Impact and Departure Dynamics of Droplets and Bubbles

Droplets and bubbles are important for understanding natural phenomena such as falling raindrops, airborne disease transmission, and plant respiration systems, and also for engineering contexts such as semiconductor fabrication, nuclear power plants, and electronics cooling. However, still, more understanding is needed of these complex dynamics problems. This dissertation will talk about the droplet impact and bubble departure dynamics that are happening on various surfaces. In Chapters 2 and 3, we will explore how raindrops can transmit plant pathogens. When the raindrop impacts the infected wheat leaf, the micron-sized dry spore can liberate from the surface in two different ways: dry dispersal and wet dispersal. The dry spore can liberate from the surface by the inertia of the drop, after that, the air vortex generated by the drop impact can carry the dry spores above the laminar boundary layer, with the potential for long-distance transport. For the wet dispersal, spore-laden droplets can be generated after raindrop impact, but how these spore-laden droplets can make neighboring plant diseases is still a mystery. We have shown that the splashed droplets can stick to the adjacent healthy leaf depending on the inertia of the impacting droplet, anisotropic leaf orientation, and whether it is treated with fungicide or not. In Chapter 4, We design a micropillar aluminum substrate that preferentially grows frost on top of the pillars. When deposited droplets impact the frost-tipped pillars, the dynamic pressure causes the water to wick within the frost faster than it can impale the gaps between the pillars. Upon freezing, this safely suspends the resulting ice sheet in the air-trapping Cassie state, without any surface coatings required. For the last part (Chapter 5), we investigated the bubble coalescence dynamics that can depart the bubble with a micrometer size. We made the micro-structured surfaces tailored to nucleation sites to enable the coalescence-induced departure of micro-bubbles. A scaling model reveals two different modes of bubble departure following the coalescence-induced depinning: capillary-inertial jumping for micrometric bubbles and a buoyant-inertial departure for millimetric ones. Eventually, this small bubble departure can delay film boiling which can be the barrier to the boiling heat transfer.