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.

本文阐述了通过施加电场激活气泡逸出、合并及离去的实验结果。本研究采用了一种简单的电场湿控器件对气泡动力学进行了探究。当将铜电极线插入去离子水中，并将其置于单个或多个气泡的一侧时，气泡在电压开启的条件下倾向于从其初始位置持续逸出。观察到气泡不同侧的接触角存在不平衡，进一步促进了气泡的逸出。为了研究气泡两侧的电荷时间差异，该差异产生了润湿梯度及毛细力，从而推动气泡远离电极，本研究开发了一个包含机电框架的分析模型。对正弦波、斜坡波和方波三种交流电流波形（幅值为60 V，频率为2 Hz）进行了比较测试。结果表明，所有波形均促进了气泡的逸出；其中，方波波形具有最远的逸出能力，其次是正弦波和斜坡波。通过从上俯瞰，观察到气泡以三角形、正方形、五边形和六边形形状排列，证明了当电极置于几何中心时，气泡倾向于向外移动。在电极一侧放置电极，并在另一侧排列多个气泡的实验中，观察到靠近电极的气泡比远离电极的气泡反应更快，从而导致合并。一旦气泡尺寸增大，其离去方式或为克服薄膜水中的附着力，或为厚膜水中的浮力。通过电场控制气泡动力学，包括逸出、合并和离去，为在众多流体力学和传热研究中移动气泡或增加离去频率提供了一种主动且可逆的方法。