Bouncing of a Droplet on Superhydrophobic Surface in AC Electrowetting

Abstract

The movie shows a droplet jumping from the superhydrophobic surface under an alternating electrical actuation close to the resonance of the droplet vibration, which enables a sessile droplet to accumulate sufficient energy at the droplet surface to overcome the adhesion and the gravity. After jumping, the droplet bounces on the superhydrophobic surface, where the minimized adhesion and hysteresis make the decay of the bouncing height considerably slow. The resonant electrical actuation is the key for the droplet to accumulate sufficient energy for jumping; thereby the deformation and the jumping of a droplet are substantially affected by applied frequencies. Superhydrophobic surfaces are preferable in maximizing the jumping height because of the effective energy conversion from the surface energy to the kinetic energy. In addition, the stability and the reproducibility of the droplet jumping can be ameliorated because of the minimized adhesion superhydrophobic surfaces provide. The stability and the reproducibility of commercially available superhydrophobic surfaces used in this demonstration are fairly acceptable for the controlled jumping of a droplet on demand (CJDD), which is expected to play a key role in realizing three-dimensional droplet manipulations in digital microfluidics. The increasing demand for the three-dimensional configuration in microfluidics is also applicable to the digital microfluidics to manipulate droplets in three-dimensions. The three-dimensional droplet manipulation could be the radical solution to the difficult problems in current digital microfluidics such as the cross-contamination and the degradation of electrodes, and it will significantly broaden the scope of applications in digital microfluidics due to the expanded degree-of-freedom in z-direction. The controlled droplet jumping made by the resonant AC electrowetting shown in this movie could be a historical milestone in digital microfluidics, as suggesting a potential way to realize three-dimensional digital microfluidics by simply transporting droplets in z-direction based on current single plate EWOD configurations.