Simulation of the Slip Velocity Effect in an AC Electrothermal Micropump

Abstract

The principal aim of this study was to analyze the effect of slip velocity at the microchannel wall on an alternating current electrothermal (ACET) flow micropump fitted with several pairs of electrodes. Using the finite element method (FEM), the coupled momentum, energy, and Poisson equations with and without slip boundary conditions have been solved to compute the velocity, temperature, and electrical field in the microchannel. The effects of the frequency and the voltage, and the electrical and thermal conductivities, respectively, of the electrolyte solution and the substrate material, have been minutely analyzed in the presence and absence of slip velocity. The slip velocity was simulated along the microchannel walls at different values of slip length. The results revealed that the slip velocity at the wall channel has a significant impact on the flow field. The existence of slip velocity at the wall increases the shear stress and therefore enhances the pumping efficiency. It was observed that higher average pumping velocity was achieved for larger slip length. When a glass substrate was used, the effect of the presence of the slip velocity was more manifest. This study shows also that the effect of slip velocity on the flow field is very important and must be taken into consideration in an ACET micropump.

Keywords: AC electrothermal; microfluidics; micropump; slip velocity.

Figure 1

Geometry of an alternating current electrothermal (ACET) micropump involving the electrodes. (a) 3D view, (b) 2D view of the reduced computational domain.

Figure 2

2D unstructured mesh with triangular elements; the mesh is refined near the wall of microchannel.

Figure 3

Velocity contours, without slip velocity for V_rms = 1 V (a) and V_rms = 3 V (b) and with slip velocity for V_rms = 1 V (c) and V_rms = 3 V (d). The slip length and the frequency are taken as equal to L_s = 1µm and f = 100 kHz, respectively. The slip velocity has an effect on the distribution of fluid velocity. An extensive increase in the velocity field due to the increase of the shear stress is noted.

Figure 3

Velocity contours, without slip velocity for V_rms = 1 V (a) and V_rms = 3 V (b) and with slip velocity for V_rms = 1 V (c) and V_rms = 3 V (d). The slip length and the frequency are taken as equal to L_s = 1µm and f = 100 kHz, respectively. The slip velocity has an effect on the distribution of fluid velocity. An extensive increase in the velocity field due to the increase of the shear stress is noted.

Figure 4

Slip velocity profile on the top and bottom walls for V_rms = 3 V and L_s = 1 µm compared to the no-slip boundary condition.

Figure 5

(a) Transversal velocity profile at x = 20 μm for V_rms = 1, 2, and 3 V; (b) maximum velocity and slip velocity profiles as a function of $V_{\mathrm{rms}}^4$ with and without slip velocity.

Figure 6

Slip velocity profile on the top and bottom walls for V_rms = 2 or 3 V and L_s = 1 µm.

Figure 7

(a) Average pumping velocity versus V_rms, with and without slip velocity, for L_s = 0.5 µm and 1 µm. (b) Isovalues of u_ave as a function of L_s and V_rms.

Figure 8

The effect of electrolyte conductivity on average velocity without slip velocity and with slip velocity for (a) L_s = 0.5 µm and (b) L_s = 1 µm.

Figure 9

(a) Variation of the average velocity versus the rsm voltage for two kinds of substrates. (b) Axial profile of the bottom wall velocity for L_s = 1 µm and for two kinds of substrates.

Figure 10

(a) Average pumping velocity versus the AC frequency and (b) slip velocity along the bottom wall of microchannel for several AC frequencies.