Tribology LAB, Dipartimento di Meccanica, Matematica e Management, Politecnico di Bari, Bari, Italy.
Langmuir. 2013 Jan 15;29(2):599-609. doi: 10.1021/la304072p. Epub 2012 Dec 31.
We investigate the effect of statistical properties of the surface roughness on its superhydrophobicity. In particular, we focus on the liquid-solid interfacial structure and its dependence on the coupled effect of surface statistical properties and drop pressure. We find that, for self-affine fractal surfaces with Hurst exponent H > 0.5, the transition to the Wenzel state first involves the short wavelengths of the roughness and, then, gradually moves to larger and larger scales. However, as the drop pressure is increased, at a certain point of the loading history, an abrupt transition to the Wenzel state occurs. This sudden transition identifies the critical drop pressure p(W), which destabilizes the composite interface. We find that p(W) can be strongly enhanced by increasing the mean square slope of the surface, or equivalently the Wenzel roughness parameter r(W). Our investigation shows that, even in the case of randomly rough surface, r(W) is still the most crucial parameter in determining the superhydrophobicity of the surface. An analytical approach is, then, proposed to show that, for any given value of Young's contact angle θ(Y), a threshold value (r(W))(th) = 1/(-cos θ(Y)) exists, above which the composite interface is strongly stabilized and the surface presents robust superhydrophobic properties. Interestingly, this threshold value is identical to the one that would be obtained in pure Wenzel regime to guarantee perfect superhydrophobicity.
我们研究了表面粗糙度的统计特性对超疏水性的影响。特别关注的是液-固界面结构及其与表面统计特性和液滴压力的耦合效应的关系。我们发现，对于具有赫斯特指数 H > 0.5 的自仿射分形表面，向 Wenzel 状态的转变首先涉及到粗糙度的短波长，然后逐渐向更大的尺度移动。然而，随着液滴压力的增加，在加载历史的某个点，会突然发生向 Wenzel 状态的转变。这种突然的转变确定了临界液滴压力 p(W)，它会使复合界面失稳。我们发现，通过增加表面均方斜率，或者等效地增加 Wenzel 粗糙度参数 r(W)，可以显著增强 p(W)。我们的研究表明，即使在随机粗糙表面的情况下，r(W)仍然是决定表面超疏水性的最关键参数。然后，提出了一种分析方法来表明，对于给定的杨氏接触角θ(Y)值，存在一个阈值(r(W))(th) = 1/(-cos θ(Y))，超过这个阈值，复合界面会被强烈稳定，表面会呈现出稳定的超疏水性。有趣的是，这个阈值与在纯 Wenzel 区域中获得的保证完美超疏水性的阈值相同。
