Dwivedi Raghvendra Kumar, Muralidhar K
Department of Mechanical Engineering Indian Institute of Technology Kanpur, Kanpur 208016, India.
Phys Rev E. 2022 Oct;106(4-2):045111. doi: 10.1103/PhysRevE.106.045111.
Modeling the electrowetting process of a liquid droplet placed on a hydrophobic surface in an ambient environment has several challenges over and above those of basic spreading [F. Mugele, Soft Matter 5, 3377 (2009)10.1039/b904493k]. At an external voltage below the value that causes contact angle saturation, transient spreading is augmented by contact angle reduction defined by the Young-Lippmann equation. In addition, the macroscopic equilibrium contact angle and, therefore, the spreading rate could be altered by the surface hysteresis. Beyond the saturation point, spreading reveals additional features of higher complexity [Q. Vo and T. Tran, J. Fluid Mech. 925, A19 (2021)10.1017/jfm.2021.677]. These details have been examined from experiments as well as numerical simulation in the present work. Below the saturation point, the contact angle model of Dwivedi et al. [Phys. Rev. Fluids 7, 034002 (2022)10.1103/PhysRevFluids.7.034002] with the correction related to the electric field is seen to be applicable. Beyond saturation, the experimentally determined instantaneous contact angle distribution shows two distinct functionalities with respect to the contact line velocity. The first prevails from the onset of spreading until the spreading factor attains a peak value. The second trend is initiated with the retraction of the contact line. Except for differences in parametric values, the form of the contact angle model remains unchanged. Simulations in the postsaturation regime are shown to match experimental data in terms of the transient spreading factor, drop shapes, and the instantaneous contact angle. The role of the ground wire is found to be important and the three-phase contact line formed on it has been included in simulations. Spreading dynamics of the droplet have also been studied when the ground wire is kept at a distance of 40 μm from the apex of the drop. Simulations as well as experiments, show the propagation of a capillary wave between the ground wire and the three-phase contact line. For spreading over an uncoated polydimethylsiloxane (PDMS) surface, the contact line is trapped at local pinning sites, leading to additional distortions in the instantaneous shapes acquired by the interface.
对置于环境中的疏水表面上的液滴进行电润湿过程建模，除了基本铺展问题外，还存在若干挑战[F. Mugele，《软物质》5，3377 (2009)10.1039/b904493k]。在低于导致接触角饱和值的外部电压下，由杨氏 - 利普曼方程定义的接触角减小会增强瞬态铺展。此外，宏观平衡接触角以及铺展速率可能会因表面滞后而改变。超过饱和点后，铺展呈现出更高复杂性的其他特征[Q. Vo和T. Tran，《流体力学杂志》925，A19 (2021)10.1017/jfm.2021.677]。在本工作中，已通过实验以及数值模拟对这些细节进行了研究。在饱和点以下，迪维维迪等人[《物理评论流体》7，034002 (2022)10.1103/PhysRevFluids.7.034002]的与电场相关修正的接触角模型被认为是适用的。超过饱和点后，实验确定的瞬时接触角分布相对于接触线速度呈现出两种不同的函数关系。第一种在铺展开始直至铺展因子达到峰值时占主导。第二种趋势随着接触线的回缩而开始。除了参数值的差异外，接触角模型的形式保持不变。后饱和状态下的模拟结果在瞬态铺展因子、液滴形状和瞬时接触角方面与实验数据相匹配。发现接地线的作用很重要，并且在模拟中已包含在其上形成的三相接触线。当接地线与液滴顶点保持40μm的距离时，也研究了液滴的铺展动力学。模拟和实验均表明在接地线和三相接触线之间存在毛细波的传播。对于在未涂层的聚二甲基硅氧烷（PDMS）表面上的铺展，接触线被困在局部钉扎位置，导致界面获取的瞬时形状出现额外的扭曲。
