| Summary: | 碩士 === 國立中央大學 === 化學工程與材料工程學系 === 102 === This study contains two topics: separation of liquid bridge and meniscus -climbing object.
＊PART I: Separation of Liquid Bridge
Anti-fingerprint or anti-smudge mechanism often involves a liquid bridge formed between two dissimilar surfaces. As the two surfaces are separating, the tendency of the liquid to wet one surface renders the other anti-smudge. In this work, the wetting characteristics of the liquid bridge on two asymmetric surfaces during their separation are investigated both experimentally and theoretically. In general, the contact lines on both surfaces withdraw at the beginning of separation. Before the rupture of the liquid bridge, however, five types of wetting competition are observed: (i) While the contact line remains receding on one surface, it becomes pinned on the other; (ii) The contact line on both surfaces are pinned; (iii) While the contact line is pinned on the one surface, it starts to expand on the other; (iv) While the contact line remains receding on one surface, it start to expand on the other; (v) The contact line on both surfaces are expanded. Our experimental results are in good agreement with the simulation outcomes by Surface Evolver. Evidently, the winning surface is accompanied with the signature of contact angle increment or base diameter expansion before liquid bridge rupture. Further simulation analyses reveal that wetting competition depends on both intrinsic contact angle and contact angle hysteresis. After compression and relaxation of two surfaces, the one with higher contact angle and larger hysteresis may be more wettable than the other with lower contact angle and smaller hysteresis.
＊PART II: Meniscus-Climbing Object
In nature, some terrestrial insects evolved to live exclusively on the water surface in order to adapt to the environment, such as water striders. They rely on surface tension for static weight support, and use a variety of means to propel themselves along the surface. When these small insects pass from the water surface to land, they have to overcome the slippery meniscus water surfaces that border the water’s edge. They are unable to climb meniscus by using their own legs, therefore developed the meniscus-climbing technique. By fixing their body posture, the water surface is thus deformed to generate lateral surface tension, and then the insects are propelled upon the meniscus surface without moving their legs. This phenomenon is so called “meniscus-climbing”.
In this study, we investigate the behavior of the objects which have meniscus-climbing ability by testing a variety of materials, shapes and densities of the objects. In experiment, we put these objects below the pendant drop successively, and try to observe the influences caused by these conditions to the meniscus-climbing behavior by the high-speed camera. The result shows that the different conditions of objects make significant difference of climbing situations. Specifically, we observe that the objects with climbing ability rotate themselves into vertical direction just before the climbing behavior. Furthermore, the longer object could climb higher. And in the condition of the same area but different shapes, we find the larger aspect ratio, the higher object climbing. Finally, by comparing different densities of the objects, we observe that the lower density, the higher object climbing.
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