State Key Laboratory of Fine Chemicals & Liaoning Key Laboratory of Clean Utilization of Chemical Resources, Institute of Chemical Engineering, Dalian University of Technology , Dalian 116024, P. R. China.
Department of Mechanical Engineering, University of Colorado Boulder , Boulder, Colorado 80309-0427, United States.
ACS Appl Mater Interfaces. 2017 Apr 19;9(15):13770-13777. doi: 10.1021/acsami.7b01812. Epub 2017 Apr 6.
Nanostructured superhydrophobic surfaces have been actively explored to promote favorable droplet dynamics for a wide range of technological applications. However, the tendency of condensed droplets to form as pinned states greatly limits their applicability in enhancing condensation heat transfer efficiency. Despite recent progresses, the understanding of physical mechanisms governing the wetting transition of condensed droplets is still lacking. In this work, a nanostructured superhydrophobic surface with tapered nanogaps is fabricated to demonstrate the coordination of surface wetting property, topography, and the condensing condition on the wetting state and dynamic behavior of condensed droplets. Combining the environmental scanning electron microscopy and optical visualization methods, we systematically show the morphology of nucleated droplets in nanostructures and the droplet dynamic evolution throughout the growth stages, which provides the direct evidence of condensing condition-induced droplet wetting transition. When the surface subcooling is smaller than 0.3 K, the droplets formed as the Cassie-Baxter state, followed by coalescence-induced droplet jumping. With the increase of surface subcooling up to 0.6 K, however, droplet formation occurs randomly inside nanogaps, resulting in the loss of superhydrophobicity. These new observations along with the new insights about the coordination of surface properties and condensing conditions on droplet wetting transition are useful for guiding the development of novel surfaces for improving droplet removal and phase-change heat transfer.
纳米结构的超疏水表面被广泛研究，以促进各种技术应用中液滴的有利动力学行为。然而，凝结液滴倾向于形成固定状态，这极大地限制了它们在提高凝结换热效率方面的应用。尽管最近取得了一些进展，但对控制凝结液滴润湿转变的物理机制的理解仍然不足。在这项工作中，我们制备了具有锥形纳米间隙的纳米结构超疏水表面，以展示表面润湿性、形貌和凝结条件对润湿状态和凝结液滴动态行为的协同作用。通过环境扫描电子显微镜和光学可视化方法，我们系统地展示了纳米结构中成核液滴的形态和液滴在生长阶段的动态演化，提供了凝结条件诱导液滴润湿转变的直接证据。当表面过冷度小于 0.3 K 时，液滴形成卡西米尔-贝茨（Cassie-Baxter）状态，随后发生合并诱导的液滴跳跃。然而，随着表面过冷度增加到 0.6 K，液滴在纳米间隙内随机形成，导致超疏水性丧失。这些新的观察结果以及对表面特性和凝结条件在液滴润湿转变中的协调作用的新见解，有助于指导开发新型表面以提高液滴去除和相变传热的效率。
