Coexistence and transition between Cassie and Wenzel state on pillared hydrophobic surface
Coexistence and transition between Cassie and Wenzel state on pillared hydrophobic surface
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DOI：
10.1073/pnas.0902027106
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发表时间：
2009-05-26
影响因子：
11.1
通讯作者：
Zeng, Xiao Cheng
中科院分区：
文献类型：
--
作者：
Koishi, Takahiro;Yasuoka, Kenji;Zeng, Xiao Cheng
粗糙疏水性表面上的水滴通常表现出以下两种状态之一：（i）Wenzel状态[Wenzel RN（1936）Ind Eng Chem 28：988-994]，其中水滴与粗糙表面完全接触（称为润湿接触）或（ii）Cassie状态[Cassie，ABD，巴克斯特S（1944）Trans法拉第Soc 40：546-551]，其中水滴与粗糙表面的峰以及捕获在表面凹槽之间的“气穴”接触（复合接触）。在这里，我们展示了大规模的分子动力学模拟的Wenzel状态和Cassie状态的水滴之间的过渡周期性纳米柱疏水表面。可以强烈影响转变的物理条件包括纳米柱的高度、柱之间的间距、固有接触角和水纳米滴的撞击速度（“降雨”模拟）。存在一个临界柱高度，超过该高度，柱表面上的水滴可以处于Wenzel状态或Cassie状态，具体取决于它们的初始位置。基于热力学-力学方法和动力学降雨模拟计算了Wenzel态和Cassie态之间的自由能垒。势垒的范围从在临界柱高度处的粗糙表面的k（B）T（0）的十分之几（其中k（B）是玻尔兹曼常数，T-0是环境温度）到柱高度大于水滴长度尺度的表面的接近8 k（B）T（0）。对于高度粗糙的表面，从Wenzel到Cassie状态的势垒比从Cassie到Wenzel状态的势垒高得多。因此，一旦液滴被深深地困在凹槽内，就很难重新定位在高柱的顶部。
Water droplets on rugged hydrophobic surfaces typically exhibit one of the following two states: (i) the Wenzel state [Wenzel RN (1936) Ind Eng Chem 28:988-994] in which water droplets are in full contact with the rugged surface (referred as the wetted contact) or (ii) the Cassie state [Cassie, ABD, Baxter S (1944) Trans Faraday Soc 40:546-551] in which water droplets are in contact with peaks of the rugged surface as well as the "air pockets'' trapped between surface grooves (the composite contact). Here, we show large-scale molecular dynamics simulation of transition between Wenzel state and Cassie state of water droplets on a periodic nanopillared hydrophobic surface. Physical conditions that can strongly affect the transition include the height of nanopillars, the spacing between pillars, the intrinsic contact angle, and the impinging velocity of water nanodroplet ("raining'' simulation). There exists a critical pillar height beyond which water droplets on the pillared surface can be either in the Wenzel state or in the Cassie state, depending on their initial location. The free-energy barrier separating the Wenzel and Cassie state was computed on the basis of a statistical-mechanics method and kinetic raining simulation. The barrier ranges from a few tenths of k(B)T(0) (where k(B) is the Boltzmann constant, and T-0 is the ambient temperature) for a rugged surface at the critical pillar height to approximate to 8 k(B)T(0) for the surface with pillar height greater than the length scale of water droplets. For a highly rugged surface, the barrier from the Wenzel-to-Cassie state is much higher than from Cassie-to-Wenzel state. Hence, once a droplet is trapped deeply inside the grooves, it would be much harder to relocate on top of high pillars.