Spreading dynamics and oil film entrapment of sessile drops submerged in oil driven by DC electrowetting

The effects of oil viscosity and drop size on the spreading behavior of sessile drops submerged in oil under various DC electrowetting actuation conditions are investigated systematically in this study. Settling time (i.e., time to reach 90% of the equilibrium radius) is found to be linearly proportional to the spherical radius of a drop and oil viscosity. The friction coefficient, which is almost linearly proportional to oil viscosity and is rarely affected by the applied voltage and drop size, is obtained by fitting a theoretical model to the results. Interestingly, sessile drops can jump in oil with low viscosity (0.65 cSt) when the applied voltage is turned off after the drops reach the equilibrium radius. This finding is attributed to the conversion of stored surface energy in the equilibrium state to kinetic energy for jumping when a stretched drop is released. The oil entrapment process and the instability of the entrapped oil film are also investigated by observing the bottom part of the spreading drops. The size of the oil drops generated by oil-film instability decreases as applied voltage increases and is rarely affected by oil viscosity.