Ordered domains in sheared dense suspensions: the link to viscosity and the disruptive effect of friction

TL;DR

This study uses large-scale simulations to explore how shear-induced ordering in dense suspensions affects viscosity, revealing that friction disrupts order and significantly increases viscosity by forming system-spanning stress networks.

Contribution

It provides a detailed analysis of shear-induced ordering and the disruptive role of friction in dense suspensions through large-scale particle simulations.

Findings

Shear induces ordering at high shear rates, reducing viscosity.

Friction disrupts order, leading to increased viscosity.

Defect structures and stress networks are key to rheological behavior.

Abstract

Monodisperse suspensions of Brownian colloidal spheres crystallize at high densities, and ordering under shear has been observed at densities below the crystallization threshold. We perform large-scale simulations of a model suspension containing over $10^5$ particles to quantitatively study the ordering under shear and to investigate its link to the rheological properties of the suspension. We find that at high rates, for $Pe>1$, the shear flow induces an ordering transition that significantly decreases the measured viscosity. This ordering is analyzed in terms of the development of layering and planar order, and we determine that particles are packed into hexagonal crystal layers (with numerous defects) that slide past each other. By computing local $\psi_6$ and $\psi_4$ order parameters, we determine that the defects correspond to chains of particles in a square-like lattice. We compute the individual particle contributions to the stress tensor and discover that the largest contributors to the shear stress are primarily located in these lower density, defect regions. The defect structure enables the formation of compressed chains of particles to resist the shear, but these chains are transient and short-lived. The inclusion of a contact friction force allows the stress-bearing structures to grow into a system-spanning network, thereby disrupting the order and drastically increasing the suspension viscosity.