Shear-reversal in non-Brownian suspensions : experiments and simulations

Description

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. In particular, the influence of the microstructure has been qualitatively evidenced years ago in shear-reversal experiments. 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.In recent shear-reversal experiments, we have shown that the viscosity transient allows to quantitatively identify the contribution of the shear induced microstructure to the suspensions viscosity. As the volume increases, this contribution is increasingly predominant.To get a deeper insight on each step of the transient, we have performed numerical simulations of suspensions submitted to shear-reversal. The simulations account for short range lubrication interactions together with direct contact forces between particles, including surface roughness, contact elasticity and solid friction. The separated contributions of hydrodynamics and contact forces to the stress are identifyed during the transient and the influence of the contact law parameters are evaluated. In particular, we show that the contribution of the shear-induced microstructure to the viscosity that was determined in experiments provides an approximate measurement of the contribution of contact forces to the steady viscosity. In addition, at shear reversal, besides a short time contact force relaxation , the viscosity undergoes a finite jump at a more larger strain scale. This jump depends both on the roughness of the particle surface and on the friction coefficient between particles. A physical picture of this jump can be given from simple low Reynolds number hydrodynamics principles.

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