Interfacial Fluid Mechanics: From Flows, to Coalescence, and Self-Assembly

Abstract

Multiphase fluid systems can be generally defined as systems that contain at least two thermodynamic phases that exhibit fluid-like properties. The presence of a fluid-fluid interface in these systems gives rise to interesting interfacial fluid mechanics. Combining experiments, theory, and simulations, I will explore five interfacial fluid mechanics problems: (1) an invading two-phase flow in a sinusoidal channel, (2) a parallel two-phase flow in a microchannel, (3) the coalescence of viscous Newtonian drops, (4) the coalescence of non-Newtonian drops, and (5) the self-assembly of electrically charged drops at an oil-air interface. The first part of this thesis is set inside of channels. When two immiscible fluids are flowing through a microchannel, the wettability of the fluids with the boundary has a significant effect on the type of flow that ensues. I will show a theoretical model that describes the shape of the interface that forms in the context of a sinusoidal channel, when the more wetting phase is trapped along the boundary. Wettability can also be used as a tool to produce a specific type of flow. Using experiments, I will show that the surfaces of a channel can be modified such that two immiscible liquids can flow parallel to each other. I will use pressure arguments to show that the stability of this interface is highly sensitive to the parameters of this flow system. The second part of this thesis is set at the free surface, specifically at the drop-air interface. I will discuss the coalescence of drops on a substrate both in the context of viscous Newtonian drops and non-Newtonian polymeric drops. I will show that the three-dimensional shape of the dynamic interface exhibits a 3D self-similarity in the Newtonian case. Furthermore, I will show that the polymers have no effect on the coalescence dynamics in the highly wetting regime. Finally, I will discuss the self-assembly of electrically charged drops floating at an oil-air interface. I will show how electrostatic interactions leads to the self-assembly of drops and will identify the depth of the oil bath as an important parameter.