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Title: Symmetry-breaking pattern formation in thin films with application to foaming in viscous fluids
Author: Shen, Li
ISNI:       0000 0004 9350 5250
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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Foaming is a multi-disciplinary and multi-phase process. The understanding of the formation and collapse process of foams is of significant interest in engineering research and development. In this work, we look at foam, bubbles and thin films in three different ways; the physical chemistry effects at close distances, the surface phenomena using capillary waves on liquid interfaces and the onset of fluid dynamical instability triggered near rupture. Examining the experimental links between surface (shear) viscosity and the stability of a foam or emulsion theoretically, we consider the small-amplitude capillary waves in the presence of a surfactant solution of dilute concentrations. The associated Marangoni and surface viscosity effects are modelled via the Boussinesq-Scriven formulation and the resulting integro-differential initial value problem is solved analytically and surface viscosity is found to contribute an overall damping effect on the amplitude of the capillary wave with varying degrees depending on the lengthscale of the system. Numerically, we find the critical damping wavelength to increase for increasing surface concentration but the rate of increase remains different for both the surface viscosity and the Marangoni effect. Near rupture, patterns can be observed experimentally on the surface of a soap bub- ble. These quasi-stable transient structures are associated with the instabilities of the complex Marangoni flow on the curved thin film in the presence of a surfactant solution. We report a generalised Cahn-Hilliard-Swift-Hohenberg model derived using asymptotic theory which describes the quasi-elastic wrinkling pattern formation and the consequent coarsening dynamics in a curved surfactant-laden thin film. By testing the theory against experiments on soap bubbles, we find quantitative agreement with the analytical predic- tions of the nucleation and the early coarsening phases. Furthermore, the theory can be extended to arbitrarily curved surfactant-laden films nearing a critical thickness and is a first step in understanding bubble rupture.
Supervisor: Dini, Daniele ; van Wachem, Berend ; Denner, Fabian Sponsor: Shell University Technology Centre (UTC) ; Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral