Multimode Multidrop Serial Coalescence Effects during Condensation on Hierarchical Superhydrophobic Surfaces
Multimode Multidrop Serial Coalescence Effects during Condensation on Hierarchical Superhydrophobic Surfaces
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DOI：
10.1021/la304264g
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发表时间：
2013-01-22
期刊：
影响因子：
3.9
通讯作者：
Varanasit, Kripa K.
中科院分区：
文献类型：
--
作者：
Rykaczewski, Konrad;Paxson, Adam T.;Varanasit, Kripa K.
通过降低最大液滴离开直径显著低于毛细管长度，通过自发液滴运动来提高冷凝速率的前景引起了人们对超疏水表面（SHS）上冷凝的极大兴趣。导致自发液滴运动的移动的聚结最初被报道仅发生在由纳米级和微米级拓扑特征组成的分级SHS上。然而，随后的研究表明，移动的聚结也发生在单独的纳米结构SHS。因此，最近的重点一直是了解纳米结构表面上的冷凝过程，而不是分层SHS。在这项工作中，我们调查的微观形貌的分层SHS的液滴聚结动力学和润湿状态在冷凝过程中的影响。我们发现，孤立的移动的和im移动的两滴之间的合并，几乎完全集中在以前的研究，是罕见的。我们确定了几个新的液滴脱落模式，这是由切向推进的移动的滴的帮助。这些液滴脱落模式包括在连续聚结事件期间合并的多个液滴，其最终形成离开或保持锚定到表面的液滴。我们直接涉及合并后的下降粘附形成的纳米级以及微米Wenzel和Cassie-Baxter润湿状态的下降。我们确定了最佳的微尺度特征间距的分层SHS，这促进了最高数量的微滴离开。这种最佳的表面结构由微尺度特征组成，所述微尺度特征间隔得足够近，以使得较大的液滴能够转变成微Cassie状态，但同时在特征之间提供足够的间隔，以发生移动的聚结。
The prospect of enhancing the condensation rate by decreasing the maximum drop departure diameter significantly below the capillary length through spontaneous drop motion has generated significant interest in condensation on superhydrophobic surfaces (SHS). The mobile coalescence leading to spontaneous drop motion was initially reported to occur only on hierarchical SHS, consisting of both nanoscale and microscale topological features. However, subsequent studies have shown that mobile coalescence also occurs on solely nanostructured SHS. Thus, recent focus has been on understanding the condensation process on nanostructured surfaces rather than on hierarchical SHS. In this work, we investigate the impact of microscale topography of hierarchical SHS on the droplet coalescence dynamics and wetting states during the condensation process. We show that isolated mobile and immobile coalescence between two drops, almost exclusively focused on in previous studies, are rare. We identify several new droplet shedding modes, which are aided by tangential propulsion of mobile drops. These droplet shedding modes comprise of multiple droplets merging during serial coalescence events, which culminate in formation of a drop that either departs or remains anchored to the surface. We directly relate postmerging drop adhesion to formation of drops in nanoscale as well as microscale Wenzel and Cassie-Baxter wetting states. We identify the optimal microscale feature spacing of the hierarchical SHS, which promotes departure of the highest number of microdroplets. This optimal surface architecture consists of microscale features spaced close enough to enable transition of larger droplets into micro-Cassie state yet, at the same time, provides sufficient spacing in-between the features for occurrence of mobile coalescence.