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Electrokinetically driven microfluidic devices that are used for biological cell/particle manipulation (e.g., cell sorting, separation) involve electrokinetic transport of these particles in microchannels whose dimension is comparable with particles' size. This paper presents an analytical study on electrokinetic transport of a charged spherical particle in a charged parallel-plate microchannel. Under the thin electric double-layer assumption, solutions in closed-form solutions for the particle velocity and disturbed electrical and fluid velocity fields are obtained for plane-symmetric (along the channel centerline) and asymmetric (off the channel centerline) motions of a sphere in a parallel-plate microchannel. The effects of relative particle size and eccentricity (i.e., off the centerline distance) on a particle's translational and rotational velocities are analyzed.

Dielectrophoresis (DEP) has been shown to have significant potential for the characterization of cells and could become an efficient tool for rapid identification and assessment of microorganisms. The present work is focused on the trapping, characterization, and separation of two species of Cryptosporidium (C. parvum and C. muris) and Giardia lambia (G. lambia) using a microfluidic experimental setup. Cryptosporidium oocysts, which are 2-4 lm in size and nearly spherical in shape, are used for the preliminary stage of prototype development and testing. G. lambia cysts are 8-12 lm in size. In order to facilitate effective trapping, simulations were performed to study the effects of buffer conductivity and applied voltage on the flow and cell transport inside the DEP chip. Microscopic experiments were performed using the fabricated device and the real part of Clausius-Mossotti factor of the cells was estimated from critical voltages for particle trapping at the electrodes under steady fluid flow. The dielectric properties of the cell compartments (cytoplasm and membrane) were calculated based on a single shell model of the cells. The separation of C. muris and G. lambia is achieved successfully at a frequency of 10 MHz and a voltage of 3 Vpp (peak to peak voltage).

The present work reports numerical simulation and experimental validation of novel designs of microfluidic mixers that can be employed for biological mixing applications. Numerical simulations involving various geometrical models were performed for design optimization. The effect of the presence of embedded obstacles was studied in detail, in order to understand the effect of channel occlusion on micromixing. The mixing performance of various channel designs was compared, and crossover in the mixing performance of the designs was observed in response to a change in the flow Reynolds number (Re). The improvement in micromixing efficiency was discussed in connection with the variations in local values of the Reynolds number and Dean number. It was observed that the presence of obstacles contributes to a significant increase in local Re in the vicinity of sharp-edged obstacles, thereby enhancing the efficiency of mixing. In addition, the local Dean number is observed to increase significantly inside spiral microfluidic designs. We validate the optimized microfluidic mixer designs by performing micromixing experiments and image analysis based on regions of interest along the length of the channels. Numerical predictions were observed to be in reasonable agreement with experimental results. Finally, we demonstrated the biological applicability of an optimized micromixer design for on-chip detection of calcium levels in blood serum. The passive mixing designs presented in this work are useful for chip-scale implementations of cell-drug biology, where some of the key cell signaling processes appear at second time scales.

This study reports a theoretical and experimental study on the irreversible deposition of colloidal particles from electrokinetic microfluidic flow. The electrokinetic particle transport model presented in this study is based on the stochastic Langevin equation, incorporating the electrical, hydrodynamic, Derjaguin-Landau-Verwey-Overbeek colloidal interactions and random Brownian motion of colloidal particles. Brownian dynamics simulation is used to compute the particle deposition in terms of the surface coverage. Direct videomicroscopic observation using the parallel-plate flow cell technique is employed to determine the deposition kinetics of polystyrene latex particles in NaCl electrolytes. The theoretical predictions are compared with experimental results, and a reasonable agreement is found.