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Interfacial dynamics in complex fluids : studies of drop and free-surface deformation in polymer solutions Zhou, Diwen

Abstract

Interfacial flows in complex fluids are an important subject, scientifically rich and technologically important. The main scientific attraction comes from the fact that the microstructure of the bulk fluids may evolve during interfacial flow, and thereby generating non-Newtonian stresses that act on the interface. Thus, interfacial motion and conformation of the microstructure are coupled. Such flow situations arise in many industrial applications, including processing of polymer blends, foaming, and emulsification. In this thesis, I describe three projects aimed at exploring interfacial dynamics of viscoelastic polymeric liquids. The first project consists of finite-element simulations of drop deformation in converging flows in an axisymmetric conical geometry. The moving interface is captured using a diffuse-interface model and accurate interfacial resolution is ensured by adaptive refinement of the grid. The drop experiences a predominantly elongational flow. The amount of deformation sustained by the drop depends, besides the geometry and kinematics of the flow, on the rheology of both the drop and the matrix fluids. The second and third projects concern the same process of selective withdrawal, in which stratified layers of immiscible fluids are withdrawn from a tube placed a certain distance from the interface. We have chosen to work with an air-liquid system, with the suction tube embedded in the Newtonian or viscoelastic liquid. The second project is an experimental study, where we used video recording and imaging processing to analyze how the interfacial deformation is influenced by the non-Newtonian rheology of the liquid. We discover three regimes, subcritical, critical and supercritical. The third project consists of sharp-interface, moving-grid finite-element simulations of selective withdrawal for Newtonian and viscoelastic Giesekus liquids. The experiments and computations are in reasonable agreement. The work of this thesis has led to two main outcomes. The first is a detailed understanding of how viscoelastic stress can lead to unusual and sometimes counter-intuitive effects on interfacial deformation. The second is a potentially important new method for measuring elongational viscosity of low-viscosity liquids. This is worth further investigation considering the poor performance of existing methods.

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