Vandadi Aref, Zhao Lei, Cheng Jiangtao
Department of Mechanical and Energy Engineering, University of North Texas Denton TX 76207 USA.
Department of Mechanical Engineering, Virginia Polytechnic Institute and State University Blacksburg VA 24061 USA
Nanoscale Adv. 2018 Dec 20;1(3):1136-1147. doi: 10.1039/c8na00237a. eCollection 2019 Mar 12.
Recently the development of superhydrophobic surfaces with one-tier or hierarchical textures has drawn increasing attention because enhanced condensation heat transfer has been observed on such biomimetic surfaces in well-tailored supersaturation or subcooling conditions. However, the physical mechanisms underlying condensation enhancement are still less understood. Here we report an energy-based analysis on the formation and growth of condensate droplets on two-tier superhydrophobic surfaces, which are fabricated by decorating carbon nanotubes (CNTs) onto microscale fluorinated pillars. Thus-formed hierarchical surfaces with two tier micro/nanoscale roughness are proved to be superior to smooth surfaces in the spatial control of condensate droplets. In particular, we focus on the self-pulling process of condensates in the partially wetting morphology (PW) from surface cavities due to intrinsic Laplace pressure gradient. In this analysis, the self-pulling process of condensate tails is resisted by adhesion energy, viscous dissipation, contact line dissipation and line tension in a combined manner. This process can be facilitated by adjusting the configuration and length scale of the first-tier texture. The optimum design can not only lower the total resistant energy but also favor the out-of-plane motion of condensate droplets anchored in the first-tier cavity. It is also shown that engineered surface with hierarchical roughness is beneficial to remarkably mitigating contact line dissipation from the perspective of molecular kinetic theory (MKT). Our study suggests that scaling down surface roughness to submicron scale can facilitate the self-propelled removal of condensate droplets.
近年来，具有单层或分级纹理的超疏水表面的发展引起了越来越多的关注，因为在精心定制的过饱和或过冷条件下，人们观察到在这种仿生表面上冷凝传热得到了增强。然而，冷凝增强背后的物理机制仍不太清楚。在此，我们报告了对双层超疏水表面上冷凝液滴形成和生长的基于能量的分析，该表面是通过将碳纳米管（CNT）装饰在微米级氟化柱上制成的。由此形成的具有两层微/纳米级粗糙度的分级表面在冷凝液滴的空间控制方面被证明优于光滑表面。特别是，我们关注由于固有拉普拉斯压力梯度导致的部分润湿形态（PW）下冷凝物从表面腔中的自拉动过程。在该分析中，冷凝物尾部的自拉动过程受到粘附能、粘性耗散、接触线耗散和线张力的综合抵抗。通过调整第一层纹理的构型和长度尺度可以促进这一过程。最佳设计不仅可以降低总阻力能，还有利于锚定在第一层腔中的冷凝液滴的平面外运动。研究还表明，从分子动力学理论（MKT）的角度来看，具有分级粗糙度的工程表面有利于显著减轻接触线耗散。我们的研究表明，将表面粗糙度缩小到亚微米尺度可以促进冷凝液滴的自推进去除。
