Studies in Interfacial Fluid Mechanics: From Diffusiophoresis in Multivalent Electrolytes to Drop Motions on Fibers in a Crosswind

Abstract

In this thesis I study several problems that arise in experimental interfacial fluid mechanics. There are four main chapters. The first treats a flow problem in electrokinetics. The other three chapters explore aspects of the motion of drops on fibers exposed to a cross flow. In Chapter 2 we study the valence dependence of diffusiophoresis, a small-scale phenomenon that can be used to manipulate particles. We investigate the motion of charged particles in one-dimensional salt gradients for a variety of multivalent electrolytes. We develop a one-dimensional model and obtain good agreement between our experimental and modeling results with no fitting parameters. Our results indicate that the valence combination of ions defines the ambipolar diffusivity, which controls the time-scale over which the gradient diffuses and hence dictates the diffusiophoretic speed of the particles. In Chapter 3 we show that drops wetting parallel fibers in a uniform cross flow can interact aerodynamically, both with their downstream and upstream neighbors, leading to a variety of behaviors, including alignment, repulsion, and sometimes coalescence. Using particle image velocimetry, we visualize the wake patterns and rationalize the behaviors. We obtain a diagram of the different interaction regimes which depend on the distance between the fibers. We build on this knowledge in Chapter 4 as we search for conditions that promote coalescence. The parameter space we investigate straddles critical and sub-critical conditions for vortex shedding in flow past a circular cylinder and two regimes of steady asymmetry present for flow past a sphere that were identified by linear stability analysis. We relate observed behaviors to the flow transitions and drop geometry. Our findings will help improve designs for coalescence filters and fog harvesters. Finally, in Chapter 5, we study mixing inside and between drops. As drops move on the fibers, they deposit thin films which are intercepted by other drops. We use dye to visualize the exchange of fluid and rationalize the mixing with analytical and numerical models for the deposited films and the internal flows of the drops. Our findings are relevant to applications where one seeks to saturate a liquid absorbent with a gas-phase contaminant.