Bio


Our research is broadly defined by multiphysics problems in fluid dynamics and transport engineering. Our work contributes to the understanding of these problems primarily through theoretical tools such as techniques of applied mathematics as well as massively-parallel simulations. Numerical simulations enable quantitative visualization of the detailed physical processes which can be difficult to detect experimentally. They also provide quantitative data that guide the development of reduced-order models, which would naturally induce insight for design, optimization and control. Most of our work involves complementary interactions with experimental groups within and outside of Stanford. Specific current research topics include:

(1) Electro-convection and microscale chaos near electrochemical interfaces

(2) Particle-laden flows with applications in solar receivers

(3) Applications of superhydrophobic surfaces for drag reduction of turbulent flows

(4) Micro-bubble generation by breaking waves

(5) Electrokinetics of micropores and nanopores

Academic Appointments


Program Affiliations


  • Institute for Computational and Mathematical Engineering (ICME)

Professional Education


  • PhD, Stanford University, Mechanical Engineering (2009)
  • M.S., Stanford University, Mechanical Engineering (2004)
  • B.S., Sharif University of Technology, Mechanical Engineering (2002)

2018-19 Courses


Stanford Advisees


All Publications


  • Chaotic induced-charge electro-osmosis. Physical review letters Davidson, S. M., Andersen, M. B., Mani, A. 2014; 112 (12): 128302-?

    Abstract

    We present direct numerical simulations of the coupled Poisson-Nernst-Planck and Navier-Stokes equations for an electrolyte around a polarizable cylinder subject to an external electric field. For high fields, a novel chaotic flow phenomenon is discovered. Our calculations indicate significant improvement in the prediction of the mean flow relative to standard asymptotic models. These results open possibilities for chaos-enhanced mixing in microdevices and provide insight into barriers to efficient electrokinetic micropumps with broad applications in electrochemical and lab-on-a-chip systems.

    View details for PubMedID 24724683

  • Overlimiting Current and Shock Electrodialysis in Porous Media LANGMUIR Deng, D., Dydek, E. V., Han, J., Schlumpberger, S., Mani, A., Zaltzman, B., Bazant, M. Z. 2013; 29 (52): 16167-16177

    Abstract

    Most electrochemical processes, such as electrodialysis, are limited by diffusion, but in porous media, surface conduction and electroosmotic flow also contribute to ionic flux. In this article, we report experimental evidence for surface-driven overlimiting current (faster than diffusion) and deionization shocks (propagating salt removal) in a porous medium. The apparatus consists of a silica glass frit (1 mm thick with a 500 nm mean pore size) in an aqueous electrolyte (CuSO4 or AgNO3) passing ionic current from a reservoir to a cation-selective membrane (Nafion). The current-voltage relation of the whole system is consistent with a proposed theory based on the electroosmotic flow mechanism over a broad range of reservoir salt concentrations (0.1 mM to 1.0 M) after accounting for (Cu) electrode polarization and pH-regulated silica charge. Above the limiting current, deionized water (≈10 μM) can be continuously extracted from the frit, which implies the existence of a stable shock propagating against the flow, bordering a depleted region that extends more than 0.5 mm across the outlet. The results suggest the feasibility of shock electrodialysis as a new approach to water desalination and other electrochemical separations.

    View details for DOI 10.1021/la4040547

    View details for Web of Science ID 000329332000017

    View details for PubMedID 24320737

  • Direct numerical simulation of electroconvective instability and hydrodynamic chaos near an ion-selective surface PHYSICS OF FLUIDS Druzgalski, C. L., Andersen, M. B., Mani, A. 2013; 25 (11)

    View details for DOI 10.1063/1.4818995

    View details for Web of Science ID 000329184100005

  • Current-Induced Membrane Discharge PHYSICAL REVIEW LETTERS Andersen, M. B., van Soestbergen, M., Mani, A., Bruus, H., Biesheuvel, P. M., Bazant, M. Z. 2012; 109 (10)

    Abstract

    Possible mechanisms for overlimiting current (OLC) through aqueous ion-exchange membranes (exceeding diffusion limitation) have been debated for half a century. Flows consistent with electro-osmotic instability have recently been observed in microfluidic experiments, but the existing theory neglects chemical effects and remains to be quantitatively tested. Here, we show that charge regulation and water self-ionization can lead to OLC by "current-induced membrane discharge" (CIMD), even in the absence of fluid flow, in ion-exchange membranes much thicker than the local Debye screening length. Salt depletion leads to a large electric field resulting in a local pH shift within the membrane with the effect that the membrane discharges and loses its ion selectivity. Since salt co-ions, H(+) ions, and OH(-) ions contribute to OLC, CIMD interferes with electrodialysis (salt counterion removal) but could be exploited for current-assisted ion exchange and pH control. CIMD also suppresses the extended space charge that leads to electro-osmotic instability, so it should be reconsidered in both models and experiments on OLC.

    View details for DOI 10.1103/PhysRevLett.109.108301

    View details for Web of Science ID 000308346900011

    View details for PubMedID 23005334

  • Analysis and optimization of numerical sponge layers as a nonreflective boundary treatment JOURNAL OF COMPUTATIONAL PHYSICS Mani, A. 2012; 231 (2): 704-716
  • Physics and Computation of Aero-Optics ANNUAL REVIEW OF FLUID MECHANICS, VOL 44 Wang, M., Mani, A., Gordeyev, S. 2012; 44: 299-321
  • Deionization shocks in microstructures PHYSICAL REVIEW E Mani, A., Bazant, M. Z. 2011; 84 (6)

    Abstract

    Salt transport in bulk electrolytes is limited by diffusion and advection, but in microstructures with charged surfaces (e.g., microfluidic devices, porous media, soils, or biological tissues) surface conduction and electro-osmotic flow also contribute to ionic fluxes. For small applied voltages, these effects lead to well known linear electrokinetic phenomena. In this paper, we predict some surprising nonlinear dynamics that can result from the competition between bulk and interfacial transport at higher voltages. When counterions are selectively removed by a membrane or electrode, a "deionization shock" can propagate through the microstructure, leaving in its wake an ultrapure solution, nearly devoid of coions and colloidal impurities. We elucidate the basic physics of deionization shocks and develop a mathematical theory of their existence, structure, and stability, allowing for slow variations in surface charge or channel geometry. Via asymptotic approximations and similarity solutions, we show that deionization shocks accelerate and sharpen in narrowing channels, while they decelerate and weaken, and sometimes disappear, in widening channels. These phenomena may find applications in separations (deionization, decontamination, biological assays) and energy storage (batteries, supercapacitors) involving electrolytes in microstructures.

    View details for DOI 10.1103/PhysRevE.84.061504

    View details for Web of Science ID 000298669200006

    View details for PubMedID 22304094

  • Overlimiting Current in a Microchannel PHYSICAL REVIEW LETTERS Dydek, E. V., Zaltzman, B., Rubinstein, I., Deng, D. S., Mani, A., Bazant, M. Z. 2011; 107 (11)

    Abstract

    We revisit the classical problem of diffusion-limited ion transport to a membrane (or electrode) by considering the effects of charged sidewalls. Using simple mathematical models and numerical simulations, we identify three basic mechanisms for overlimiting current in a microchannel: (i) surface conduction carried by excess counterions, which dominates for very thin channels, (ii) convection by electro-osmotic flow on the sidewalls, which dominates for thicker channels, and (iii) transitions to electro-osmotic instability on the membrane end in very thick channels. These intriguing electrokinetic phenomena may find applications in biological separations, water desalination, and electrochemical energy storage.

    View details for DOI 10.1103/PhysRevLett.107.118301

    View details for Web of Science ID 000294569600007

    View details for PubMedID 22026706

  • 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

  • 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

  • Prediction of Sound Generated by Complex Flows at Low Mach Numbers AIAA JOURNAL Khalighi, Y., Mani, A., Ham, F., Moin, P. 2010; 48 (2): 306-316

    View details for DOI 10.2514/1.42583

    View details for Web of Science ID 000274803400007

  • 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

  • Suitability of artificial bulk viscosity for large-eddy simulation of turbulent flows with shocks JOURNAL OF COMPUTATIONAL PHYSICS Mani, A., Larsson, J., Moin, P. 2009; 228 (19): 7368-7374
  • Computational study of optical distortions by separated shear layers and turbulent wakes JOURNAL OF FLUID MECHANICS Mani, A., Moin, P., Wang, M. 2009; 625: 273-298
  • 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

  • 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

  • Resolution requirements for aero-optical simulations JOURNAL OF COMPUTATIONAL PHYSICS Mani, A., Wang, M., Moin, P. 2008; 227 (21): 9008-9020
  • Statistical description of the free-space propagation of highly aberrated optical beams JOURNAL OF THE OPTICAL SOCIETY OF AMERICA A-OPTICS IMAGE SCIENCE AND VISION Mani, A., Wang, M., Moin, P. 2006; 23 (12): 3027-3035

    Abstract

    The free-space propagation of initially aberrated optical beams is considered with an emphasis on aero-optical applications. An exact statistical solution of the paraxial wave equation is derived that can be used to obtain statistics of the beam such as beam center, spread, and higher-order statistics as algebraic functions of propagation distance, wavelength, and statistics of the initial wavefront. Correlations between the proposed description and intensity-based statistics, such as the Strehl ratio, are investigated. It is found that the root-mean-square (rms) of the gradient of the wavefront plays an important role in causing coherence degradation and that the rms of the wavefront error is not always an appropriate measure of the degradation. To illustrate the use of this statistical tool, index of refraction data from a numerical simulation of compressible flow over a cylinder are employed to perform an aero-optical analysis.

    View details for Web of Science ID 000242326400005

    View details for PubMedID 17106458