Study of microscale interfacial phenomena during phase change processes: Analysis of micro-drops and thin films

Interfacial phenomena in the three phase contact line region, where a liquid-vapor interface intersects a solid surface, are of importance to many phenomena such as contact angle, adsorption, surface patterning, spreading, thin film stability, phase change, etc.; The objectives of this work are to experimentally analyze the coupled phenomena of spreading and phase change for a partially wetting system. We studied five experimental systems: S1---spreading of apolar n-butanol drops during phase change processes, S2 and S3---effects of surface modification on n-butanol spreading and dewetting of the meniscus, S4---spreading of polar 2-propanol drop, S5---coalescence of 2-propanol droplets, and S6---evaporation and spreading of surfactant laden micro-drops of water.; Using system S1, we found that the interfacial velocity for the drop during condensation and evaporation is a function of the apparent contact angle and the rate of change of radius of curvature.; Using systems S2 and S3, the vivid difference in the behavior of the retracting meniscus with respect to its variation in apparent contact angle and curvature was found to depend on the wettability.; Using system S4, an improved image analyzing technique based on the reflectivity measurements using interferometry was developed, which gives fundamental insights about the physics of interfacial phenomena of a spreading drop during phase change.; The reflectivity technique was also used in system S5 to study the shapes of condensing drops during their coalescence on the quartz surface. We found that the curvature and its gradient govern the growth and coalescence of the condensing drops.; In system S6, the stability of the adsorbed film was analyzed using the concept of minimization of the interfacial free energy. During a phase change process, the thin film of the surfactant solution was unstable and it broke into micro-drops. The drop profiles were experimentally measured and analyzed during the phase change process as the contact line advanced and receded. The important finding of the experiments is the abrupt change in the velocity of the contact line, when the adsorbed film of the surfactant solution is just hydrated or desiccated, during the phase change processes.