Interplay of induced charge electroosmosis and electrothermal flow in insulator-based dielectrophoresis

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.

Figure 1

(a) Schematic illustrating the formation of ICEO (highlighted by the looped arrows) around the corners of insulators in an iDEP microdevice because of the action of electric field upon the diffuse charge induced by the leaked electric field (see the background lines) into the insulator; (b) schematic illustrating the formation of ETF (highlighted by the looped arrows) around the corners because the action of electric field upon the Joule heating-induced fluid property gradients (see the background color, the darker the higher temperature) creates an electrothermal force (see the vector plot); (c) schematic showing the geometry and computational domain of the constricted microchannel used in this work.

Figure 2

Experimental images of tracing particles in a constricted microchannel in buffer solutions of different concentrations under different ac voltages. The dc voltage was fixed at 20 V and applied downwards. The arrow on each image indicates the particle moving direction inside the constriction, which switches from along the dc electric field (top to bottom) to against it with the increase of buffer concentration or ac voltage. The looped arrows on the top-left image highlight the fluid circulations of ICEO while those on both the top-right and bottom-left images highlight the fluid circulations of ETF.

Figure 3

Comparison of the experimentally (upper row) and numerically (lower row, the background color shows the magnitude of the particle velocity, the darker the larger) obtained particle streak lines in buffer solutions of different concentrations under a fixed 20-V dc-biased 500-V ac voltage. The arrow on each image indicates the particle moving direction inside the constriction, which switches from along the dc electric field (top to bottom) to against it with the increase of the buffer concentration because of the accompanying changes in the particle and wall zeta potentials. The looped arrows on the left-most and right-most images highlight the fluid circulations of ICEO and ETF, respectively.

Figure 4

Effect of buffer concentration on nonlinear electrokinetic flows in a constricted microchannel under a fixed 20-V dc-biased 500-V ac voltage: (a) Numerically predicted maximum induced zeta potential ${\zeta }_i$ (which occurs at approximately the starting point of the arc corner of the constriction entrance) and maximum fluid temperature $T$ (which occurs in the middle of the constriction); (b) comparison of the experimentally (symbols) and numerically (solid line) obtained nonlinear fluid velocities (excluding the linear dc electroosmotic flow) along with the numerically predicted velocities of ICEO (with a negative velocity, dashed line) and ETF (with a positive velocity, dotted line) alone. The positive velocity is defined as that along the dc electric field.

Figure 5

Comparison of the experimentally (upper row) and numerically (lower row, the background color shows the magnitude of the particle velocity, the darker the larger) obtained particle streak lines in 0.1-mM buffer solution under different ac voltages. The dc voltage is fixed at 20 V. The arrow on each image indicates the particle moving direction inside the constriction, which switches from along the dc electric field (top to bottom) to against it with the increase of ac voltage because of the development of ETF. The looped arrows on the left-most and right-most images highlight the fluid circulations of ICEO and ETF, respectively.

Figure 6

Effect of the ac voltage on the nonlinear electrokinetic flow of 0.1-mM buffer in a constricted microchannel under a fixed 20-V dc: (a) Numerically predicted maximum induced zeta potential ${\zeta }_i$ and maximum fluid temperature $T$; (b) comparison of the experimentally (symbols) and numerically (solid line) obtained nonlinear fluid velocities (excluding the linear dc electroosmotic flow) along with the numerically predicted velocities of ICEO (with a negative velocity, dashed line) and ETF (with a positive velocity, dotted line) alone. The positive velocity is defined as that along the dc electric field.