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Channel flow instabilities of complex fluids

Castillo Sánchez, Hugo A.; (2019) Channel flow instabilities of complex fluids. Doctoral thesis (Ph.D), University College London. Green open access

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Abstract

Complex fluids are present in our daily life: food, paints, plastics, pharmaceuticals, to name just a few, and thus, their study is of great interest to the industry. In many industrial processes, raw materials are transformed into finished products with specific rheological properties. Such a transformation may consist of many intermediate physical processes (extruding, melting, coating, pumping, among others). However, the complex nature of these materials might be problematic in many of these operations. More specifically, when complex fluids are under flow conditions, an unexpected behaviour may be observed: for instance, the presence of non-ideal velocity fields during steady flow. In fluid mechanics, we refer to these undesired behaviours as \textit{flow instabilities}; they can occur in a variety of commercially important processing operations. This thesis is a study of instabilities in inertialess viscoelastic flows, also known as \textit{purely elastic instabilities}. The main research content of the thesis is split into three main chapters. In the first, we extend previous theoretical work on a newly discovered elastic instability in planar channel flow of highly shear-thinning viscoelastic fluids. Our motivation was to answer whether the mechanism of such instability observed is truly elastic or principally as a result of strong shear-thinning; we found that both components were critical for the existence of the instability. In the second chapter, we carried out a similar analysis to study instabilities in channel flow of fluids that exhibit much more complex rheological behaviour: thixotropic-viscoelasto-plasticity (TVEP). For the most unstable flows, we showed that the growth rate of instability scales with the rate of recovery of thixotropic structure. Finally, in a third chapter, we extended the scope of the fluids we could consider by including the shear-banding phenomenon in the flows of TVEP fluids; this allowed us to identify distinct bulk and interfacial modes of instability.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Channel flow instabilities of complex fluids
Event: University College London
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2019. Original content in this thesis is licensed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) Licence (https://creativecommons.org/licenses/by/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
Keywords: fluid mechanics, rheology, polymers, viscoelasticity, thixotropy, plasticity, micellar solutions, stability analysis, applied mathematics
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Mathematics
URI: https://discovery.ucl.ac.uk/id/eprint/10087846
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