Jumping of a droplet on a superhydrophobic surface in AC electrowetting

Consistent droplet bouncing driven by AC electrowetting was achieved by introducing a superhydrophobic surface instead of conventional hydrophobic surfaces. A superhydrophobic surface is very effective to reduce interfacial energy barrier or adhesion, allowing complete detachment of a droplet from the substrate. While a fixed electric potential (100 V(rms)) was applied, the shape deformation and the droplet bouncing were significantly influenced by the frequency of the AC electrowetting. Consistent droplet bouncing only occurred at very narrow frequency ranges (e.g., 30-31 Hz for 8 mu L droplets), indicating that resonance dominates the droplet bouncing. Interestingly, the resonance was 1/2 sub-harmonics, where every other actuation was skipped, when the droplet was in the air. Theoretical evaluation of the resonant frequency based on the linear theory implies that the fundamental resonance between the AC electrowetting and the vertical vibration of the shape oscillation could be important to produce consistent droplet bouncing.