(178n) Shear-Induced Diffusion of Rough and Frictional Particles in Concentrated Suspensions

The shear-induced diffusion of concentrated suspensions of non-Brownian spheres in a viscous shearing flow was simulated. These simulations account for both the roughness of the particles and frictional interactions during their contacts. The particle roughness is modeled as the range of the Hertzian repulsive force for normal interactions between particles. Friction between particles is incorporated via a Cundall-Strack tangential spring method. In the absence of a clear relationship between friction and roughness, the range of the Hertzian force and the friction coefficient were varied independently.

The simulations show that, for a sufficiently weak friction, the diffusion of roughened particles in concentrated suspensions is smaller than that of smooth particles [1]. This contrasts with predictions based on pairwise collision analysis, which suggest that particle displacement increases with increasing roughness. The surprising result is explained by the observation that a large particle roughness promotes organization of the concentrated particles into layered structures aligned in the flow direction. The particles tend to remain in their layers for long periods of time and jumps between the layers are rare events.

As friction between particles increases, the particle diffusivity in concentrated suspensions increases [2]. In contrast, friction has a negligible impact on displacements during pairwise collisions, indicating that the friction effect on diffusivity in concentrated suspensions is an inherently a many-body effect. A collision between frictional particles leads to particles rolling on each other whereas collision between frictionless particles is dominated by a sliding motion. As a result, rotational velocity of frictional particles during a collision is larger than that of frictionless particles. The rotational velocity of colliding particles perturbs translational velocities of other particles in suspension, through hydrodynamic interactions and contacts, which in turn increases the diffusivity.

The results indicate that roughness and friction can both have a significant impact on diffusivity and microstructure of the suspension, pointing to the need for better characterization of roughness as well as models linking roughness and friction.

[1] H. Zhang, P. Pham, B. Metzger, D.I. Kopelevich, & J.E. Butler. Effect of particle roughness on shear-induced diffusion, Phys. Rev. Fluids. 8, 064303 (2023).

[2] H. Zhang, D. I. Kopelevich, & J. E. Butler, Frictional Effects on Shear-Induced Diffusion in Suspensions of Non-Brownian Particles, J. Fluid Mech., 1001, A42 (2024).