Department of Mechanical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA.
Nanoscale. 2018 Apr 5;10(14):6426-6436. doi: 10.1039/C8NR00354H.
Besides the Wenzel state, liquid droplets on micro/nanostructured surfaces can stay in the Cassie state and consequently exhibit intriguing characteristics such as a large contact angle, small contact angle hysteresis and exceptional mobility. Here we report molecular dynamics (MD) simulations of the wetting dynamics of Cassie-state water droplets on nanostructured ultrahydrophobic surfaces with an emphasis on the genesis of the contact line friction (CLF). From an ab initio perspective, CLF can be ascribed to the collective effect of solid-liquid retarding and viscous damping. Solid-liquid retarding is related to the work of adhesion, whereas viscous damping arises from the viscous force exerted on the liquid molecules within the three-phase (liquid/vapor/solid) contact zone. In this work, a universal scaling law is derived to generalize the CLF on nanostructured ultrahydrophobic surfaces. With the decreasing fraction of solid-liquid contact (i.e., the solid fraction), CLF for a Cassie-state droplet gets enhanced due to the fact that viscous damping is counter-intuitively intensified while solid-liquid retarding remains unchanged. Nevertheless, the overall friction between a Cassie-state droplet and the structured surface is indeed reduced since the air cushion formed in the interstices of the surface roughness underneath the Cassie-state droplet applies negligible resistance to the contact line. Our results have revealed the genesis of CLF from an ab initio perspective, demonstrated the effects of surface structures on a moving contact line and justified the critical role of CLF in the analysis of wetting-related situations.
除了 Wenzel 状态外，微/纳米结构表面上的液滴还可以处于 Cassie 状态，从而表现出有趣的特性，例如大接触角、小接触角滞后和卓越的移动性。在这里，我们报告了 Cassie 状态下水滴在具有超疏水性纳米结构表面上的润湿动力学的分子动力学（MD）模拟，重点研究了接触线摩擦（CLF）的起源。从一个从头开始的角度来看，CLF 可以归因于固-液阻滞和粘性阻尼的集体效应。固-液阻滞与粘附功有关，而粘性阻尼则源于三相（液/气/固）接触区中液体分子上的粘性力。在这项工作中，推导出了一个普遍的标度定律来概括纳米结构超疏水性表面上的 CLF。随着固-液接触分数（即固体质分数）的降低，Cassie 状态液滴的 CLF 得到增强，因为粘性阻尼出乎意料地增强，而固-液阻滞保持不变。然而，由于 Cassie 状态液滴下方的表面粗糙度间隙中形成的气垫对接触线施加的阻力可以忽略不计，因此 Cassie 状态液滴与结构化表面之间的整体摩擦力确实减小了。我们的结果从一个从头开始的角度揭示了 CLF 的起源，展示了表面结构对移动接触线的影响，并证明了 CLF 在润湿相关情况分析中的关键作用。
