Rokoni Arif, Kim Dong-Ook, Sun Ying
Department of Mechanical Engineering and Mechanics, Drexel University, Philadelphia, PA 19104, USA.
Soft Matter. 2019 Aug 28;15(32):6518-6529. doi: 10.1039/c9sm01055f. Epub 2019 Jul 26.
Wicking in hierarchical micro/nanostructured surfaces has attracted significant attention due to its potential applications in thermal management, moisture capturing, drug delivery, and oil recovery. Although some studies have shown that hierarchical structures enhance wicking over micro-structured surfaces, others have found very limited wicking improvement. In this study, we demonstrate the importance of micropatterns in wicking enhancement in hierarchical surfaces using ZnO nanorods grown on silicon micropillars of varying spacings and heights. The wicking front over hierarchical surfaces is found to follow a two-stage motion, where wicking is faster around micropillars, but slower in between adjacent pillar rows and the latter stage dictates the wicking enhancement in hierarchical surfaces. The competition between the added capillary action and friction due to nanostructures in these two different wicking stages results in a strong dependence of wicking enhancement on the height and spacing of the micropillars. A scaling model for the propagation coefficient is developed for wicking in hierarchical surfaces considering nanostructures in both wicking stages and the model agrees well with the experiments. This microstructure-controlled two-stage wicking characteristic sheds light on a more effective design of hierarchical micro/nanostructured surfaces for wicking enhancement.
分级微纳结构表面的毛细作用因其在热管理、水分捕获、药物递送和石油开采等方面的潜在应用而备受关注。尽管一些研究表明分级结构比微结构表面能增强毛细作用，但也有研究发现毛细作用的改善非常有限。在本研究中，我们利用生长在不同间距和高度的硅微柱上的氧化锌纳米棒，证明了微图案在分级表面毛细作用增强中的重要性。发现分级表面上的毛细前沿遵循两阶段运动，其中微柱周围的毛细作用较快，但相邻柱排之间较慢，且后一阶段决定了分级表面的毛细作用增强。在这两个不同的毛细阶段，由于纳米结构而增加的毛细作用与摩擦力之间的竞争导致毛细作用增强强烈依赖于微柱的高度和间距。考虑到两个毛细阶段中的纳米结构，为分级表面的毛细作用建立了一个传播系数的标度模型，该模型与实验结果吻合良好。这种由微观结构控制的两阶段毛细作用特性为更有效地设计用于增强毛细作用的分级微纳结构表面提供了思路。
