Department of Chemical and Materials Engineering, National Central University , Jhongli 320, Taiwan.
Department of Chemical Engineering, National Taiwan University , Taipei 106, Taiwan.
Langmuir. 2017 Jun 20;33(24):6182-6191. doi: 10.1021/acs.langmuir.7b01120. Epub 2017 Jun 5.
The motion of droplets on typical surfaces is generally halted by contact line pinning associated with contact angle hysteresis. In this study, it was shown that, on a zwitterionic sulfobetaine silane (SBSi)-coated surface, aqueous drops with appropriate solutes can demonstrate hysteresis-free behavior, whereas a pure water drop shows spontaneous spreading. By adding solutes such as polyethylene glycol, 2(2-butoxy ethoxy) ethanol, or sodium n-dodecyl sulfate, an aqueous drop with a small contact angle (disappearance of spontaneous spreading) was formed on SBSi surfaces. The initial drop shape was readily relaxed back to a circular shape (hysteresis-free behavior), even upon severe disturbances. Moreover, it was interesting to observe the self-propulsion of such a drop on horizontal SBSi surfaces in the absence of externally provided stimuli. The self-propelled drop tends to follow a random trajectory, and the continuous movement can last for at least 10 min. This self-propelled random motion can be attributed to the combined effects of the hysteresis-free surface and the Marangoni stress. The former comes from the total wetting property of the surface, while the latter originates from surface tension gradient due to fluctuating evaporation rates along the drop border.
在典型的表面上，液滴的运动通常会因与接触角滞后相关的接触线钉扎而停止。在这项研究中，研究表明，在两性离子磺酸甜菜碱硅烷 (SBSi) 涂层表面上，具有适当溶质的水液滴可以表现出无滞后行为，而纯水液滴则表现出自发铺展。通过添加诸如聚乙二醇、2(2-丁氧基乙氧基)乙醇或十二烷基硫酸钠等溶质，在 SBSi 表面上形成具有小接触角（自发铺展消失）的水液滴。初始液滴形状很容易恢复到圆形（无滞后行为），即使受到严重干扰也是如此。此外，有趣的是观察到在没有外部提供刺激的情况下，这种液滴在水平 SBSi 表面上的自推进。自推进的液滴倾向于遵循随机轨迹，并且连续运动至少可以持续 10 分钟。这种自推进的随机运动可归因于无滞后表面和马兰戈尼应力的综合影响。前者来自表面的完全润湿特性，而后者则源于由于液滴边界沿蒸发速率波动而导致的表面张力梯度。
