Applied Mechanics of Materials Laboratory, Department of Mechanical Engineering, Temple University, 1947 North 12th Street, Philadelphia, PA 19122, USA.
Soft Matter. 2018 Feb 28;14(9):1517-1529. doi: 10.1039/c7sm02208e.
We studied the wetting behavior of multiscale self-similar hierarchical wrinkled surfaces. The hierarchical surface was fabricated on poly(dimethylsiloxane) (PDMS) substrates by manipulating the sequential strain release and combined plasma/ultraviolet ozone (UVO) treatment. The generated structured surface shows an independently controlled dual-scale roughness with level-1 small-wavelength wrinkles (wavelength of 700-1500 nm and amplitude of 50-500 nm) resting on level-2 large-wavelength wrinkles (wavelength of 15-35 μm and amplitude of 3.5-5 μm), as well as accompanying orthogonal cracks. By tuning the aspect ratio of hierarchical wrinkles, the degree of wetting anisotropy in hierarchical wrinkled surfaces, defined as the contact angle difference between the parallel and perpendicular directions to the wrinkle grooves, is found to change between 3° and 9°. Through both experimental characterization (confocal fluorescence imaging) and theoretical analyses, we showed that the wetting state in the hierarchical wrinkled surface is in the Wenzel wetting state. We found that the measured apparent contact angle is larger than the theoretically predicted Wenzel contact angle, which is found to be attributed to the three-phase contact line pinning effect of both wrinkles and cracks that generates energetic barriers during the contact line motion. This is evidenced by the observed sudden drop of over 20° in the static contact angles along both perpendicular and parallel directions after slight vibration perturbation. Finally, we concluded that the observed small degree of wetting anisotropy in the hierarchical wrinkled surfaces mainly arises from the competition between orthogonal wrinkles and cracks in the contact line pinning.
我们研究了多尺度自相似分级褶皱表面的润湿行为。通过操纵顺序应变释放和等离子体/紫外臭氧（UVO）联合处理，在聚二甲基硅氧烷（PDMS）基底上制备了分级表面。所生成的结构化表面具有独立控制的双尺度粗糙度，具有一级小波长皱纹（波长为 700-1500nm，振幅为 50-500nm），其位于二级大波长皱纹（波长为 15-35μm，振幅为 3.5-5μm）上，以及伴随的正交裂纹。通过调整分级皱纹的纵横比，可以改变分级褶皱表面的润湿各向异性程度，定义为平行和垂直于皱纹槽方向的接触角差。通过实验特征描述（共焦荧光成像）和理论分析，我们表明分级褶皱表面中的润湿状态处于 Wenzel 润湿状态。我们发现，测量的表观接触角大于理论预测的 Wenzel 接触角，这归因于褶皱和裂纹的三相接触线钉扎效应，该效应在接触线运动过程中产生能量障碍。这可以通过轻微振动扰动后在垂直和水平两个方向上观察到的静态接触角突然下降超过 20°得到证明。最后，我们得出结论，观察到的分级褶皱表面的小润湿各向异性主要源于接触线钉扎中正交皱纹和裂纹之间的竞争。
