Grewal H S, Piao Shuxue, Cho Il-Joo, Jhang Kyung-Young, Yoon Eui-Sung
Center for BioMicrosystems, Korea Institute of Science and Technology, Seoul, Republic of Korea.
Soft Matter. 2016 Jan 21;12(3):859-66. doi: 10.1039/c5sm01649e. Epub 2015 Nov 9.
Surface modification is a promising method to solve the tribological problems in microsystems. To modify the surface, we fabricated hierarchical patterns with different pitches of nano-scale features and different surface chemistries. Micro- and nano-patterns with similar geometrical configurations were also fabricated for comparison. The nano-tribological behavior of the patterns was investigated using an atomic force microscope at different relative humidity levels (5% to 80%) and applied normal loads (40 nN to 120 nN) under a constant sliding velocity. The results showed significant enhancement in the de-wetting and tribological performance of the hierarchical patterns compared with those of flat and micro- and nano-patterned surfaces. The PTFE-coated hierarchical patterns showed similar dynamic contact angles (advancing and receding) to those of the real lotus leaf. The influence of relative humidity on adhesion and friction behavior was found to be significant for all the tested surfaces. The tribological performance was improved as the pitch of the nano-scale geometry of the hierarchical pattern increased, even though the wetting property was not influenced significantly. A model was proposed based on the role of intermolecular force to explain the effect of the pitch of the hierarchical patterns on the adhesion and friction behavior. According to the model based on the molecular force, the contact between a ball and the patterned surface was a multi-asperity contact, contrary to the single-asperity contact predicted by the Johnson-Kendall-Roberts (JKR) and Maugis-Dugdale (MD) models. The strong intermolecular forces, which are activated in the confined spaces between the adjacent nano-pillars and the ball, contributed to the contact area and hence the adhesion and friction forces.
表面改性是解决微系统中摩擦学问题的一种很有前景的方法。为了对表面进行改性，我们制造了具有不同纳米尺度特征间距和不同表面化学性质的分级图案。还制造了具有相似几何构型的微米和纳米图案用于比较。使用原子力显微镜在不同相对湿度水平（5%至80%）和恒定滑动速度下施加不同法向载荷（40 nN至120 nN），研究了这些图案的纳米摩擦学行为。结果表明，与平坦表面以及微米和纳米图案化表面相比，分级图案的去湿和摩擦学性能有显著提高。涂有聚四氟乙烯的分级图案显示出与真实荷叶相似的动态接触角（前进角和后退角）。发现相对湿度对所有测试表面的粘附和摩擦行为有显著影响。尽管润湿性没有受到显著影响，但随着分级图案纳米尺度几何结构间距的增加，摩擦学性能得到改善。基于分子间力的作用提出了一个模型，以解释分级图案间距对粘附和摩擦行为的影响。根据基于分子力的模型，球与图案化表面之间的接触是多粗糙峰接触，这与约翰逊 - 肯德尔 - 罗伯茨（JKR）模型和莫吉斯 - 杜格代尔（MD）模型预测的单粗糙峰接触相反。在相邻纳米柱与球之间的受限空间中激活的强分子间力，有助于增加接触面积，从而增加粘附力和摩擦力。
