Dynamic Electrowetting-on-Dielectric (DEWOD) on Unstretched and Stretched Teflon

Dynamic electrowetting-on-dielectric (DEWOD) of the unstretched and stretched Teflon is reported in the experiments with water drop impact and rebound. We explore experimentally and theoretically the situation with the capacitance different from the standard static electrowetting. Deionized water drops impact onto either an unstretched hydrophobic Teflon surface or Teflon stretched up to 250% strain normally to the impact direction. The surface roughness of the unstretched Teflon increased after stretching from 209.9 to 245.6 nm resulting in the increase in the equilibrium water contact angle from 96 ± 4° to 147 ± 5°, respectively. The electric arrangement used in the drop impact experiments on DEWOD results in a dramatically reduced capacitance and requires a much higher voltage to observe EW in comparison with the standard static case of a drop deposited on a dielectric layer and attached to an electrode. In the dynamic situation we found that as the EW sets in it can greatly reduce the superhydrophobicity of the unstretched and stretched Teflon. At 0 kV, the water drop rebound height (hmax) is higher for the stretched Teflon (hmax ≈ 5.13 mm) and lower for the unstretched Teflon (hmax ≈ 4.16 mm). The EW response of unstretched Teflon is weaker than that of the stretched one. At the voltage of 3 kV, the water drop sticks to the stretched Teflon without rebound, whereas water drops still partially rebound (hmax ≈ 2.8 mm) after a comparable impact onto the unstretched Teflon. We found a sharp dynamic EW response for the stretched Teflon. The contact angle of deionized water ranged from 147 ± 5° (superhydrophobic) to 67 ± 5° (partially hydrophilic) by applying external voltage of 0 and 3 kV, respectively. Dynamic electrowetting introduced in this work for the first time can be used to control spray cooling, painting, and coating and for drop transport in microfluidics.