All Publications

  • 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


    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


    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


    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)


    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


    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