Xiangchun Xuan

Dielectrophoretic focusing of particles in a microchannel constriction using DC‐biased AC flectric fields

This paper presents a fundamental study of particle electrokinetic focusing in a single microchannel constriction. Through both experiments and simulations, we demonstrate that such dielectrophoresis‐induced particle focusing can be implemented in a much smaller magnitude of DC‐biased AC electric fields (10 kV/m in total) as compared to pure DC electric fields (up to 100 kV/m). This is attributed to the increase in the ratio of cross‐stream particle dielectrophoretic velocity to streamwise electrokinetic velocity as only the DC field component contributes to the latter. The effects of the 1 kHz frequency AC to DC electric field ratio on particle trajectories and velocity variations through the microchannel constriction are also examined, which are found to agree with the simulation results.

DC-dielectrophoretic separation of microparticles using an oil droplet obstacle

A new dielectrophoretic particle separation method is demonstrated and examined in the following experimental study. Current electrodeless dielectrophoretic (DEP) separation techniques utilize insulating solid obstacles in a DC or low-frequency AC field, while this novel method employs an oil droplet acting as an insulating hurdle between two electrodes. When particles move in a non-uniform DC field locally formed by the droplet, they are exposed to a negative DEP force linearly dependent on their volume, which allows the particle separation by size. Since the size of the droplet can be dynamically changed, the electric field gradient, and hence DEP force, becomes easily controllable and adjustable to various separation parameters. By adjusting the droplet size, particles of three different diameter sizes, 1 microm, 5.7 microm and 15.7 microm, were successfully separated in a PDMS microfluidic chip, under applied field strength in the range from 80 V cm-1 to 240 V cm-1. A very effective separation was realized at the low field strength, since the electric field gradient was proved to be a more significant parameter for particle discrimination than the applied voltage. By utilizing low strength fields and adaptable field gradient, this method can also be applied to the separation of biological samples that are generally very sensitive to high electric potential.

Distinguishing the viability of a single yeast cell with an ultra-sensitive radio frequency sensor

We propose and demonstrate a simple, ultra sensitive radio frequency (RF) sensor to detect a single yeast cell and distinguish its viability in a microfluidic channel. On-chip interference is used to cancel background probing signals to improve sensor sensitivity. Individual viable and nonviable yeast cells (approximately 5.83 +/- 0.85 microm in diameter) are measured with clear sensing and identification of these cells.

Microfluidic separation of live and dead yeast cells using reservoir-based dielectrophoresis

Separating live and dead cells is critical to the diagnosis of early stage diseases and to the efficacy test of drug screening, etc. This work demonstrates a novel microfluidic approach to dielectrophoretic separation of yeast cells by viability. It exploits the cell dielectrophoresis that is induced by the inherent electric field gradient at the reservoir-microchannel junction to selectively trap dead yeast cells and continuously separate them from live ones right inside the reservoir. This approach is therefore termed reservoir-based dielectrophoresis (rDEP). It has unique advantages as compared to existing dielectrophoretic approaches such as the occupation of zero channel space and the elimination of any mechanical or electrical parts inside microchannels. Such an rDEP cell sorter can be readily integrated with other components into lab-on-a-chip devices for applications to biomedical diagnostics and therapeutics.

DC electrokinetic particle transport in an l-shaped microchannel

Electrokinetic transport of particles through an L-shaped microchannel under DC electric fields is theoretically and experimentally investigated. The emphasis is placed on the direct current (DC) dielectrophoretic (DEP) effect arising from the interactions between the induced spatially nonuniform electric field around the corner and the dielectric particles. A transient multiphysics model is developed in an arbitrary Lagrangian-Eulerian (ALE) framework, which comprises the Navier-Stokes equations for the fluid flow and the Laplace equation for the electrical potential. The predictions of the DEP-induced particle trajectory shift in the L-shaped microchannel are in quantitative agreement with the obtained experimental results. Numerical studies also show that the DEP effect can alter the angular velocity and even the direction of the particles rotation. Further parametric studies suggest that the L-shaped microfluidic channel may be utilized to focus and separate particles by size via the induced DEP effect.

Particle electrophoresis and dielectrophoresis in curved microchannels

Studies of particle electrophoresis have so far been limited to primarily theoretical or numerical analyses in straight microchannels. Very little work has been done on particle electrophoretic motions in real microchannels that may have one or multiple turns for reducing the devices size or achieving other functions. This article presents an experimental and numerical study of particle electrophoresis in curved microchannels. Polystyrene microparticles are found to migrate across streamlines and flow out of a spiral microchannel in a focused stream near the outer wall. This transverse focusing effect arises from the dielectrophoretic particle motion induced by the nonuniform electric field intrinsic to curved channels. The experimental observations agree quantitatively with the numerical predictions.

Transient electrophoretic motion of a charged particle through a converging-diverging microchannel: Effect of direct current-dielectrophoretic force

Transient electrophoretic motion of a charged particle through a converging–diverging microchannel is studied by solving the coupled system of the Navier–Stokes equations for fluid flow and the Laplace equation for electrical field with an arbitrary Lagrangian–Eulerian finite‐element method. A spatially non‐uniform electric field is induced in the converging–diverging section, which gives rise to a direct current dielectrophoretic (DEP) force in addition to the electrostatic force acting on the charged particle. As a sequence, the symmetry of the particle velocity and trajectory with respect to the throat is broken. We demonstrate that the predicted particle trajectory shifts due to DEP show quantitative agreements with the existing experimental data. Although converging–diverging microchannels can be used for super fast electrophoresis due to the enhancement of the local electric field, it is shown that large particles may be blocked due to the induced DEP force, which thus must be taken into account in the study of electrophoresis in microfluidic devices where non‐uniform electric fields are present.

Joule heating effects on electroosmotic flow in insulator-based dielectrophoresis

Insulator‐based dielectrophoresis (iDEP) is an emerging technology that has been successfully used to manipulate a variety of particles in microfluidic devices. However, due to the locally amplified electric field around the in‐channel insulator, Joule heating often becomes an unavoidable issue that may disturb the electroosmotic flow and affect the particle motion. This work presents the first experimental study of Joule heating effects on electroosmotic flow in a typical iDEP device, e.g. a constriction microchannel, under DC‐biased AC voltages. A numerical model is also developed to simulate the observed flow pattern by solving the coupled electric, energy, and fluid equations in a simplified two‐dimensional geometry. It is observed that depending on the magnitude of the DC voltage, a pair of counter‐rotating fluid circulations can occur at either the downstream end alone or each end of the channel constriction. Moreover, the pair at the downstream end appears larger in size than that at the upstream end due to DC electroosmotic flow. These fluid circulations, which are reasonably simulated by the numerical model, form as a result of the action of the electric field on Joule heating‐induced fluid inhomogeneities in the constriction region.

Electrokinetic focusing and filtration of cells in a serpentine microchannel

Focusing cells into a single stream is usually a necessary step prior to counting and separating them in microfluidic devices such as flow cytometers and cell sorters. This work presents a sheathless electrokinetic focusing of yeast cells in a planar serpentine microchannel using dc-biased ac electric fields. The concurrent pumping and focusing of yeast cells arise from the dc electrokinetic transport and the turn-induced acdc dielectrophoretic motion, respectively. The effects of electric field (including ac to dc field ratio and ac field frequency) and concentration (including buffer concentration and cell concentration) on the cell focusing performance were studied experimentally and numerically. A continuous electrokinetic filtration of E. coli cells from yeast cells was also demonstrated via their differential electrokinetic focusing in a serpentine microchannel.

Analysis of electrokinetic flow in microfluidic networks

A general model for electrokinetic flow in a one-to-multi-branch microchannel system is developed. This model can be extended to more complex microfluidic networks. The liquid flow may be generated by applying pressure gradients (pressure-driven flow) or electric fields (electro-osmotic flow) to the microchannels. Phenomenological coefficients in non-equilibrium thermodynamics are employed to describe the effects of channel size and surface electrokinetic properties on microfluidic characteristics. Analytical solutions of the flow rate and the streaming potential (for pressure-driven flow) or the electric current (for electro-osmotic flow) are obtained for each branch-channel, in addition to the distributions of pressure and electric potential. The flow behaviors in such a microchannel network can thus be predicted by these analytical solutions. As examples, a two-section heterogeneous microchannel and a one-to-two-branch microchannel system are analyzed using the derived model.