Partial Coalescence of Oil and Water: Spreading Behavior and Material Synthesis

Interactions of oil and water form the basis of numerous natural systems including oil spills, food dressings, cosmetic compounds and pharmaceutical drugs. The impact of a single drop of oil onto an air-water interface represents one of the most elemental interactions between oil and water, and thus presents a model problem to understanding the underlying physics involving such droplet-interface interactions. Previous reports have focused on the outcomes of a drop impact onto an identical bulk, characterizing the interaction of a two phase system. There is also ample literature on high velocity impacts of drops onto interfaces, with evaluations of the effect of the Weber number(We) on the final outcome. However, the comprehensive behavior of a drop upon gentle contact with an interface in a three phase system needs further examination. The current study essays to shed light on the complex dynamics taking place during the gentle impact of an oil drop onto an air-water interface in the context of oil spills. Using high speed imaging, the dynamics of drop deformation during the merger of an oil drop with a bulk liquid in a three phase system is investigated experimentally. The merger is found to either proceed in a single event of complete coalescence or in a cascade of multiple events of partial coalescence where a small portion of the drop detaches itself from the merging oil film at every stage. The mechanism adopted by the drop in coalescing with the bulk liquid is found to depend greatly on the physical properties of the oil drop as characterized by the Ohnesorge number. The transient topological features transpiring during the intermediate stages of transition from a drop to a film scale as a function of the Ohp and are theoretically validated using appropriate force balances. Additionally, the spreading behavior of the oils following two distinct impact scenarios - (1) upon impact onto the air-water interface and (2) succeeding the emergence of a drop from below the surface is investigated. It is found that the spreading characteristics during coalescence and emergence are heavily dependent on the viscosity of the oil and bulk liquid respectively, ultimately determining the extent of film coverage over the liquid substrate. The spreading characteristics are scaled against the physical properties of the oil and bulk liquids, and to validate the spreading behavior of the oils, a theoretical model based on the damped harmonic motion is presented. Finally, through material synthesis, attempts are made to control the dewetting events which lead to the emergence of oils in underwater oil spills. It is demonstrated that through manipulation of the production of the initial parent drop, compounds such as armored drops, and single and double emulsions can be created to suit the needs of applications like drug delivery, and emulsion stabilization.