Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, ROC.
J Phys Chem B. 2010 Jun 10;114(22):7509-15. doi: 10.1021/jp100258m.
Superhydrophobic surfaces generally involve completely nonwetting or partially wetting roughness. Because the contact angle is closely related to the liquid-gas interfacial tension, the shape of the liquid-gas interfaces within the grooves plays a key role in determining the droplet wetting behavior. We consider a droplet with volume, V, atop holes with radius, r, and obtain the analytical expression of the bottom liquid-air shape based on surface free energy minimization. It is found that the bottom shape in terms of the interfacial angle, theta(1), depends on the hole size through V/r(3) in addition to the intrinsic contact angle, theta(*). For a given droplet volume, the smaller the hole size (r(3)/V --> 0), the more flat the interface (theta(1) --> 0). In addition, the flatness of the interface grows with reducing the intrinsic contact angle. Numerical simulations of Surface Evolver are performed to confirm our theory. Moreover, wetting experiments in which the gravity effect cannot be neglected are conducted, and the results are consistent with those by numerical simulations. Our result points out that such wall-free capillarity may be useful in extracting liquid from microfluidic device spontaneously.
超疏水表面通常涉及完全不润湿或部分润湿的粗糙度。由于接触角与气液界面张力密切相关，因此凹槽内的气液界面形状在确定液滴润湿行为方面起着关键作用。我们考虑了一个体积为 V 的液滴位于半径为 r 的孔顶上，并基于表面自由能最小化得到了底部液-气形状的解析表达式。结果发现，底部形状（以界面角θ(1)表示）不仅取决于固有接触角θ(*), 还取决于孔的大小（通过 V/r(3)）。对于给定的液滴体积，孔越小（r(3)/V--->0），界面越平坦（θ(1)--->0）。此外，界面的平坦度随固有接触角的减小而增加。使用 Surface Evolver 进行了数值模拟以验证我们的理论。此外，还进行了不能忽略重力效应的润湿实验，实验结果与数值模拟结果一致。我们的结果表明，这种无壁毛细作用可能有助于自发地从微流控装置中提取液体。
