Friction and the flow of concentrated suspensions

Abstract

Suspensions are ubiquitous in industrial processing, yet fundamental understanding of how they flow remains limited. Recent progress on shear-thickening suspensions of non-Brownian particles establishes the importance of direct mechanical contact and friction between particles. This represents a paradigm shift, linking wet suspensions to dry granular materials through a static jamming volume fraction. In this thesis I explore further the implications of mechanical contact in three ways. Firstly in time-dependent flows, I show that large shear-rate fluctuations arise from a competition between rapid microscopic contact dynamics and the slow dynamics controlling how the suspension is sheared. I develop a dynamical-systems approach that graphically shows how an instability arises, indicates how to control the instability, and allows the extraction of a contact relaxation time that is inaccessible to conventional rheometry. Next, more complex interparticle interactions are considered. I take the relevant effect to be a stress-dependent constraint on relative interparticle motion, e.g., sliding, twisting or rolling. Constraints lower the jamming volume fraction and can either form or break with stress. I show that an interplay between two constraint types can capture all classes of flow curve, with predictions compared against my own experimental or literature data. In particular, a yield stress behaviour is reproduced for rolling constraints being broken while sliding is constrained. Finally, I investigate the protocol dependence of yield-stress suspension rheology. The complex experimental phenomenology is shown to be consistent with an adhesively-bonded compressive frictional contact network. The yield stress is hence related to jamming and constraints, rather than just resulting from interparticle attraction. This finding continues the transition of non-Brownian suspension rheology from the colloidal to the granular frame and suggests novel ways to tune the yielding behaviour through the interparticle friction coefficient or flow protocols.