Electric-field induced droplet stable vertical vibration: Experiments and numerical simulations

In this study, stable vertical vibration of a single droplet was stimulated through Electrowetting on Dielectric (EWOD) on a Polydimethylsiloxane-coated Indium Tin Oxide glass (PDMS-ITO) surface in an oil atmosphere. The mechanism of the vibration is clarified as a self -adjusted energy earning and losing equilibrium, and a small friction at the contact line is crucial for this stable vibration. We investigated the impact of needle tip electrode height, different voltage forms (AC and DC voltages), and voltage magnitude on the droplet vibration state. As decreasing the needle tip electrode height, there is a transition of the droplet from a vibrating state to a static equilibrium state. During the vibration, the vibration period remains constant regardless of tip height, but the contact angle changes synchronously with the voltage. The electric signal in the circuit is also measured to provide solid evidence for this synchronicity. Finally, the saturation voltage of the contact angle and amplitude is about 120 V. The intrinsic connection between contact angle saturation and amplitude saturation is clarified by studying the surface energy of the droplet. A theoretical model is constructed to numerically simulate the shape and amplitude of the droplet's vibrations.