Rheology of a binary suspension

Abstract

Suspensions are widely encountered in industrial processes. So it is important to fundamentally understand how they ow. While progress has been made on monodisperse suspensions, most realistic suspensions are composed of multiple constituent particles. Among a variety of multi-component suspensions, this work concerns a specific class consisting of large repulsive grains suspended in a viscoelastic gel. In such system, we highlight a unique solid-liquid transition which is triggered by external flow. Rheo-imaging reveals the correlation between the rheological transition and the structural change. This state transition is the focus of this thesis.

We first establish a model binary suspension of large repulsive particles and small attractive particles. We mix two species of silica particles together, with the smaller (Brownian) ones being hydrophobically attractive and the larger (non- Brownian) ones stabilised via surface charge. Using a water-glycerol-ethanol mixture as the suspending solvent, we match the refractive index which, along with the fluorescent labelling of both particles and solvent, enables confocal microscopy. Remarkably, this model system is well-characterised, tuneable and transparent (imageable).

Through extensive rheological studies, we observe a ow-switched transition between a solid state and a liquid state. Specifically, the binary suspension solidi es upon cessation of vigorous flow, while prolonged gentle flow results in a liquid state which permanently persists at rest. We demonstrate that this state transition is reversible and has memory.

Rheo-confocal microscopy reveals distinct structure in the two states. The solid state consists of a gel matrix of small particles with large particles embedded inside, whereas in the liquid state, the small particles phase separate into disjoint, globular blobs. While there exists two states, detailed observation identi es three ow regimes. By varying the particle composition, we construct a state diagram to map out the extent of these regimes.

The three regimes are demarcated by two transition boundaries, which are closely related to the macroscopic property of each state. We verify that the solid state is essentially a particle-fi lled gel and the lower boundary is its yield stress. Moreover, we show that the blobs in the liquid state are solid `droplets' whose strength directly determines the upper transition boundary. Beyond the state diagram, we further explore the parameter space of the binary system. We con rm that the small-small attraction and the large particle size are two key factors to the state transition. However, we still do not understand the microscopic mechanism. We illustrate several possible research lines, whose results may ll important gaps for the fundamental understanding.

Our system contributes more than a model system. By comparing the rheology and microstructure, we reveal the similarity between a Li-ion battery slurry and our suspension. Moreover, the ow-switched solid-liquid transition, which is reversible and has memory, sheds new light on the smart material design. We identify our suspensions as a memory mechanorheological (McR) uid, whose rheology is mechanically tuneable and can persist upon removal of mechanical stimuli.