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. / Doctor of Philosophy / Dynamic interaction of droplets and bubbles with different surfaces is ubiquitous: an impacting rain droplet on a plant leaf is responsible for transmitting thousands of plant pathogens, or decreasing the departure size of bubbles on the surface of heat exchangers would increase their efficiency. It is now well-understood that the departure of condensed droplets on water repellent surfaces exhibits superior heat transfer compared to all other modes of condensation and also enables self-cleaning, delayed frosting, and anti-fogging surface technology.In Chapters 2 and 3, we are studying the dynamic interaction of raindrops and wheat leaves. By depositing water droplets on diseased leaves, we found out a raindrop can transmit wheat pathogens. This simple but important phenomenon would adversely affect the quality of our wheat which is the most widely grown crop in the world, contributing to a large amount of portion the global food supply. Chapter 4 sheds light on another example of the dynamic interaction of raindrops and an icy surface. We designed a pillared aluminum substrate that preferentially grows condensation frosting on top of the pillars. With this passive anti-frosting technology, we are able to trap water droplets and ice in the suspending water droplets in the air-trapping Cassie state without using a fragile nanotextured structure or a complex re-entrant structure. Upon freezing, this safely suspends the resulting ice sheet in the air-trapping Cassie state, without any surface coatings required. Under a cold and humid environment, Cassie water freezes into Cassie ice which is advantageous for its low surface adhesion. In Chapter 5, we show that rationally micro-structured surfaces tailor nucleation sites to enable the coalescence-induced departure of micro-bubbles. With this technique, we are able to remove surface bubbles at smaller sizes that would result in enhancing the critical heat flux of nucleate boiling. We have used a blend of experiments and scaling to understand the underlying physics of this phase-change problem.

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A model is developed for the dynamics of an acoustically driven bubble in a channel. The bubble is assumed to be smaller than the transverse dimension of the channel and spherical in shape. The channels considered are infinite in length and formed by either parallel planes or tubes with triangular, rectangular, or hexagonal cross sections. For surfaces that are rigid or pressure release, the boundary conditions on the channel walls in each of these geometries can be satisfied using the method of images. Effects due to confinement by the channel walls are thus determined by an analysis of coupled bubble interactions in line and plane arrays. An existing model for the coupled dynamics of spherical bubbles provides the basis for the model. Liquid compressibility is an essential feature of the model, both in terms of radiation damping and the finite propagation speed of acoustic waves radiated by the bubble. Solutions for the frequency response are obtained analytically by perturbation for low drive amplitudes and weak nonlinearity, and by numerical solution for high drive amplitudes and strong nonlinearity. The perturbation solutions for the radial motion at the drive frequency and its second harmonic are obtained in closed form for a bubble between parallel planes. The response of a bubble between rigid parallel planes is found to be mass controlled, whereas for a rigid tube it is found to be radiation damping controlled. The dynamics of a bubble located near the center of a tube are found to depend on the area but not the specific geometry of the cross section. At drive amplitudes below which subharmonic generation occurs, the numerical solutions for high drive amplitudes reveal the same general properties as the perturbation solutions for low drive amplitudes. All of the solutions can be extended to tubes with arbitrary wall impedance if the radiation impedance on the bubble is known, for example calculated by normal mode expansion. / text

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In the coalescence zone, we obtained ozone concentration profiles of pairs of bubbles in vertical and oblique alignments, which show rapid decrease in the interaction period and more gradual decline in the late stages of coalescence. / In the grid region, the hydrodynamics of a stream of gas injected vertically into a bed of solids was observed in a semi-circular column containing particles of several sizes and densities. This was found to be similar to those reported in the literature. Furthermore, experiments were carried out in a 15 cm diameter column to characterize the magnitude of gas transfer from spout to dense phase and from forming bubbles to dense phase. The results show that the transfer from the spout is mainly due to convective gas outflow with some molecular diffusion. While during bubble formation, the transfer seems to be molecular diffusion only, being similar in magnitude to single bubbles in fluidized beds.

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Bibliography: leaves 315-326 / xxv, 370 leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Faculty of Engineering, 1993