Heima Yuta, Teshima Hideaki, Zhang Xuehua, Li Qin-Yi, Takahashi Koji
Department of Aeronautics and Astronautics, Kyushu University, Motooka 744, Nishi-Ku, Fukuoka 819-0395, Japan.
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Motooka 744, Nishi-Ku, Fukuoka 819-0395, Japan.
Langmuir. 2025 Jan 14;41(1):917-925. doi: 10.1021/acs.langmuir.4c04227. Epub 2024 Dec 24.
Despite the importance of the effect of subnanoscale roughness on contact line behavior, it is difficult to directly observe the local behavior of contact lines at the micro- and nanoscale, leaving significant gaps in our current understanding. In this research, we investigate contact line motions and their relationship with nanoscale surface topography using coherence scanning interferometry. Our experiments were conducted on the substrates with different wettability without changing nanoscale surface topography. Titanium dioxide was used as a substrate, the wettability of which was varied under UV-light irradiation. A ridge-like structure with a height of approximately 1 nm was observed to cause contact line deformation toward the droplet side, regardless of the direction of the contact line motion. This was explained in terms of an imbalance in the local capillary pressure at the nanoscale contact line. We also found that the deformation becomes larger on the more hydrophilic surface, which was rationalized by theoretical prediction based on analysis of the work done by the force acting on the contact line and the change in surface free energy associated with the deformation of the liquid/gas interface. Furthermore, it was revealed by contact angle measurements that the maximum pinning forces on a hydrophilic surface were less than half of those on a hydrophobic surface. We attributed the weak pinning force on the hydrophilic surface to cascading depinning, where the initial depinning event triggers a chain reaction of subsequent depinning events, driven by the conversion of excess surface energy to kinetic energy. Our experimental works provide new insights of the relationship between the subnanoscale surface roughness and macroscopic contact line motion.
尽管亚纳米级粗糙度对接触线行为的影响很重要，但在微米和纳米尺度上直接观察接触线的局部行为却很困难，这使得我们目前的理解存在重大空白。在本研究中，我们使用相干扫描干涉术研究接触线运动及其与纳米级表面形貌的关系。我们的实验是在具有不同润湿性的基底上进行的，而不改变纳米级表面形貌。使用二氧化钛作为基底，其润湿性在紫外光照射下会发生变化。观察到一个高度约为1 nm的脊状结构会导致接触线向液滴一侧变形，而与接触线运动方向无关。这是根据纳米级接触线处局部毛细管压力的不平衡来解释的。我们还发现，在更亲水的表面上变形会更大，这通过基于作用在接触线上的力所做的功以及与液/气界面变形相关的表面自由能变化的分析进行理论预测得到了合理化解释。此外，通过接触角测量发现，亲水表面上的最大钉扎力不到疏水表面上的一半。我们将亲水表面上较弱的钉扎力归因于级联脱钉，即初始脱钉事件引发后续脱钉事件的连锁反应，由多余表面能向动能的转化驱动。我们的实验工作为亚纳米级表面粗糙度与宏观接触线运动之间的关系提供了新的见解。
