Activating Bubble's Escape, Coalescence, and Departure under an Electric Field Effect.

In this paper, we present the results of applying an electric field to activate bubbles' escape, coalescence, and departure. A simple electrowetting-on-dielectric device was utilized in this bubble dynamics study. When a copper electrode wire inserted into deionized water was positioned on one side of single or multiple bubbles, the bubble tended to continuously escape from its initial position as the voltage was turned on. Contact angle imbalance at different sides of the bubble was observed, which further promoted the bubble's escape. An analysis model with an electromechanical framework was developed to study the charging time difference on two sides of the bubble, which generated a wettability gradient and capillary force to propel it away from the electrode. Sine, ramp, and square alternating current waveforms with 60 V amplitude and 2 Hz frequency were tested for comparison. It was shown that all waveforms promoted the bubble's escape; the square wave shape manifested the farthest escape capability, followed by sine and ramp waves. An upper view of several bubbles aligning in triangle, square, pentagon, and hexagon shapes demonstrated that the bubbles tended to move outward when the electrode is placed at the geometric centers. Experiments with an electrode on one side and several bubbles positioned in a line were conducted. In these cases, the bubbles closer to the electrode reacted faster than those farther from the electrode, resulting in coalescence. Once the bubble size became larger, it departed either by overcoming the disjoining pressure in a thin film of water or via the buoyancy force in a thick film of water. Controlling bubble dynamics by the electric field, including escape, coalescence, and departure provides an active and reversible approach to move bubbles or increase departure frequency in many fluid mechanics and heat transfer studies.