Interfacial flows with heat and mass transfer

Abstract

This dissertation presents various systems that involve interfacial flows coupled with heat and mass transfer. The work is divided into four separate parts studying equilibrium behavior of CO2 bubble dissolution in a flow-focusing microfluidic channel, surfactant effects on the coalescence cascade of liquid drops, evaporative cooling with a thin film flow of water, and the diffusiophoresis of charged particles driven by CO2 dissolution. We present experiments and support the observations with theoretical models.

In the first chapter, we study the effect of surfactant and gas exchange on the dissolution of a CO2 bubble in a microfluidic channel. When a CO2 bubble is generated in a flow-focusing microfluidic channel, it rapidly dissolves and reaches a steady state radius. This apparent equilibrium behavior of the bubble is due to the inflow of air which was originally dissolved in the aqueous phase. Moreover, different surface densities of surfactant molecules that cover the gas bubble controls the initial composition of the bubble, which leads to different equilibrium radii. The multicomponent model predicting equilibrium behavior is studied for both spherical and cylindrical bubbles.

The second chapter covers the coalescence cascade of surfactant drops. When surfactant concentration is varied in a falling drop and a bath, we observe different regimes of the coalescence cascade. We introduce a new regime for the coalescence cascade where there is no rebound of daughter drops, which we refer to as the damped coalescence cascade. A perturbation of the local surfactant concentration after one pinch-off generates radially outward Marangoni flow, which drags air away from the thin gap then suppresses the rebound.

In the third chapter, we introduce an evaporative cooling device using a thin film flow of water on a superhydrophilic surface. By using superhydrophilic surfaces and a staggered structure, we achieved steady thin films of water flowing through all external surfaces of the device, which cooled down the device by 6℃.

In the last chapter, diffusiophoresis of charged particles driven by CO2 dissolution is studied. We present experiments with a CO2 bubble in a Hele-Shaw cell and suggest some modelling ideas including multicomponent dissolution.