Controlling states of water droplets on nanostructured surfaces by design
Controlling states of water droplets on nanostructured surfaces by design
复制标题
通过设计控制纳米结构表面上的水滴状态
DOI：
10.1039/c7nr06896d
复制
发表时间：
2017
期刊：
影响因子：
6.7
通讯作者：
Zeng Xiao Cheng
中科院分区：
文献类型：
--
作者：
Zhu Chongqin;Gao Yurui;Huang Yingying;Li Hui;Meng Sheng;Francisco Joseph S.;Zeng Xiao Cheng
最近已经证明了表现出超疏水性和超疏油性的表面。具体而言，基于悬垂/倒梯形微结构的卓越设计使得水滴能够仅在微柱的尖端处接触这些表面，处于被称为Cassie状态的状态。然而，在某些条件下，Cassie状态可能转变为不期望的Wenzel状态。在这里，我们从大规模的分子动力学模拟表明，Cassie和Wenzel状态之间的过渡可以通过精确设计的梯形纳米结构的表面上进行控制。可以利用表面的本征接触角和表面的底角来控制过渡。对于给定的底角，可以实现三种状态：Wenzel状态，其中当固有接触角小于某个临界值时，水滴只能以Wenzel状态存在; Cassie状态，其中当固有接触角大于另一个临界值时，水滴只能以Cassie状态存在;以及当内禀接触角在两个临界值之间时，Wenzel和Cassie状态都可以存在的Wenzel-Cassie状态。一个强大的第一个临界值的底角依赖性被揭示，而第二个临界值显示出更少的依赖于底角。通过计算Cassie到Wenzel态转变的自由能垒，定量评价了Cassie态对于各种底角（和固有接触角）的稳定性。
Surfaces that exhibit both superhydrophobic and superoleophobic properties have recently been demonstrated. Specifically, remarkable designs based on overhanging/inverse-trapezoidal microstructures enable water droplets to contact these surfaces only at the tips of the micro-pillars, in a state known as the Cassie state. However, the Cassie state may transition into the undesirable Wenzel state under certain conditions. Herein, we show from large-scale molecular dynamics simulations that the transition between the Cassie and Wenzel states can be controlled via precisely designed trapezoidal nanostructures on a surface. Both the base angle of the trapezoids and the intrinsic contact angle of the surface can be exploited to control the transition. For a given base angle, three regimes can be achieved: the Wenzel regime, in which water droplets can exist only in the Wenzel state when the intrinsic contact angle is less than a certain critical value; the Cassie regime, in which water droplets can exist only in the Cassie state when the intrinsic contact angle is greater than another critical value; and the bistable Wenzel–Cassie regime, in which both the Wenzel and Cassie states can exist when the intrinsic contact angle is between the two critical values. A strong base-angle dependence of the first critical value is revealed, whereas the second critical value shows much less dependence on the base angle. The stability of the Cassie state for various base angles (and intrinsic contact angles) is quantitatively evaluated by computing the free-energy barrier for the Cassie-to-Wenzel state transition.