Microfluidic flow reversal at low frequency by AC electrothermal effect

This paper describes a flow reversal phenomenon for fluids with moderate conductivity. Fluids with conductivities of 2 x 10(-4) S/m, 0.02 S/m and up to 0.1 S/m were experimented at frequencies ranging from 1 to 110 kHz. Flow reversal was observed only at similar to 1 kHz and 5.3 V (rms) for sigma = 0.02 S/m, and our analysis indicates that AC electrothermal effect could be responsible. Analysis of the system impedance and simulation of power consumption show that the distribution of electric power consumption is dependent on conductivity and AC frequency. At low frequencies, possibly more electric power is consumed at surface/electrolyte interface rather than within the fluid, which consequently changes the location of temperature maximum and the directions of temperature gradients. The direction of AC electrothermal force is reoriented, causing the flow reversal. Numerical simulation is also performed and agrees within the experiments.