Mo Wu

All Publications

• Impedance matching in optically induced dielectrophoresis: Effect of medium conductivity on trapping force. Applied physics letters Zaman, M. A., Wu, M., Ren, W., Hesselink, L. 2024; 125 (5): 051108

  Abstract

  An impedance analysis for optically induced dielectrophoresis is presented. A circuit model is developed for this purpose. The model parameters are fully defined in terms of the geometrical and material properties of the system. It is shown that trapping force can only be generated when the material properties follow certain impedance matching conditions. The impedance match factor is introduced to succinctly quantify the phenomenon. It is used to calculate bounds on the allowed electrical conductivity of the suspension medium. Results from the proposed model are found to be in good agreement with full-wave numerical simulations. By computing the acceptable set of material parameters with little computational cost, the presented analysis can streamline ODEP system design for various applications.

  View details for DOI 10.1063/5.0223354

  View details for PubMedID 39100735

  View details for PubMedCentralID PMC11296733

• Dielectrophoretic bead-droplet reactor for solid-phase synthesis. Nature communications Padhy, P., Zaman, M. A., Jensen, M. A., Cheng, Y. T., Huang, Y., Wu, M., Galambos, L., Davis, R. W., Hesselink, L. 2024; 15 (1): 6159

  Abstract

  Solid-phase synthesis underpins many advances in synthetic and combinatorial chemistry, biology, and material science. The immobilization of a reacting species on the solid support makes interfacing of reagents an important challenge in this approach. In traditional synthesis columns, this leads to reaction errors that limit the product yield and necessitates excess consumption of the mobile reagent phase. Although droplet microfluidics can mitigate these problems, its adoption is fundamentally limited by the inability to controllably interface microbeads and reagent droplets. Here, we introduce Dielectrophoretic Bead-Droplet Reactor as a physical method to implement solid-phase synthesis on individual functionalized microbeads by encapsulating and ejecting them from microdroplets by tuning the supply voltage. Proof-of-concept demonstration of the enzymatic coupling of fluorescently labeled nucleotides onto the bead using this reactor yielded a 3.2-fold higher fidelity over columns through precise interfacing of individual microreactors and beads. Our work combines microparticle manipulation and droplet microfluidics to address a long-standing problem in solid-phase synthesis with potentially wide-ranging implications.

  View details for DOI 10.1038/s41467-024-49284-z

  View details for PubMedID 39039069

  View details for PubMedCentralID PMC11263596

• Spectral tweezers: Single sample spectroscopy using optoelectronic tweezers. Applied physics letters Zaman, M. A., Wu, M., Ren, W., Jensen, M. A., Davis, R. W., Hesselink, L. 2024; 124 (7): 071104

  Abstract

  A scheme that combines optoelectronic tweezers (OET) with spectroscopic analysis is presented. Referred to as spectral tweezers, the approach uses a single focused light beam that acts both as the trapping beam for OET and the probe beam for spectroscopy. Having simultaneous manipulation and spectral characterization ability, the method is used to isolate single micro-samples from clusters and perform spectral measurements. Experimental results show that a characteristic spectral signature can be obtained for a given sample. The proposed approach can be easily integrated into the optical setups used for conventional OETs with only a few additional optical components, making it a convenient tool for bio-analytical applications.

  View details for DOI 10.1063/5.0191871

  View details for PubMedID 38356894

  View details for PubMedCentralID PMC10864034

• Microparticle electrical conductivity measurement using optoelectronic tweezers. Journal of applied physics Ren, W., Zaman, M. A., Wu, M., Jensen, M. A., Davis, R. W., Hesselink, L. 2023; 134 (11): 113104

  Abstract

  When it comes to simulate or calculate an optoelectronic tweezer (OET) response for a microparticle suspended in a given medium, a precise electrical conductivity (later referred to as conductivity) value for the microparticle is critical. However, there are not well-established measurements or well-referenced values for microparticle conductivities in the OET realm. Thus, we report a method based on measuring the escape velocity of a microparticle with a standard OET system to calculate its conductivity. A widely used 6mum polystyrene bead (PSB) is used for the study. The conductivity values are found to be invariant around 2*10-3S/m across multiple different aqueous media, which helps clarify the ambiguity in the usage of PSB conductivity. Our convenient approach could principally be applied for the measurement of multiple unknown OET-relevant material properties of microparticle-medium systems with various OET responses, which can be beneficial to carry out more accurate characterization in relevant fields.

  View details for DOI 10.1063/5.0169565

  View details for PubMedID 37736285

• Resolution improvement of optoelectronic tweezers using patterned electrodes. Applied physics letters Zaman, M. A., Wu, M., Ren, W., Jensen, M. A., Davis, R. W., Hesselink, L. 2023; 123 (4): 041104

  Abstract

  An optoelectronic tweezer (OET) device is presented that exhibits improved trapping resolution for a given optical spot size. The scheme utilizes a pair of patterned physical electrodes to produce an asymmetric electric field gradient. This, in turn, generates an azimuthal force component in addition to the conventional radial gradient force. Stable force equilibrium is achieved along a pair of antipodal points around the optical beam. Unlike conventional OETs where trapping can occur at any point around the beam perimeter, the proposed scheme improves the resolution by limiting trapping to two points. The working principle is analyzed by performing numerical analysis of the electromagnetic fields and corresponding forces. Experimental results are presented that show the trapping and manipulation of micro-particles using the proposed device.

  View details for DOI 10.1063/5.0160939

  View details for PubMedID 37502178

  View details for PubMedCentralID PMC10371355

• Topological visualization of the plasmonic resonance of a nano C-aperture. Applied physics letters Zaman, M. A., Ren, W., Wu, M., Padhy, P., Hesselink, L. 2023; 122 (8): 081107

  Abstract

  The plasmonic response of a nano C-aperture is analyzed using the Vector Field Topology (VFT) visualization technique. The electrical currents that are induced on the metal surfaces when the C-aperture is excited by light is calculated for various wavelengths. The topology of this two-dimensional current density vector is analyzed using VFT. The plasmonic resonance condition is found to coincide with a distinct shift in the topology which leads to increased current circulation. A physical explanation of the phenomenon is discussed. Numerical results are presented to justify the claims. The analyses suggest that VFT can be a powerful tool for studying the physical mechanics of nano-photonic structures.

  View details for DOI 10.1063/5.0143309

  View details for PubMedID 36846092

• Controlled Transport of Individual Microparticles Using Dielectrophoresis. Langmuir : the ACS journal of surfaces and colloids Zaman, M. A., Padhy, P., Wu, M., Ren, W., Jensen, M. A., Davis, R. W., Hesselink, L. 2022

  Abstract

  A dielectrophoretic device employing a planar array of microelectrodes is designed for controlled transport of individual microparticles. By exciting the electrodes in sequence, a moving dielectrophoretic force is created that can drag a particle across the electrodes in a straight line. The electrode shapes are designed to counter any lateral drift of the trapped particle during transport. This facilitates single particle transport by creating a narrow two-dimensional corridor for the moving dielectrophoretic force to operate on. The design and analysis processes are discussed in detail. Numerical simulations are performed to calculate the electromagnetic field distribution and the generated dielectrophoretic force near the electrodes. The Langevin equation is used for analyzing the trajectory of a microparticle under the influence of the external forces. The simulations show how the designed electrode geometry produces the necessary lateral confinement required for successful particle transport. Finally, experimental results are presented showing controlled bidirectional linear transport of single polystyrene beads of radius 10 and 5 μm for a distances 840 and 1100 μm, respectively. The capabilities of the proposed platform make it suitable for micro total analysis systems (μTAS) and lab-on-a-chip (LOC) applications.

  View details for DOI 10.1021/acs.langmuir.2c02235

  View details for PubMedID 36541659

• Modeling Brownian Microparticle Trajectories in Lab-on-a-Chip Devices with Time Varying Dielectrophoretic or Optical Forces. Micromachines Zaman, M. A., Wu, M., Padhy, P., Jensen, M. A., Hesselink, L., Davis, R. W. 2021; 12 (10)

  Abstract

  Lab-on-a-chip (LOC) devices capable of manipulating micro/nano-sized samples have spurred advances in biotechnology and chemistry. Designing and analyzing new and more advanced LOCs require accurate modeling and simulation of sample/particle dynamics inside such devices. In this work, we present a generalized computational physics model to simulate particle/sample trajectories under the influence of dielectrophoretic or optical forces inside LOC devices. The model takes into account time varying applied forces, Brownian motion, fluid flow, collision mechanics, and hindered diffusion caused by hydrodynamic interactions. We develop a numerical solver incorporating the aforementioned physics and use it to simulate two example cases: first, an optical trapping experiment, and second, a dielectrophoretic cell sorter device. In both cases, the numerical results are found to be consistent with experimental observations, thus proving the generality of the model. The numerical solver can simulate time evolution of the positions and velocities of an arbitrarily large number of particles simultaneously. This allows us to characterize and optimize a wide range of LOCs. The developed numerical solver is made freely available through a GitHub repository so that researchers can use it to develop and simulate new designs.

  View details for DOI 10.3390/mi12101265

  View details for PubMedID 34683316

• Microparticle transport along a planar electrode array using moving dielectrophoresis. Journal of applied physics Zaman, M. A., Padhy, P., Ren, W., Wu, M., Hesselink, L. 2021; 130 (3): 034902

  Abstract

  We present a device that can achieve controlled transport of colloidal microparticles using an array of micro-electrodes. By exciting the micro-electrodes in regular sequence with an AC voltage, a time-varying moving dielectrophoretic force-field is created. This force propels colloidal microparticles along the electrode array. Using this method, we demonstrate bidirectional transport of polystyrene micro-spheres. Electromagnetic simulation of the device is performed, and the dielectrophoretic force profile around the electrode array is mapped. We develop a Brownian dynamics model of the trajectory of a particle under the influence of the time-varying force-field. Numerical and experimental results showing controlled particle transport are presented. The numerical model is found to be in good agreement with experimental data. The developed numerical framework can be useful in designing and modeling lab-on-a-chip devices that employ external non-contact forces for micro-/nanoparticle manipulation.

  View details for DOI 10.1063/5.0049126

  View details for PubMedID 34334807