LV ET AL . VOL. 9 ’ NO. 12 ’ 12311–12319 ’ 2015 www.acsnano.org 12311 November 13, 2015 C 2015 American Chemical Society Dewetting Transitions of Dropwise Condensation on Nanotexture- Enhanced Superhydrophobic Surfaces Cunjing Lv, Pengfei Hao, * Xiwen Zhang, and Feng He Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China U nderstanding and realizing sponta- neous wetting transitions (e.g., from the sticky Wenzel state to the non- sticking water-repellent CassieÀBaxter state) and self-removal of droplets on water- repellent materials is highly desired and of critical importance for a wide range of practical applications, such as self-cleaning, anti-icing, antifouling, water harvest, and drag reduction, 1À14 and particularly in heat exchange technologies. 15À18 It is well known that dropwise condensation achieves heat and mass transfer coefficients over an order of magnitude higher than its filmwise counterpart 15,16 because small individual droplets can regularly form and shed off the surface before a thick liquid is formed, thereby minimizing the thermal resistance to heat transfer across the condensate layer. Recently, there has been significant inter- est in developing superhydrophobic sur- faces for promoting dropwise condensa- tion. 2,3,15À31 Such surfaces benefit greatly from the combination of nano/microstructures and the inherent hydrophobicity of their chem- istry, which allows attaining extreme non- wetting properties with vapor trapped underneath (Cassie state) and coalescence- induced self-propelled dropwise condensa- tion. 2,3,15,19À23,28À31 Unfortunately, even the most optimal natural superhydrophobic ma- terial, lotus leaves, ultimately become sticky (Wenzel state) to condensed water, 4 which strongly suppresses its water repellence and use in practical applications. It is well known from experiments, due to impalement of the vapor pockets, inducing strong pinning of the contact lines, that the transition from the Cassie to Wenzel state is an irrevers- ible event. 32,33 Although the transition from Wenzel to Cassie state can be in- duced by some external assistance (e.g., applying pressure/force to the droplet, 34,35 electric voltage 36À38 and magnetic forces, 39 * Address correspondence to haopf@tsinghua.edu.cn. Received for review September 6, 2015 and accepted November 13, 2015. Published online 10.1021/acsnano.5b05607 ABSTRACT Although realizing dewetting transitions of droplets spontaneously on solid textured surfaces is quite challenging, it has become a key research topic in many practical applications that require highly efficient removal of liquid. Despite intensive efforts over the past few decades, due to impalement of vapor pockets inducing strong pinning of the contact lines, how to realize the self- removal of small droplets trapped in the textures remains an urgent problem. We report an in situ spontaneous dewetting transition of condensed droplets occurring on pillared surfaces with two-tier roughness, from the valleys to the tops of the pillars, owing to the nanotexture-enhanced superhydrophobicity, as well as the topology of the micropillars. Three wetting transition modes are observed. It is found that a further decreased Laplace pressure on the top side of the individual droplets accounts for such a surprising transition and self-removal of condensed water. An explicit model is constructed, which quite effectively predicts the Laplace pressure of droplets trapped by the textures. Our model also reveals that the critical size of the droplet for transition scales as the spacing of the micropillars. These findings are expected to be crucial to a fundamental understanding, as well as a remarkable strategy to guide the fabrication, of optimum super-water-repellant surfaces. KEYWORDS: spontaneous transition . nano/microstructures . superhydrophobic . dropwise condensation . individual droplets . Laplace pressure ARTICLE