Study of passive and active driven motion of droplets on engineered substrates

Abstract

Droplet motion is an everyday phenomenon with potential benefits to multiple industrial and biological applications. It can be achieved via various methods, and the understanding and altering of the underlying mechanism are important to the accurate control of the droplet behaviour and motion. This thesis focuses on three different mechanisms that induce the droplet motion: roughness gradient by micro-structure fabrication, thermocapillary motion with self-rewetting fluid and vapor-mediated droplet motion.

Firstly, the motion of microscale water droplets on the hydrophobic microstructured surfaces with structural wettability contrast has been studied. The velocity and displacement of the droplets moving across the wettability contrasts have been monitored and their relations to the morphological parameters of the micro-structure have been systematically investigated. Besides, the dynamic behaviour of the droplets has been investigated and explained by the mathematical mode proposed.

Secondly, the thermocapillary motion of self-rewetting droplets has been reported. The behaviour of self-rewetting droplets departed greatly from the droplets of ordinary mixture and pure fluids. A unique oscillatory behaviour was observed for self-rewetting droplets, which was related to the nonmonotonic dependence of surface tension on temperature. Influencing parameters were studied and IR thermography assisted to reveal the internal convection.

Last, the motion of sessile mixture or pure droplets induced by vapour was investigated. The spatial concentration change via the mass transfer through the liquid-vapour interface near contact line leads to unbalanced surface tension, which leads to droplet motion. Depending on the concentration of both droplets and the vapour, repulsive or attractive motion can be observed. A phase map as well as a critical concentration boundary was proposed for the mixture of PG and water droplets, which can help to predict the direction of droplet motion.