Role of solvent on the rheological properties of non-Brownian suspensions

The rheology of suspension is characterized by a linear dependence of the shear stress σ on the shear rate (Boyer et al. 2011), as a function of the microscopy friction μ_p. The coupling between the flow, the normal forces, and μ_p, 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 study, we use Tuning-Fork Microscopy (TFM) to measure μ_p between solid polystyrene (PS) particles immersed in a solvent, and we compare our results to numerical simulations (Chèvremont et al. 2019, Lobry et al. 2019). We show that PEG and NaI lead to a constant μ_p (resp. 0 and 0.2) and Silicon (Si) oil leads to a decrease of μ_p with increasing load. Our results are consistent with the rheology: for Si oil, μ_p depends on contact elasticity, thus on the applied load. PS-PEG is Newtonian but exhibits shear-thinning when the volume fraction φ gets close to Random Loose Packing: beads swelling induces a repulsive force preventing particle contact (Chatté et al. 2018), 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 viscosity is dominated by particle contacts and increases with the shear rate.