Evaporation, spreading and impact of droplets

• Main Authors: Paola G. Pittoni, 皮保拉
• Other Authors: Lin, Shi-Yow
• Format: Others
• Language: en_US
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/83661229712630221802

博士 === 國立臺灣科技大學 === 化學工程系 === 102 === In this work, the dynamics of evaporating, spreading and impacting droplets were studied in experiments at low and high temperatures. First, the correlations between dissimilar behaviors of the triple line during drop evaporative processes and unlike surface topographies were deeply analyzed and discussed. Dynamic, advancing and receding contact angles were found to be strongly affected by the substrate topography and roughness. This dependence brought out the imperative to carefully investigate and report the inner characteristics of the system to keep using concepts such as the receding contact angle as meaningful specifications of a given gas-liquid-solid system. Then, it was pointed out how modifications of the substrate may actually occur during the evaporative process itself. The triple line movements, in fact, may be influenced by surface alterations due to diffusion or chemical reactions taking place in the course of the measurements. Specifically, on this topic, the triple line dynamics of a molten metal drop spreading and evaporating at high temperature were illustrated. Afterward, we wondered if the triple line behaviors observed in evaporating drops could be noted also during highly dynamic processes and for short time scales. Pinned and free receding behaviors were indeed recorded for drops impacting and spreading on substrates characterized by peculiar topographies and hydrophobicities. So, the relations between the movements of the triple line and these specific surface morphologies were deeply investigated. The pinned state due to random dilute cavities in a generally smooth surface was described here for the first time in the case of impinging droplet experiments. Phenomenology and possible causes of the failure of the superhydrophobicity were exposed analyzing the impact behaviors of drops impinging on two highly hydrophobic surfaces. On this matter, a theoretical description of the experimental results was proposed, revealing the essential roles played by dynamic pressure, hammer pressure and liquid penetration time. Finally, the influence of hydrophobicity and roughness of the solid substrates on the occurrence and formation mechanisms of bubbles entrapped into impacting water drops were studied.