Adrien Izzet - (ESPCI) - Rheology, tribology, and acoustic lubrication of dense non-Brownian suspensions

Suspensions are dispersions of solid particles in a fluid. Their rheology implies a linear dependence of the shear stress σ on the shear rate, as a function of the microscopy friction μp between the particles. The coupling between the flow, the normal forces, and μp, induces a strong dependence of the rheology on σ. While the theoretical framework is well-established, measurements of the microscopic properties are missing. In this presentation, I will show how we use Tuning-Fork Microscopy (TFM) to measure μp between solid polystyrene (PS) particles immersed in a solvent, and how our results compare with the numerical simulations found in the literature and moreover, provide novel insights on the origin of the rheological behaviors of suspensions. This study focuses on three typical solvents: I will show that PEG and NaI (aqueous solution) lead to a constant μp (resp. 0 and 0.2) and Silicon (Si) oil leads to a decrease of μp with increasing load σ. The TFM results are consistent with the rheology: for Si oil, μp depends on contact elasticity and thus on the applied load; PS-PEG has a Newtonian behavior but exhibits shear-thinning when the volume fraction φ gets close to Random Loose Packing: the swelling of the beads induces a repulsive force preventing particle contact, except at high φ. PS-NaI exhibits shear-thickening that is more visible at large φ: even at low σ, the suspension flow is in its inertial regime, so the suspension viscosity is dominated by particle contacts and increases with the shear rate. Following these experiments, I will also present recent results from a new experiment in which a PS-Si oil suspension is activated by ultrasounds. While μp can decrease with increasing shear stress, it is also possible to tune it down by propagating ultrasounds into the suspension: in the presence of ultrasound, at constant stress, the microscopic friction coefficient is replaced by a lower effective microscopic coefficient. This leads to a decrease of the viscosity of the system at fixed imposed stress and solid fraction