Dynamics of electrowetting-driven spreading and receding of a droplet

Electrowetting is a powerful method to achieve external wetting control, by exploiting the potential-dependence of the liquid contact angle with respect to a solid substrate. We study the effect of electrolyte concentration $c_0$ on electrowetting-induced spreading and receding of a droplet on the basal plane of graphite, a very smooth conducting substrate. We find that for positive applied potentials, the static electrowetting contact angle is independent of $c_0$ over three decades of concentration but that the spreading time to reach this final state varies as $c_0^{-1}$. In contrast, for negative potentials, the electrowetting angle varies as $c_0^{1/2}$ and the spreading time varies as $c_0^{-1/2}$. We show that both spreading and receding times are governed by the formation of the electrical double layer (EDL) on the substrate, and that electrowetting can be used as a macroscopic probe into the structure of the EDL.