题目:Microscale thin film stability and dynamics
Interfacial flows exist widely in nature and industrial processes, and often involve thin film constructions. Here we represent two experimental realizations in which high-speed video imaging technique is used to accurately measure film profiles, for investigating dominant forces that governing thin film stability, and the fluid dynamics in film evolution. In the first example the stability of a thin film trapped between a rising bubble and a liquid-liquid meniscus interface is studied, to demonstrate the validity of using surface forces to predict film stability, with an emphasis that a short-ranged hydrophobic attraction determines the stability of thin water films. In the second system drop impact onto a solid surface is considered. Owing to viscous lubrication pressure in the intervening air layer, the first contact does not occur at a point but along a ring, leading to the entrapment of a small bubble under drop center. We use high-speed interferometry at5 million frames per secondto reveal the details of this process, including air disc size at first contact, air compression dynamics, as well as the liquid-solid contact mechanism. The effects of surface roughness on the first impact are also investigated.