Rohrs Chae, Azimi Arash, He Ping
Department of Mechanical Engineering , Lamar University , Beaumont , Texas 77710 , United States.
Langmuir. 2019 Nov 26;35(47):15421-15430. doi: 10.1021/acs.langmuir.9b03002. Epub 2019 Nov 11.
A liquid droplet on a micropatterned substrate equalizes into either the Cassie-Baxter (also called Cassie for short) or the Wenzel state. This paper investigates the wetting phenomena on ideal micropatterned surfaces consisting of straight micropillars at different pillar dimensions and spacings (the word "ideal" refers to being chemically homogeneous and free of submicron-scale roughness all over the micropatterned surface). Two modeling approaches are used: (1) a thermodynamic approach analyzing the Gibbs energy of the droplet-solid-gas system and (2) a computational fluid dynamics (CFD) approach studying the three-dimensional dynamic wetting process to validate the results of the first approach. The thermodynamic approach incorporates three creative submodels proposed in this paper: (i) a sagging model explaining the pillar edge effect, (ii) a touchdown model transitioning the droplet's partial penetrating condition toward its full penetrating condition, i.e., the Wenzel state, and (iii) a liquid-volume model dynamically computing the liquid volume between the pillar valleys while in the partial penetrating condition or in the Wenzel state. The results of the thermodynamic approach reveal (1) a small energy barrier between the Cassie and Wenzel states, (2) no partial penetration and sagging of the liquid in the Cassie state on the ideal straight micropillared surface, and (3) that the apparent contact angle in the most stable Wenzel state can be 5° or more lower than the prediction of the Wenzel equation when the pillar height is equal or greater than 75 μm. To the best of our knowledge, this paper presents the theoretical explanation of this Wenzel deviation on micropatterned surfaces for the first time in the literature. Utilizing the state-of-the-art continuum model developed by the authors in previous studies, the CFD approach investigates the same wetting conditions and confirms the same findings.
微图案化基底上的液滴会转变为 Cassie-Baxter 状态（简称为 Cassie 状态）或 Wenzel 状态。本文研究了由不同柱体尺寸和间距的直微柱组成的理想微图案化表面上的润湿现象（“理想”一词指的是在整个微图案化表面上化学性质均匀且不存在亚微米级粗糙度）。采用了两种建模方法：(1) 一种热力学方法，分析液滴 - 固体 - 气体系统的吉布斯自由能；(2) 一种计算流体动力学 (CFD) 方法，研究三维动态润湿过程以验证第一种方法的结果。热力学方法纳入了本文提出的三个创新性子模型：(i) 一个下垂模型，用于解释柱体边缘效应；(ii) 一个着陆模型，将液滴的部分穿透状态转变为完全穿透状态，即 Wenzel 状态；(iii) 一个液体体积模型，在部分穿透状态或 Wenzel 状态下动态计算柱体谷之间的液体体积。热力学方法的结果表明：(1) Cassie 状态和 Wenzel 状态之间存在一个小的能量势垒；(2) 在理想的直微柱表面上，Cassie 状态下液体不会出现部分穿透和下垂现象；(3) 当柱体高度等于或大于 75 μm 时，最稳定的 Wenzel 状态下的表观接触角可能比 Wenzel 方程的预测值低 5°或更多。据我们所知，本文首次在文献中对微图案化表面上的这种 Wenzel 偏差给出了理论解释。利用作者在先前研究中开发的最先进的连续介质模型，CFD 方法研究了相同的润湿条件并证实了相同的结果。
