Shear-driven ordering and disordering of frictional sphere suspensions in two 2D

Dense granular suspensions are of great relevance in natural, biological, and industrial systems. Most studies of dense suspensions of frictional spheres are restricted to bidisperse/polydisperse systems to avoid ordering. However, in certain contexts ordering can be useful, e.g., for optical, photonic or plasmonic properties. Thus, it is important to study what microscopic constraints such as hydrodynamics and friction can affect the ordering of particles. In this study we analyze the ordering process in two dimensions, and how it is disrupted with local activation of friction. The lubricated-flow discrete element method (lf-DEM) simulation comprise of inertia-less spheres with short-range repulsion, short-range lubrication and stress-activated friction. We implement both sliding and rolling friction. Initially, we focus on the effect of sliding friction by itself, revealing two qualitatively different regimes depending on the value of shear stress.

At low shear stresses, when hydrodynamics is dominant, the particles form layered assemblies. The particles oscillate between hexagonal and square order in order to flow in a worm-like cooperation. This results in a moderate degree of long-range order in the flowing state. In contrast, at higher shear stress, as friction gets activated, the system transitions to a less ordered state with transient hexagonal close-packed microdomains. The worm-like cooperative motion is largely absent, along with a general absence of long-range order.

We explore the transition between these two states in rate-controlled and stress-controlled simulations, revealing abrupt and continuous changes in global hexatic order parameter, respectively. The distinction between the two states gradually disappears at lower packing fractions as the low-shear regime becomes increasingly disordered.

Introducing rolling friction further constrains the motion of these particles and disrupts order in a qualitatively distinct manner compared to sliding friction alone. This model sets up the groundwork for more realistic studies in shear-induced ordering in granular materials and colloids with frictional constraints.