk conductivity measurement used as input parameter for the simulations, in place of the predicted bulk conductivity (K(0)), guarantees agreement with data in the thin EDL region. Results also indicate that the charge regulation boundary condition, complemented with an adequate bulk electrolyte model, provides better agreement with experimental trends than the specified zeta potential or specified surface net charge boundary conditions. Further, it is shown that currents due to advection (by electroosmotic flow) are in all cases studied less than 25% of the total current in the system.

    View details for DOI 10.1016/j.jcis.2008.06.008

    View details for Web of Science ID 000258553900034

    View details for PubMedID 18639884

  • Sample zone dynamics in peak mode isotachophoresis ANALYTICAL CHEMISTRY Khurana, T. K., Santiago, J. G. 2008; 80 (16): 6300-6307

    Abstract

    We present a theoretical and experimental study of analyte preconcentration via peak mode isotachophoresis (ITP). We perform perturbation analysis of the governing equations that includes electromigration, diffusion, buffer reactions, and nonlinear ionic strength effects. This analysis relaxes the inherent numerical stiffness and achieves a fast solution to the transient sample evolution problem. In this model, we have incorporated a semiempirical relation to capture dispersion phenomenon within ITP interfaces. We also present a simple, closed-form analytical model that identifies key parameters governing the preconcentration dynamics in peak mode ITP. We have validated our models through a detailed experimental study performed in constant current conditions. The relevant governing experiment parameters were varied independently; namely, the leading electrolyte concentration, trailing electrolyte concentration, and current. Through our experimental study, we have identified optimum conditions to achieve high preconcentration ratio and sample accumulation rates. Our approach to the theoretical problem and experimental study provides useful guidelines in optimizing parameters such as detector location, ITP duration, and electrolyte composition in ITP preconcentration and separation assays.

    View details for DOI 10.1021/ac800792g

    View details for Web of Science ID 000258448100021

    View details for PubMedID 18642874

  • Lymphocyte electrotaxis in vitro and in vivo JOURNAL OF IMMUNOLOGY Lin, F., Baldessari, F., Gyenge, C. C., Sato, T., Chambers, R. D., Santiago, J. G., Butcher, E. C. 2008; 181 (4): 2465-2471

    Abstract

    Electric fields are generated in vivo in a variety of physiologic and pathologic settings, including penetrating injury to epithelial barriers. An applied electric field with strength within the physiologic range can induce directional cell migration (i.e., electrotaxis) of epithelial cells, endothelial cells, fibroblasts, and neutrophils suggesting a potential role in cell positioning during wound healing. In the present study, we investigated the ability of lymphocytes to respond to applied direct current (DC) electric fields. Using a modified Transwell assay and a simple microfluidic device, we show that human PBLs migrate toward the cathode in physiologically relevant DC electric fields. Additionally, electrical stimulation activates intracellular kinase signaling pathways shared with chemotactic stimuli. Finally, video microscopic tracing of GFP-tagged immunocytes in the skin of mouse ears reveals that motile cutaneous T cells actively migrate toward the cathode of an applied DC electric field. Lymphocyte positioning within tissues can thus be manipulated by externally applied electric fields, and may be influenced by endogenous electrical potential gradients as well.

    View details for Web of Science ID 000258345300026

    View details for PubMedID 18684937

  • A depth-averaged electrokinetic flow model for shallow microchannels JOURNAL OF FLUID MECHANICS Lin, H., Storey, B. D., Santiago, J. G. 2008; 608: 43-70
  • Ballistic dispersion in temperature gradient focusing PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES Huber, D. E., Santiago, J. G. 2008; 464 (2091): 595-612
  • High flow rate per power electroosmotic pumping using low ion density solvents SENSORS AND ACTUATORS A-PHYSICAL Kim, D., Posner, J. D., Santiago, J. G. 2008; 141 (1): 201-212
  • Preconcentration, separation, and indirect detection of nonfluorescent analytes using fluorescent mobility markers ANALYTICAL CHEMISTRY Khurana, T. K., Santiago, J. G. 2008; 80 (1): 279-286

    Abstract

    We present a method to achieve separation and indirect detection of nonfluorescent species using fluorescent mobility markers. This technique leverages isotachophoresis (ITP) for both preconcentration and separation. We employ a leading electrolyte (LE), trailing electrolyte (TE), and a set of fluorescent markers of mobilities designed to bound those of nonfluorescent analytes of interest. Fluorescent markers and nonfluorescent analytes are initially mixed homogenously and ITP is initiated. The dynamics of isotachophoresis cause the analyte and fluorescent marker mixture to segregate into respective zones between the LE and TE in the order of reducing mobility. Unlabeled analytes are detected as gaps (regions with local minimums in intensity) in the fluorescent signals of mobility markers. We have successfully demonstrated preconcentration, separation, and detection of unlabeled amino acids serine, glycine, and phenylalanine; and of acetic acid, aspartic acid, and 3-phenylpropionic acid. We show detection of 12 microM concentration of analytes with signal-to-noise ratio of 4.0 and with a high degree of repeatability. We discuss methods for encoding mobility marker identity using marker fluorescence intensity level and alternating fluorescence emission wavelengths. We present example experimental results of fluorescence intensity level encoding.

    View details for DOI 10.1021/ac701706h

    View details for Web of Science ID 000252026900042

    View details for PubMedID 18031059

  • Quick Measurement of Electroosmotic Flow Velocity Chips & Tips, Lab on a Chip Nohmi, M., Santiago, J., G. 2008
  • Toward orientation-independent design for gas recombination in closed-loop electroosmotic pumps SENSORS AND ACTUATORS B-CHEMICAL Lin, C., Yao, S., Posner, J. D., Myers, A. M., Santiago, J. G. 2007; 128 (1): 334-339
  • Investigation of internal pressure gradients generated in electrokinetic flows with axial conductivity gradients EXPERIMENTS IN FLUIDS Devasenathipathy, S., Bharadwaj, R., Santiago, J. G. 2007; 43 (6): 959-967
  • Free-surface microfluidic control of surface-enhanced Raman spectroscopy for the optimized detection of airborne molecules PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Piorek, B. D., Lee, S. J., Santiago, J. G., Moskovits, M., Banerjee, S., Meinhart, C. D. 2007; 104 (48): 18898-18901

    Abstract

    We present a microfluidic technique for sensitive, real-time, optimized detection of airborne water-soluble molecules by surface-enhanced Raman spectroscopy (SERS). The method is based on a free-surface fluidic device in which a pressure-driven liquid microchannel flow is constrained by surface tension. A colloidal suspension of silver nanoparticles flowing through the microchannel that is open to the atmosphere absorbs gas-phase 4-aminobenzenethiol (4-ABT) from the surrounding environment. As surface ions adsorbed on the colloid nanoparticles are substituted by 4-ABT, the colloid aggregates, forming SERS "hot spots" whose concentrations vary predictably along the microchannel flow. 4-ABT confined in these hot spots produces SERS spectra of very great intensity. An aggregation model is used to account quantitatively for the extent of colloid aggregation as determined from the variation of the SERS intensity measured as a function of the streamwise position along the microchannel, which also corresponds to nanoparticle exposure time. This allows us to monitor simultaneously the nanoparticle aggregation process and to determine the location at which the SERS signal is optimized.

    View details for DOI 10.1073/pnas.0708596104

    View details for Web of Science ID 000251498700007

    View details for PubMedID 18025462

  • Current distribution in polymer electrolyte membrane fuel cell with active water management JOURNAL OF POWER SOURCES Strickland, D. G., Litster, S., Santiago, J. G. 2007; 174 (1): 272-281
  • Free-solution oligonucleotide separation in nanoscale channels ANALYTICAL CHEMISTRY Pennathur, S., Baldessari, F., Santiago, J. G., Kattah, M. G., Steinman, J. B., Utz, P. J. 2007; 79 (21): 8316-8322

    Abstract

    In this paper, we report an experimental study of electrokinetic transport and separation of double-stranded deoxyribonucleic acid (dsDNA) oligonucleotides in custom-fabricated fused-silica nanochannels filled with a gel-free sodium borate aqueous buffer. Mixtures of fluorescently labeled dsDNA molecules in the range of 10-100 base pair (bp), fluorescein, and fluorescein-12-UTP (UTP) were separated in less than 120 s in channels of depth ranging from 40 to 1560 nm. We varied the channel depth and background buffer concentration to achieve a 0.006-0.2 range of Debye length-to-channel-half-depth ratio (lambdaD/h), and a 0.004-1.7 range of the ratio of length of dsDNA molecule to channel half-depth (l/h). We find observed oligonucleotide migration times depend on both l/h and lambdaD/h. Electrophoretic mobility estimates agree well with published (micrometer-scale channel) values for background electrolyte (BGE) concentrations greater than approximately 10 mM. At BGE concentrations of 1 and 5 mM, mobility estimates in our nanochannels are higher than published values. Of the cases studied, the highest separation sensitivities were achieved in 100 nm channels with 1-10 mM ion density buffers. Potential applications of this technology include rapid small-scale sequencing and other fluorescence-based oligonucleotide separation and detection assays.

    View details for DOI 10.1021/ac0710580

    View details for Web of Science ID 000250584800053

    View details for PubMedID 17883279

  • Taylor-Aris dispersion in temperature gradient focusing ELECTROPHORESIS Huber, D. E., Santiago, J. G. 2007; 28 (14): 2333-2344

    Abstract

    Microfluidic temperature gradient focusing (TGF) uses an axial temperature gradient to induce a gradient in electrophoretic flux within a microchannel. When balanced with an opposing fluid flow, charged analytes simultaneously focus and separate according to their electrophoretic mobilities. We present a theoretical and experimental study of dispersion in TGF. We model the system using generalized dispersion analysis that yields a 1-D convection-diffusion equation that contains dispersion terms particular to TGF. We consider analytical solutions for the model under uniform temperature gradient conditions. Using a custom TGF experimental setup, we compare focusing measurements with the theoretical predictions. We find that the theory well represents the focusing behavior for flows within the Taylor-Aris dispersion regime.

    View details for DOI 10.1002/elps.200600830

    View details for Web of Science ID 000248390900001

    View details for PubMedID 17578841

  • Comments on the conditions for similitude in electroosmotic flows JOURNAL OF COLLOID AND INTERFACE SCIENCE Santiago, J. G. 2007; 310 (2): 675-677

    Abstract

    This note provides a few comments on the conditions required for similitude between velocity and electric field in electroosmotic flows. The velocity fields of certain electroosmotic flows with relatively thin electric double layers (EDLs) are known to be irrotational in regions outside of the EDL. Under restricted conditions, the velocity field, V , can be expressed in terms of the electric field, E , as V =cE , where c is a scalar constant. The irrotationality solution is certainly unique and exact for Stokes flow, but may not be stable (or unique) for flows with Reynolds numbers significantly greater than unity.

    View details for DOI 10.1016/j.jcis.2007.01.088

    View details for Web of Science ID 000246459700040

    View details for PubMedID 17350645

  • Engineering model of a passive planar air breathing fuel cell cathode JOURNAL OF POWER SOURCES O'Hayre, R., Fabian, T., Litster, S., Prinz, F. B., Santiago, J. G. 2007; 167 (1): 118-129
  • On-Chip Electrophoresis Devices: Do’s, Don’ts, and Dooms Chips & Tips, Lab on a Chip Persat, A., Zangle, T., A., Posner, J., D., Santiago, J., G. 2007
  • Detection of 100 aM fluorophores using a high-sensitivity on-chip CE system and transient isotachophoresis ANALYTICAL CHEMISTRY Jung, B., Zhu, Y., Santiago, J. G. 2007; 79 (1): 345-349

    Abstract

    We present a highly sensitive capillary electrophoresis (CE) assay that combines transient, single-interface on-chip isotachophoresis (ITP) and a laser-induced confocal fluorescence detection setup. We performed experimental parametric studies to show the effects of microscope objective specifications and intensity of excitation laser on optimization of a high-sensitivity on-chip CE detection system. Using the optimized detection system, single-molecule detection of Alexa Fluor 488 was demonstrated, and signal data were validated with autocorrelation analysis. We also demonstrated a separation and detection of 100 aM fluorophores (Alexa Fluor 488 and bodipy) in a fast assay using a high-sensitivity on-chip CE detection system and an ITP/CE protocol with no manual buffer exchange steps. This is, to the knowledge of the authors, the highest electrophoretic separation sensitivity ever reported.

    View details for DOI 10.1021/ac060949p

    View details for Web of Science ID 000243143300050

    View details for PubMedID 17194159

  • An electro-osmotic fuel pump for direct methanol fuel cells ELECTROCHEMICAL AND SOLID STATE LETTERS Buie, C. R., Kim, D., Litster, S., Santiago, J. G. 2007; 10 (11): B196-B200

    View details for DOI 10.1149/1.2772083

    View details for Web of Science ID 000249323200007

  • Active water management for PEM fuel cells JOURNAL OF THE ELECTROCHEMICAL SOCIETY Litster, S., Buie, C. R., Fabian, T., Eaton, J. K., Santiago, J. G. 2007; 154 (10): B1049-B1058

    View details for DOI 10.1149/1.2766650

    View details for Web of Science ID 000248984700021

  • Rotational electrophoresis of striped metallic microrods PHYSICAL REVIEW E Rose, K. A., Meier, J. A., Dougherty, G. M., Santiago, J. G. 2007; 75 (1)

    Abstract

    Analytical models are developed for the translation and rotation of metallic rods in a uniform electric field. The limits of thin and thick electric double layers are considered. These models include the effect of stripes of different metals along the length of the particle. Modeling results are compared to experimental measurements for metallic rods. Experiments demonstrate the increased alignment of particles with increasing field strength and the increase in degree of alignment of thin versus thick electric double layers. The metal rods polarize in the applied field and align parallel to its direction due to torques on the polarized charge. The torque due to polarization has a second-order dependence on the electric field strength. The particles are also shown to have an additional alignment torque component due to nonuniform densities along their length. The orientation distributions of dilute suspensions of particles are also shown to agree well with results predicted by a rotational convective-diffusion equation.

    View details for DOI 10.1103/PhysRevE.75.011503

    View details for Web of Science ID 000243893400055

    View details for PubMedID 17358156

  • Measurement of temperature and reaction species in the cathode diffusion layer of a free-convection fuel cell JOURNAL OF THE ELECTROCHEMICAL SOCIETY Fabian, T., O'Hayre, R., Prinz, F. B., Santiago, J. G. 2007; 154 (9): B910-B918

    View details for DOI 10.1149/1.2752971

    View details for Web of Science ID 000248984600013

  • The role of ambient conditions on the performance of a planar, air-breathing hydrogen PEM fuel cell JOURNAL OF POWER SOURCES Fabian, T., Posner, J. D., O'Hayre, R., Cha, S., Eaton, J. K., Prinz, F. B., Santiago, J. G. 2006; 161 (1): 168-182
  • Water management in proton exchange membrane fuel cells using integrated electroosmotic pumping JOURNAL OF POWER SOURCES Buie, C. R., Posner, J. D., Fabian, T., Cha, S., Kim, D., Prinz, F. B., Eaton, J. K., Santiago, J. G. 2006; 161 (1): 191-202
  • Optimization of a microfluidic mixer for studying protein folding kinetics ANALYTICAL CHEMISTRY Hertzog, D. E., Ivorra, B., Mohammadi, B., Bakajin, O., Santiago, J. G. 2006; 78 (13): 4299-4306

    Abstract

    We have applied an optimization method in conjunction with numerical simulations to minimize the mixing time of a microfluidic mixer developed for protein folding studies. The optimization method uses a semideterministic algorithm to find the global minimum of the mixing time by varying the mixer geometry and flow conditions. We describe the minimization problem and constraints and give a brief overview of the optimization algorithm. We present results of the optimization, including the optimized geometry and parameter sensitivities, and we demonstrate the improvement in mixing performance with experiments using microfabricated mixers. The dye-quenching experiments of the original and optimized mixer designs show respective mixing times of 7 and 4 mus, a 40% reduction. The new design also provides more uniform mixing across streamlines that enter the mixer. The optimized mixer is the fastest reported continuous flow mixer for protein folding.

    View details for DOI 10.1021/ac051903j

    View details for Web of Science ID 000238665200014

    View details for PubMedID 16808436

  • Electroosmotic pumps fabricated from porous silicon membranes JOURNAL OF MICROELECTROMECHANICAL SYSTEMS Yao, S., Myers, A. M., Posner, J. D., Rose, K. A., Santiago, J. G. 2006; 15 (3): 717-728
  • A hybrid method for bubble geometry reconstruction in two-phase microchannels EXPERIMENTS IN FLUIDS Wang, E. N., Devasenathipathy, S., Lin, H., Hidrovo, C. H., Santiago, J. G., Goodson, K. E., Kenny, T. W. 2006; 40 (6): 847-858
  • Convective instability of electrokinetic flows in a cross-shaped microchannel JOURNAL OF FLUID MECHANICS Posner, J. D., Santiago, J. G. 2006; 555: 1-42
  • Semi-deterministic and genetic algorithms for global optimization of microfluidic protein-folding devices INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING Ivorra, B., Hertzog, D. E., Mohammadi, B., Santiago, J. G. 2006; 66 (2): 319-333

    View details for DOI 10.1002/nme.1562

    View details for Web of Science ID 000236623800006

  • On-chip millionfold sample stacking using transient isotachophoresis ANALYTICAL CHEMISTRY Jung, B., Bharadwaj, R., Santiago, J. G. 2006; 78 (7): 2319-2327

    Abstract

    We present a simple and robust isotachophoresis (ITP) method that can be integrated with microchip-based capillary electrophoresis (CE) devices to achieve millionfold sample stacking. We performed an experimental parametric study to show the effects of initial sample ion concentration, leading ion concentration, and trailing ion concentration on ITP stacking. We also discuss the usefulness and limitations of a simple one-dimensional nondispersive model and a scaling analysis for dispersion rate. We found that a single-column ITP configuration together with electroosmotic flow suppression and high leading ion concentration provide high-performance ITP and can be integrated readily with CE separation. We demonstrated detection of trace of 100 fM Alexa Fluor 488 (signal-to-noise ratio of 11) with a concentration increase of a factor of 2 x 10(6). Application of our ITP/CE protocol to the stacking and separation of negatively charged fluorescent tracers (Alexa Fluor 488 and bodipy) resulted in a concentration increase of 6.4 x 10(4) and a signal increase of 4.5 x 10(5). The ITP/CE protocol can be performed with a standard microchannel cross design or simple flow control. The method can be implemented with available off-the-shelf chip systems using off-the-shelf voltage control systems and buffer chemistries.

    View details for DOI 10.1021/ac051659w

    View details for Web of Science ID 000236686600037

    View details for PubMedID 16579615

  • ELECTROPHORESIS IN NANOCHANNELS PROCEEDINGS OF THE ASME FLUIDS ENGINEERING DIVISON SUMMER CONFERENCE, VOL 2 Pennathur, S., Baldessari, F., Kattah, M., Utz, P. J., Santiago, J. G. 2006: 589-593
  • Electrophoresis in nanochannels: brief review and speculation. Journal of nanobiotechnology Baldessari, F., Santiago, J. G. 2006; 4: 12-?

    Abstract

    The relevant physical phenomena that dominate electrophoretic transport of ions and macromolecules within long, thin nanochannels are reviewed, and a few papers relevant to the discussion are cited. Sample ion transport through nanochannels is largely a function of their interaction with electric double layer. For small ions, this coupling includes the net effect of the external applied field, the internal field of the double layer, and the non-uniform velocity of the liquid. Adsorption/desorption kinetics and the effects of surface roughness may also be important in nanochannel electrophoresis. For macromolecules, the resulting motion is more complex as there is further coupling via steric interactions and perhaps polarization effects. These complex interactions and coupled physics represent a valuable opportunity for novel electrophoretic and chromatographic separations.

    View details for PubMedID 17116262

  • Dynamics of field-amplified sample stacking JOURNAL OF FLUID MECHANICS Bharadwaj, R., Santiago, J. G. 2005; 543: 57-92
  • Electrokinetic transport in nanochannels. 1. Theory ANALYTICAL CHEMISTRY Pennathur, S., Santiago, J. G. 2005; 77 (21): 6772-6781

    Abstract

    Electrokinetic transport in fluidic channels facilitates control and separation of ionic species. In nanometer-scale electrokinetic systems, the electric double layer thickness is comparable to characteristic channel dimensions, and this results in nonuniform velocity profiles and strong electric fields transverse to the flow. In such channels, streamwise and transverse electromigration fluxes contribute to the separation and dispersion of analyte ions. In this paper, we report on analytical and numerical models for nanochannel electrophoretic transport and separation of neutral and charged analytes. We present continuum-based theoretical studies in nanoscale channels with characteristic depths on the order of the Debye length. Our model yields analytical expressions for electroosmotic flow, species transport velocity, streamwise-transverse concentration field distribution, and ratio of apparent electrophoretic mobility for a nanochannel to (standard) ion mobility. The model demonstrates that the effective mobility governing electrophoretic transport of charged species in nanochannels depends not only on electrolyte mobility values but also on zeta potential, ion valence, and background electrolyte concentration. We also present a method we term electrokinetic separation by ion valence (EKSIV) whereby both ion valence and ion mobility may be determined independently from a comparison of micro- and nanoscale transport measurements. In the second of this two-paper series, we present experimental validation of our models.

    View details for Web of Science ID 000233125400014

    View details for PubMedID 16255573

  • Electrokinetic transport in nanochannels. 2. Experiments ANALYTICAL CHEMISTRY Pennathur, S., Santiago, J. G. 2005; 77 (21): 6782-6789

    Abstract

    We present an experimental study of nanoscale electrokinetic transport in custom-fabricated quartz nanochannels using quantitative epifluorescence imaging and current monitoring techniques. One aim is to yield insight into electrical double layer physics and study the applicability of continuum theory to nanoscale electrokinetic systems. A second aim is to explore a new separation modality offered by nanoscale electrophoretic separations. We perform parametric variations of applied electric field, channel depth, background buffer concentration, and species valence to impose variations on zeta potential, effective mobility, and Debye length among other parameters. These measurements were used to validate a continuum theory-based analytical model presented in the first of this two-paper series. Our results confirm the usefulness of continuum theory in predicting electrokinetic transport and electrophoretic separations in nanochannels. Our model leverages independent measurements of zeta potential performed in a microchannel system at electrolyte concentrations of interest. These data yield a zeta potential versus concentration relation that is used as a boundary condition for the nanochannel electrokinetic transport model. The data and model comparisons together show that the effective mobility governing electrophoretic transport of charged species in nanochannels depends not only on ion mobility values but also on the shape of the electric double layer and analyte ion valence. We demonstrate a method we term electrokinetic separation by ion valence, whereby both ion valence and mobility may be determined independently from a comparison of micro- and nanoscale transport measurements.

    View details for DOI 10.1021/ac0508346

    View details for Web of Science ID 000233125400015

    View details for PubMedID 16255574

  • Multiple-species model for electrokinetic instability PHYSICS OF FLUIDS Oddy, M. H., Santiago, J. G. 2005; 17 (6)

    View details for DOI 10.1063/1.1931727

    View details for Web of Science ID 000229749500026

  • Convective and absolute electrokinetic instability with conductivity gradients JOURNAL OF FLUID MECHANICS Chen, C. H., Lin, H., Lele, S. K., Santiago, J. G. 2005; 524: 263-303
  • A Multiple-Species Model for Electrokinetic Instability Physics of Fluids Oddy, M., H., Santiago, J., G. 2005; 6 (17): 064108(1)- 064108(17)
  • Electrokinetic instabilities in thin microchannels PHYSICS OF FLUIDS Storey, B. D., Tilley, B. S., Lin, H., Santiago, J. G. 2005; 17 (1)

    View details for DOI 10.1063/1.1823911

    View details for Web of Science ID 000226358000033

  • Microfluidic mixers for UV studies of unlabeled proteins MICRO TOTAL ANALYSIS SYSTEMS 2004, VOL 1 Hertzog, D., Santiago, J., Bakajin, O. 2005: 539-541
  • Femtomole mixer for microsecond kinetic studies of protein folding ANALYTICAL CHEMISTRY Hertzog, D. E., Michalet, X., Jager, M., Kong, X. X., Santiago, J. G., Weiss, S., Bakajin, O. 2004; 76 (24): 7169-7178

    Abstract

    We have developed a microfluidic mixer for studying protein folding and other reactions with a mixing time of 8 mus and sample consumption of femtomoles. This device enables us to access conformational changes under conditions far from equilibrium and at previously inaccessible time scales. In this paper, we discuss the design and optimization of the mixer using modeling of convective diffusion phenomena and a characterization of the mixer performance using microparticle image velocimetry, dye quenching, and Forster resonance energy-transfer (FRET) measurements of single-stranded DNA. We also demonstrate the feasibility of measuring fast protein folding kinetics using FRET with acyl-CoA binding protein.

    View details for DOI 10.1021/ac048661s

    View details for Web of Science ID 000225781700003

    View details for PubMedID 15595857

  • Nucleation and growth of vapor bubbles in a heated silicon microchannel JOURNAL OF HEAT TRANSFER-TRANSACTIONS OF THE ASME Wang, E. N., Devasenathipathy, S., Santiago, J. G., Goodson, K. E., Kenny, T. W. 2004; 126 (4): 497-497
  • A laser induced cavitation pump JOURNAL OF MICROMECHANICS AND MICROENGINEERING Wang, G. R., Santiago, J. G., Mungal, M. G., Young, B., Papademetriou, S. 2004; 14 (7): 1037-1046
  • Instability of electrokinetic microchannel flows with conductivity gradients PHYSICS OF FLUIDS Lin, H., Storey, B. D., Oddy, M. H., Chen, C. H., Santiago, J. G. 2004; 16 (6): 1922-1935

    View details for DOI 10.1063/1.1710898

    View details for Web of Science ID 000221345600007

  • A review of micropumps JOURNAL OF MICROMECHANICS AND MICROENGINEERING Laser, D. J., Santiago, J. G. 2004; 14 (6): R35-R64
  • High-pressure electroosmotic pumps based on porous polymer monoliths SENSORS AND ACTUATORS B-CHEMICAL Tripp, J. A., Svec, F., Frechet, J. M., Zeng, S. L., Mikkelsen, J. C., Santiago, J. G. 2004; 99 (1): 66-73
  • Computational study of band-crossing reactions JOURNAL OF MICROELECTROMECHANICAL SYSTEMS Matta, A., Knio, O. M., Ghanem, R. G., Chen, C. H., Santiago, J. G., Debusschere, B., Najm, H. N. 2004; 13 (2): 310-322
  • A method for determining electrophoretic and electroosmotic mobilities using AC and DC electric field particle displacements JOURNAL OF COLLOID AND INTERFACE SCIENCE Oddy, M. H., Santiago, J. G. 2004; 269 (1): 192-204

    Abstract

    We have developed a method for measuring the electrophoretic mobility of submicrometer, fluorescently labeled particles and the electroosmotic mobility of a microchannel. We derive explicit expressions for the unknown electrophoretic and the electroosmotic mobilities as a function of particle displacements resulting from alternating current (AC) and direct current (DC) applied electric fields. Images of particle displacements are captured using an epifluorescent microscope and a CCD camera. A custom image-processing code was developed to determine image streak lengths associated with AC measurements, and a custom particle tracking velocimetry (PTV) code was devised to determine DC particle displacements. Statistical analysis was applied to relate mobility estimates to measured particle displacement distributions.

    View details for DOI 10.1016/S0021-9797(03)00601-5

    View details for Web of Science ID 000187574400026

    View details for PubMedID 14651913

  • Porous glass electroosmotic pumps: design and experiments JOURNAL OF COLLOID AND INTERFACE SCIENCE Yao, S. H., Hertzog, D. E., Zeng, S. L., Mikkelsen, J. C., Santiago, J. G. 2003; 268 (1): 143-153

    Abstract

    An analytical model for electroosmotic flow rate, total pump current, and thermodynamic efficiency reported in a previous paper has been applied as a design guideline to fabricate porous-structure EO pumps. We have fabricated sintered-glass EO pumps that provide maximum flow rates and pressure capacities of 33 ml/min and 1.3 atm, respectively, at applied potential 100 V. These pumps are designed to be integrated with two-phase microchannel heat exchangers with load capacities of order 100 W and greater. Experiments were conducted with pumps of various geometries and using a relevant, practical range of working electrolyte ionic concentration. Characterization of the pumping performance are discussed in the terms of porosity, tortuosity, pore size, and the dependence of zeta potential on bulk ion density of the working solution. The effects of pressure and flow rate on pump current and thermodynamic efficiency are analyzed and compared to the model prediction. In particular, we explore the important tradeoff between increasing flow rate capacity and obtaining adequate thermodynamic efficiency. This research aims to demonstrate the performance of EOF pump systems and to investigate optimal and practical pump designs. We also present a gas recombination device that makes possible the implementation of this pumping technology into a closed-flow loop where electrolytic gases are converted into water and reclaimed by the system.

    View details for DOI 10.1016/S0021-9797(03)00730-6

    View details for Web of Science ID 000186639800019

    View details for PubMedID 14611783

  • Porous glass electroosmotic pumps: theory JOURNAL OF COLLOID AND INTERFACE SCIENCE Yao, S. H., Santiago, J. G. 2003; 268 (1): 133-142

    Abstract

    This paper presents an analytical study of electroosmotic (EO) pumps with porous pumping structures. We have developed an analytical model to solve for electroosmotic flow rate, pump current, and thermodynamic efficiency as a function of pump pressure load for porous-structure EO pumps. The model uses a symmetric electrolyte approximation valid for the high-zeta-potential regime and numerically solves the Poisson-Boltzmann equation for charge distribution in the idealized pore geometry. Generalized scaling of pumping performance is discussed in the context of a parameterization that includes porosity, tortuosity, pore size, bulk ionic density, and the nonuniform conductivity distribution over charge layers. The model also incorporates an approximate ionic-strength-dependent zeta potential formulation.

    View details for DOI 10.1016/S0021-9797(03)00731-8

    View details for Web of Science ID 000186639800018

    View details for PubMedID 14611782

  • Thousandfold signal increase using field-amplified sample stacking for on-chip electrophoresis ELECTROPHORESIS Jung, B., Bharadwaj, R., Santiago, J. G. 2003; 24 (19-20): 3476-3483

    Abstract

    Field-amplified sample stacking (FASS) leverages conductivity gradients between a volume of injected sample and the background buffer to increase sample concentration. A major challenge in applying FASS to on-chip assays is the initial setup of high-conductivity gradient boundaries in the region of the injected sample volume. We have designed, fabricated, and characterized a novel FASS-capillary electrophoresis (CE) chip design that uses a photoinitiated porous polymer structure to facilitate sample injection and flow control for high-gradient FASS. This polymer structure provides a region of high flow resistance that allows the electromigration of sample ions. We have demonstrated an electropherogram signal increase by a factor of 1100 in electrophoretic separations of fluorescein and Bodipy with, respectively, 2 microM and 1 microM initial concentrations.

    View details for DOI 10.1002/elps.200305611

    View details for Web of Science ID 000186457900020

    View details for PubMedID 14595694

  • Particle imaging techniques for microfabricated fluidic systems EXPERIMENTS IN FLUIDS Devasenathipathy, S., Santiago, J. G., Wereley, S. T., Meinhart, C. D., Takehara, K. 2003; 34 (4): 504-514
  • On-chip coupling of isoelectric focusing and free solution electrophoresis for multidimensional separations ANALYTICAL CHEMISTRY Herr, A. E., Molho, J. I., Drouvalakis, K. A., Mikkelsen, J. C., Utz, P. J., Santiago, J. G., Kenny, T. W. 2003; 75 (5): 1180-1187

    Abstract

    We have developed an acrylic microfluidic device that sequentially couples liquid-phase isoelectric focusing (IEF) and free solution capillary electrophoresis (CE). Rapid separation (<1 min) and preconcentration (73x) of species were achieved in the initial IEF dimension. Using full-field fluorescence imaging, we observed nondispersive mobilization velocities on the order of 20 microm/s during characterization of the IEF step. This transport behavior allowed controlled electrokinetic mobilization of focused sample bands to a channel junction, where voltage switching was used to repeatedly inject effluent from the IEF dimension into an ampholyte-based CE separation. This second dimension was capable of analyzing all fluid volumes of interest from the IEF dimension, as IEF was 'parked' during each CE analysis and refocused prior to additional CE analyses. Investigation of each dimension of the integrated system showed time-dependent species displacement and band-broadening behavior consistent with IEF and CE, respectively. The peak capacity of the 2D system was approximately 1300. A comprehensive 2D analysis of a fluid volume spanning 15% of the total IEF channel length was completed in less than 5 min.

    View details for DOI 10.1021/ac026239a

    View details for Web of Science ID 000181259300027

    View details for PubMedID 12641239

  • Incomplete Sensitivities in Design and Control of Fluidic Channels Computer Assisted Mechanics and Engineering Sciences Mohammadi, B., Santiago, J., G. 2003; 10: 201-210
  • Microfluidic flow simulation: Stacking one-dimensional study HOUILLE BLANCHE-REVUE INTERNATIONALE DE L EAU Alexis-Alexandre, G., Mohammadi, B., Santiago, J. G., Bharadwaj, R. 2003: 18-23
  • Incomplete sensitivities for the design of minimal dispersion fluidic channels COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING Mohammadi, B., Molho, J. I., Santiago, J. G. 2003; 192 (37-38): 4131-4145
  • Particle Tracking Techniques for Microfabricated Fluidic Systems Experiments in Fluids Devasenathipathy, S., Santiago, J., G., Wereley, S., T., Meinhart, C., D. 2003; 4 (34): 504-513
  • A planar electroosmotic micropump JOURNAL OF MICROELECTROMECHANICAL SYSTEMS Chen, C. H., Santiago, J. G. 2002; 11 (6): 672-683
  • Photobleached-fluorescence imaging of microflows EXPERIMENTS IN FLUIDS Mosier, B. P., Molho, J. I., Santiago, J. G. 2002; 33 (4): 545-554
  • Design and optimization of on-chip capillary electrophoresis ELECTROPHORESIS Bharadwaj, R., Santiago, J. G., Mohammadi, B. 2002; 23 (16): 2729-2744

    Abstract

    We present a systematic, experimentally validated method of designing electrokinetic injections for on-chip capillary electrophoresis applications. This method can be used to predict point-wise and charge-coupled device (CCD)-imaged electropherograms using estimates of species mobilities, diffusivities and initial sample plug parameters. A simple Taylor dispersion model is used to characterize electrophoretic separations in terms of resolution and signal-to-noise ratio (SNR). Detection convolutions using Gaussian and Boxcar detector response functions are used to relate optimal conditions for resolution and signal as a function of relevant system parameters including electroosmotic mobility, sample injection length, detector length scale, and the length-to-detector. Analytical solutions show a tradeoff between signal-to-noise ratio and resolution with respect to dimensionless injection width and length to the detector. In contrast, there is no tradeoff with respect to the Peclet number as increases in Peclet number favor both SNR and separation solution (R). We validate our model with quantitative epifluorescence visualizations of electrophoretic separation experiments in a simple cross channel microchip. For the pure advection regime of dispersion, we use numerical simulations of the transient convective diffusion processes associated with electrokinetics together with an optimization algorithm to design a voltage control scheme which produces an injection plug that has minimal advective dispersion. We also validate this optimal injection scheme using fluorescence visualizations. These validations show that optimized voltage scheme produces injections with a standard deviation less than one-fifth of the width of the microchannel.

    View details for Web of Science ID 000177888500022

    View details for PubMedID 12210178

  • Particle tracking techniques for electrokinetic microchannel flows ANALYTICAL CHEMISTRY Devasenathipathy, S., Santiago, J. G., Takehara, K. 2002; 74 (15): 3704-3713

    Abstract

    We have applied particle tracking techniques to obtain spatially resolved velocity measurements in electrokinetic flow devices. Both micrometer-resolution particle image velocimetry (micro-PMV) and particle tracking velocimetry (PTV) techniques have been used to quantify and study flow phenomena in electrokinetic systems applicable to microfluidic bioanalytical devices. To make the flow measurements quantitative, we performed a series of seed particle calibration experiments. First, we measure the electroosmotic wall mobility of a borosilicate rectangular capillary (40 by 400 microm) using current monitoring. In addition to this wall mobility characterization, we apply PTV to determine the electrophoretic mobilities of more than 1,000 fluorescent microsphere particles in aqueous buffer solutions. Particles from this calibrated particle/ buffer mixture are then introduced into two electrokinetic flow systems for particle tracking flow experiments. In these experiments, we use micro-PIV, together with an electric field prediction, to obtain electroosmotic flow bulk fluid velocity measurements. The first example flow system is a microchannel intersection where we demonstrate a detailed documentation of the similitude between the electrical fields and the velocity fields in an electrokinetic system with uniform zeta potential, zeta. In the second system, we apply micro-PIV to a microchannel system with nonuniform zeta. The latter experiment provides a simultaneous measurement of two distinct wall mobilities within the microchannel.

    View details for DOI 10.1021/ac011243s

    View details for Web of Science ID 000177239200027

    View details for PubMedID 12175157

  • Electroosmotic flow pumps with polymer frits SENSORS AND ACTUATORS B-CHEMICAL Zeng, S. L., Chen, C. H., Santiago, J. G., Chen, J. R., Zare, R. N., Tripp, J. A., Svec, F., Frechet, J. M. 2002; 82 (2-3): 209-212
  • Measurements and modeling of two-phase flow in microchannels with nearly constant heat flux boundary conditions JOURNAL OF MICROELECTROMECHANICAL SYSTEMS Zhang, L., Koo, J. M., Jiang, L., Asheghi, M., Goodson, K. E., Santiago, J. G., Kenny, T. W. 2002; 11 (1): 12-19
  • Bleached-Fluorescence Imaging of Microflows Experiments in Fluids Mosier, B., P., Molho, J., I., Santiago, J., G. 2002; 4 (33): 545-554
  • Electroosmotic microchannel cooling system for microprocessors Electronics Cooling Goodson, K., E., Santiago, J., G., Kenny, T., Jiang, L., Zeng, S., Koo, J., M. 2002; 8: 46-47
  • Electrokinetic instability micromixing ANALYTICAL CHEMISTRY Oddy, M. H., Santiago, J. G., Mikkelsen, J. C. 2001; 73 (24): 5822-5832

    Abstract

    We have developed an electrokinetic process to rapidly stir micro- and nanoliter volume solutions for microfluidic bioanalytical applications. We rapidly stir microflow streams by initiating a flow instability, which we have observed in sinusoidally oscillating, electroosmotic channel flows. As the effect occurs within an oscillating electroosmotic flow, we refer to it here as an electrokinetic instability (EKI). The rapid stretching and folding of material lines associated with this instability can be used to stir fluid streams with Reynolds numbers of order unity, based on channel depth and rms electroosmotic velocity. This paper presents a preliminary description of the EKI and the design and fabrication of two micromixing devices capable of rapidly stirring two fluid streams using this flow phenomenon. A high-resolution CCD camera is used to record the stirring and diffusion of fluorescein from an initially unmixed configuration. Integration of fluorescence intensity over measurement volumes (voxels) provides a measure of the degree to which two streams are mixed to within the length scales of the voxels. Ensemble-averaged probability density functions and power spectra of the instantaneous spatial intensity profiles are used to quantify the mixing processes. Two-dimensional spectral bandwidths of the mixing images are initially anisotropic for the unmixed configuration, broaden as the stirring associated with the EKI rapidly stretches and folds material lines (adding high spatial frequencies to the concentration field), and then narrow to a relatively isotropic spectrum at the well-mixed conditions.

    View details for DOI 10.1021/ac0155411

    View details for Web of Science ID 000172884900007

    View details for PubMedID 11791550

  • Fabrication and characterization of electroosmotic micropumps SENSORS AND ACTUATORS B-CHEMICAL Zeng, S. L., Chen, C. H., Mikkelsen, J. C., Santiago, J. G. 2001; 79 (2-3): 107-114
  • Electroosmotic flows in microchannels with finite inertial and pressure forces ANALYTICAL CHEMISTRY Santiago, J. G. 2001; 73 (10): 2353-2365

    Abstract

    Emerging microfluidic systems have spurred an interest in the study of electrokinetic flow phenomena in complex geometries and a variety of flow conditions. This paper presents an analysis of the effects of fluid inertia and pressure on the velocity and vorticity field of electroosmotic flows. In typical on-chip electrokinetics applications, the flow field can be separated into an inner flow region dominated by viscous and electrostatic forces and an outer flow region dominated by inertial and pressure forces. These two regions are separated by a slip velocity condition determined by the Helmholtz-Smoulochowski equation. The validity of this assumption is investigated by analyzing the velocity field in a pressure-driven, two-dimensional flow channel with an impulsively started electric field. The regime for which the inner/outer flow model is valid is described in terms of nondimensional parameters derived from this example problem. Next, the inertial forces, surface conditions, and pressure-gradient conditions for a full-field similarity between the electric and velocity fields in electroosmotic flows are discussed. A sufficient set of conditions for this similarity to hold in arbitrarily shaped, insulating wall microchannels is the following: uniform surface charge, low Reynolds number, low Reynolds and Strouhal number product, uniform fluid properties, and zero pressure differences between inlets and outlets. Last, simple relations describing the generation of vorticity in electroosmotic flow are derived using a wall-local, streamline coordinate system.

    View details for Web of Science ID 000168708200030

    View details for PubMedID 11393863

  • Simulation and design of extraction and separation fluidic devices ESAIM-MATHEMATICAL MODELLING AND NUMERICAL ANALYSIS-MODELISATION MATHEMATIQUE ET ANALYSE NUMERIQUE Mohammadi, B., Santiago, J. G. 2001; 35 (3): 513-523
  • Optimization of turn geometries for microchip electrophoresis ANALYTICAL CHEMISTRY Molho, J. I., Herr, A. E., Mosier, B. P., Santiago, J. G., Kenny, T. W., Brennen, R. A., Gordon, G. B., Mohammadi, B. 2001; 73 (6): 1350-1360
  • Passive mixing in a three-dimensional serpentine microchannel JOURNAL OF MICROELECTROMECHANICAL SYSTEMS Liu, R. H., Stremler, M. A., Sharp, K. V., Olsen, M. G., Santiago, J. G., Adrian, R. J., Aref, H., Beebe, D. J. 2000; 9 (2): 190-197
  • A PIV algorithm for estimating time-averaged velocity fields JOURNAL OF FLUIDS ENGINEERING-TRANSACTIONS OF THE ASME Meinhart, C. D., Wereley, S. T., Santiago, J. G. 2000; 122 (2): 285-289
  • Mixing of a sonic transverse jet injected into a supersonic flow AIAA JOURNAL VanLerberghe, W. M., Santiago, J. G., Dutton, J. C., Lucht, R. P. 2000; 38 (3): 470-479
  • Electroosmotic capillary flow with nonuniform zeta potential ANALYTICAL CHEMISTRY Herr, A. E., Molho, J. I., Santiago, J. G., Mungal, M. G., Kenny, T. W., Garguilo, M. G. 2000; 72 (5): 1053-1057
  • Micron-resolution velocimetry techniques LASER TECHNIQUES APPLIED TO FLUID MECHANICS Meinhart, C. D., Wereley, S. T., Santiago, J. G. 2000: 57-70
  • Electroosmotic capillary flow with nonuniform zeta potential Analytical chemistry Herr, A. E., Molho, J. I., Santiago, J. G., Mungal, M. G., Kenny, T. W., Garguilo, M. G. 2000; 72 (5): 1053-7

    Abstract

    The present work is an analytical and experimental study of electroosmotic flow (EOF) in cylindrical capillaries with nonuniform wall surface charge (zeta-potential) distributions. In particular, this study investigates perturbations of electroosmotic flow in open capillaries that are due to induced pressure gradients resulting from axial variations in the wall zeta-potential. The experimental inquiry focuses on electroosmotic flow under a uniform applied field in capillaries with an EOF-suppressing polymer adsorbed onto various fractions of the total capillary length. This fractional EOF suppression was achieved by coupling capillaries with substantially different zeta-potentials. The resulting flow fields were imaged with a nonintrusive, caged-fluorescence imaging technique. Simple analytical models for the velocity field and rate of sample dispersion in capillaries with axial zeta-potential variations are presented. The resulting induced pressure gradients and the associated band-broadening effects are of particular importance to the performance of chemical and biochemical analysis systems such as capillary electrokinetic chromatography and capillary zone electrophoresis.

    View details for PubMedID 10739211

  • PIV measurements of a microchannel flow EXPERIMENTS IN FLUIDS Meinhart, C. D., Wereley, S. T., Santiago, J. G. 1999; 27 (5): 414-419
  • A particle image velocimetry system for microfluidics EXPERIMENTS IN FLUIDS Santiago, J. G., Wereley, S. T., Meinhart, C. D., Beebe, D. J., Adrian, R. J. 1998; 25 (4): 316-319
  • Velocity Measurements of a Jet Injected into a Supersonic Crossflow Journal of Propulsion and Power Santiago, J., G., Dutton, J., C. 1997; 2 (13): 264-273
  • Crossflow Vortices of a Jet Injected into a Supersonic Crossflow AIAA Journal Santiago, J., G., Dutton, J., C. 1997; 5 (35): 915-917

Books and Book Chapters


  • Taylor Dispersion in Sample Pre-Concentration Methods CRC Handbook of Electrophoresis Bharadwaj, R., Huber, D., E., Khurana, T., Santiago, Juan, G. edited by Landers, J. CRC Press. 2008; 3rd: 1085-1120
  • Electrokinetic Flow Diagnostics Micro- and Nano-Scale Diagnostic Techniques Devasenathipathy, S., Santiago, J., G. edited by Breuer, K. New York, Springer Verlag. 2004: 1
  • Liquid Flows in Microchannels CRC Handbook of MEMS Sharp, K., V., Adrian, R., J., Santiago, J., G., Molho, J., I. edited by Gad-el-Hak, M. CRC Press, New York. 2001: 6-1 to 6-38
  • Diagnostic Techniques for Microfluidics Research Developments in Laser Techniques and Applications to Fluid Mechanics Meinhart, C., D., Wereley, S., T., Santiago, J., G. edited by Adrian, R., J., Durao, D., F.G., Durst, F. Springer-Verlag, Berlin. 1999: 1

Conference Proceedings


  • Microfluidic Control of Nanoparticle Aggregation for Surfaced Enhanced Raman Spectroscopy Piorek, B., Lee, S., J., Moskovits, M., Banerjee, S., Santiago, J., G., Meinhart, C. 2007
  • Ballistic Dispersion in Temperature Gradient Focusing Huber, D., E., Santiago, J., G. 2007
  • Temperature gradient focusing in a microfluidic device Huber, D., Santiago, J. G. ASME-AMER SOC MECHANICAL ENG. 2005: 806-806
  • Microsecond mixer for kinetic studies of protein folding Hertzog, D., Michalet, X., Jager, M., Kong, X. X., Santiago, J., Weiss, S., Bakajin, O. CELL PRESS. 2005: 376A-376A
  • Optimized field amplified sample stacking for on-chip capillary electrophoresis. Bharadwaj, R., Jung, Y. S., Santiago, J. G. AMER CHEMICAL SOC. 2004: U116-U116
  • Closed-loop electroosmotic microchannel cooling system for VLSI circuits Jiang, L. N., Mikkelsen, J., Koo, J. M., Huber, D., Yao, S. H., Zhang, L., Zhou, P., Maveety, J. G., Prasher, R., Santiago, J. G., Kenny, T. W., Goodson, K. E. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. 2002: 347-355
  • Particle-image velocimetry measurements in electrokinetic flow. Devasenathipathy, S., Santiago, J. G. AMER CHEMICAL SOC. 2000: U578-U578