Focusing particles into a tight stream is critical for many microfluidic particle-handling devices such as flow cytometers and particle sorters. This work presents a fundamental study of the passive focusing of polystyrene particles in ratchet microchannels via direct current dielectrophoresis (DC DEP). We demonstrate using both experiments and simulation that particles achieve better focusing in a symmetric ratchet microchannel than in an asymmetric one, regardless of the particle movement direction in the latter. The particle focusing ratio, which is defined as the microchannel width over the particle stream width, is found to increase with an increase in particle size or electric field in the symmetric ratchet microchannel. Moreover, it exhibits an almost linear correlation with the number of ratchets, which can be explained by a theoretical formula that is obtained from a scaling analysis. In addition, we have demonstrated a DC dielectrophoretic focusing of yeast cells in the symmetric ratchet microchannel with minimal impact on the cell viability.

Curvature-induced dielectrophoresis (C-iDEP) is an established method of applying electrical energy gradients across curved microchannels to obtain a label-free manipulation of particles and cells. This method offers several advantages over the other DEP-based methods, such as increased chip area utilisation, simple fabrication, reduced susceptibility to Joule heating and reduced risk of electrolysis in the active region. Although C-iDEP systems have been extensively demonstrated to achieve focusing and separation of particles, a detailed mathematical analysis of the particle dynamics has not been reported yet. This work computationally confirms a fully analytical dimensionless study of the electric field-induced particle motion inside a circular arc microchannel, the simplest design of a C-iDEP system. Specifically, the analysis reveals that the design of a circular arc microchannel geometry for manipulating particles using an applied voltage is fully determined by three dimensionless parameters. Simple equations are established and numerically confirmed to predict the mutual relationships of the parameters for a comprehensive range of their practically relevant values, while ensuring design for safety. This work aims to serve as a starting point for microfluidics engineers and researchers to have a simple calculator-based guideline to develop C-iDEP particle manipulation systems specific to their applications.

Insulator-based dielectrophoresis (iDEP) is an emerging technique for particle manipulation in microfluidic devices. Two nonlinear electrokinetic flows have been demonstrated to take place simultaneously in iDEP: one is induced charge electroosmosis (ICEO) due to the electric polarization of the insulator, and the other is electrothermal flow (ETF) due to the amplified Joule heating of the fluid around the insulator. These flows vary differently with the applied electric field, and become strong in a fluid with a low and a high electric conductivity, respectively. They both exhibit the pattern of fluid vortices near the insulator but with opposite circulating directions. We present in this work an experimental study of the interplay of ICEO and ETF in a constricted microchannel under dc-biased ac voltages. We also develop a depth-averaged numerical model to simulate the coupled electrokinetic fluid flow with the charge and energy transport. The experimentally measured nonlinear fluid velocity agrees closely with the numerical prediction for both a wide range of buffer concentrations and a range of ac voltages. It also matches asymptotically the predicted velocity of ICEO in a low-concentration buffer under a small ac voltage and that of ETF in a high-concentration buffer, both of which are consistent with a scaling analysis. Interestingly, the nonlinear fluid velocity becomes marginal in moderate-concentration buffers under moderate ac voltages because of the opposing effects of ICEO and ETF.