Rheology of yield stress fluids: a multiscale approach

ongoing

Yield stress fluids defy our conventional notions of liquid and solid, keeping their shape as soft solids at low loads, yet yielding and flowing like liquids at larger loads. They can then suffer arbitrarily large deformations in this liquid state, but will recover a solid state if the load is removed. Their internal microstructure and macroscopic shape are thus determined directly by the processing history they experience. Such materials are all around us: in colloids, microgels, emulsions, foams, pastes, slurries, and their biological counterparts. They find widespread applications in foods, pharmaceuticals, construction, oil extraction, lubricants, coatings, etc. Despite this importance to so many engineering processes, we still do not understand how their remarkable macroscopic rheological (deformation and flow) properties emerge out of the collective dynamics of their constituent microscopic substructures: colloid particles, microgel beads, emulsion droplets, etc. Addressing key questions emerging from recent experiments, RheoYield aims to build new theories to inform and potentially transform our understanding of the rheology of yield stress fluids. Within a multiscale approach, the project will capitalise on rapid recent progress in understanding how microscopic rearrangement events cooperate to give macroscopic flow. Using theoretical and computational tools that I have recently developed, and new ones that will be developed here, RheoYield aims to: 1. Identify the microscopic changes that take place in a soft solid as it slowly yields into a fluidised state. 2. Understand the profound influence of boundary physics on bulk yielding. 3. Develop the first microscopically founded continuum constitutive model that captures all the key features of yield stress rheology. 4. Establish a microscopically founded computational fluid dynamics of yield stress fluids. 5. Develop basic new science underpinning strategies for the optimised control of yield stress rheology.