Yan Run, Pham Robin, Chen Chung-Lung
Department of Mechanical and Aerospace Engineering, University of Missouri, Lafferre Hall, Columbia, Missouri 65211, United States.
Department of Mechanical Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 30010, Taiwan.
Langmuir. 2020 Dec 29;36(51):15558-15571. doi: 10.1021/acs.langmuir.0c02903. Epub 2020 Dec 17.
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.
在本文中，我们展示了施加电场以激活气泡逸出、合并和脱离的结果。在这项气泡动力学研究中使用了一种简单的介电层上电润湿装置。当将插入去离子水中的铜电极线置于单个或多个气泡的一侧时，随着电压开启，气泡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不平衡，这进一步促进了气泡的逸出。开发了一个具有机电框架的分析模型来研究气泡两侧的充电时间差，该时间差产生了润湿性梯度和毛细力以将其推离电极。测试了振幅为60 V、频率为2 Hz的正弦、斜坡和方波交流波形以作比较。结果表明，所有波形都促进了气泡的逸出；方波形状表现出最远的逸出能力，其次是正弦波和斜坡波。几个呈三角形、正方形、五边形和六边形排列的气泡的俯视图表明，当电极置于几何中心时，气泡倾向于向外移动。进行了电极在一侧且几个气泡排成一行的实验。在这些情况下，离电极较近的气泡比较远的气泡反应更快，从而导致合并。一旦气泡尺寸变大，它要么通过克服水薄膜中的分离压力，要么通过水厚膜中的浮力而脱离。通过电场控制气泡动力学，包括逸出、合并和脱离，为在许多流体力学和传热研究中移动气泡或增加脱离频率提供了一种主动且可逆的方法。
