Shear-reversal in suspensions of rough frictional particles: a numerical study
Résumé
Suspensions of non-Brownian particles display a complex rheology that originates in a subtle balance between hydrodynamic interactions and direct forces between particles, together with the so-called shear-induced microstructure. The influence of the microstructure has been evidenced years ago in shear-reversal experiments, and a more recent work identifyed quantitatively its contribution to the suspensions viscosity (F. Blanc et al. Local transient rheological behavior of concentrated suspensions. Journal of Rheology, 55,835.(2011)). At the particle scale, experimental studies pointed the role played by the surface roughness of the particles in promoting direct contact between particles, and recent numerical computations suggest that contact friction between particles could be part of the explanation for the large viscosity of non-Brownian suspensions (S. Gallier et al. Rheology of sheared suspensions of rough frictional particles, Journal of Fluid Mechanics 757 (2014), 514–549). The present numerical study aims to estimate the influence of the contact law between particles on the suspension response to shear-reversal. The simulations are based on the Force Coupling Method, adapted to account for short range lubrication interactions together with direct contact forces between particles, including surface roughnes, contact elasticity and solid friction. The separated contributions of hydrodynamics and contact forces to the viscosity and normal stress differences are identifyed during the transient and the influence of the contact law parameters are evaluated. In particular, at shear reversal, a short time elastic relaxation is evidenced. At a more larger strain scale, the viscosity undergoes a finite jump. In the limit of small roughness height, this jump is directly related to the friction forces. Finally, such a transient provides an approximate measurement of the contribution of contact forces to the steady stress in the suspension, which is of interest from an experimental point of view.