Hydrophobicity Evolution on Rough Surfaces

Hydrophobicity is abundant in nature and obtainable in industrial applications by roughening hydrophobic surfaces and engineering micropatterns. Classical wetting theory explains how surface roughness can enhance water repellency, assuming a droplet to have a flat bottom on top of micropatterned surfaces. However, in reality, a droplet can partially penetrate into micropatterns to form a round-bottom shape. Here, we systematically investigate the evolution of evaporating droplets on micropatterned surfaces with X-ray microscopy combined with three-dimensional finite element analyses and propose a theory that explains the wetting transition with gradually increasing penetration depth. We show that the penetrated state with a round bottom is inevitable for a droplet smaller than the micropattern-dependent critical size. Our finding reveals a more complete picture of hydrophobicity involving the partially penetrated state and its role in the wetting state transition and can be applied to understand the stability of water repellency of rough hydrophobic surfaces.疏水性（Hydrophobicity）广泛存在于自然界中，也可通过对疏水表面进行粗糙化处理并设计微结构图案，在工业应用中获得该特性。经典润湿理论解释了表面粗糙度如何提升疏水性，该理论假设液滴在微图案表面上的底部呈平面状。然而在实际情况中，液滴会部分渗入微结构图案中，从而形成底部呈圆弧状的形态。本文中，我们结合X射线显微镜（X-ray microscopy）与三维有限元分析（three-dimensional finite element analyses）方法，系统研究了微图案表面上蒸发液滴的演化过程，并提出了一套可解释渗透深度逐渐增加时润湿转变过程的理论。我们的研究表明，当液滴尺寸小于由微图案决定的临界尺寸时，具有圆底的渗透态是不可避免的。本研究揭示了包含部分渗透态在内的更为完整的疏水性图景，并阐明了该状态在润湿状态转变过程中的作用，该成果可用于理解粗糙疏水表面的拒水稳定性。