Interfacial Electrical Shear Stresses Induce Electrohydrodynamic Flows Inside Droplets Actuated by Electrowetting

In this paper, we numerically investigated electrohydrodynamic flows generated inside droplets undergoing electrowetting. We prove that interfacial shear stresses at the droplet interface are capable, alone, of generating internal electrohydrodynamic flow inside the droplet contrary to previous reports which refer such flows to electrothermal effects. The obtained fluid flow pattern agrees well with previously reported experimental results at the low frequency range of 8∼15 kHz where the effect of all other phenomena (electrothermal, Marangoni, oscillation) are negligible. We studied the effect of applied voltage, frequency, and electrical conductivity of the droplet on the magnitude of the velocity of generated flows. The results obtained will be useful in choosing best conditions to enhance mixing inside droplets while at the same time avoiding temperature rise associated with electrothermal flows which are incompatible with some biological applications.