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This paper presents a compilation of experimental and numerical results that both show that particles in a suspension under shear flow come into contact through surface roughness. Accounting for those frictional contacts captures well the measured viscosity values as well as the shear-thinning observed in many concentrated non-Brownian suspensions. More specifically, the precise analysis of the asymmetric microstructure of the suspension clearly shows the role played by surface asperities in the interparticle contact. Numerical simulation results are then recalled and show the major impact of solid friction between particles on the viscosity. It is then pointed out that shear reversal experiments are a good way to assess this impact and to deduce useful information on the frictional properties of contacts. Finally, when considering together contacts via asperities and the close link between friction and rheology, a natural explanation for the shear-thinning observed in most suspensions can be proposed. According to it, as the shear stress increases, the friction coefficient decreases which leads to a viscosity decrease. This scenario first implemented in numerical simulations is verified experimentally through coupled AFM and rheometry measurements. The asymmetry of the shear-induced microstructure of non-Brownian and non-colloidal suspensions reflects the presence of solid contacts between particles, enabled by the surface asperities of the particles. The peculiar characteristics of these asperity contacts open the way to explain the shear-thinning observed for many suspensions by a decrease in the friction coefficient with increasing shear stress. [ABSTRACT FROM AUTHOR]

The rheology of suspensions showing discontinuous shear thickening (DST) is well documented in conventional rheometer with rotating tools, but their study in capillary flow is still lacking. We present results obtained in a homemade capillary rheometer working in an imposed pressure regime. We show that the shape of the experimental curve giving the volume flow rate versus the wall stress in a capillary can be qualitatively reproduced from the curve γ ˙ (τ) obtained in rotational geometry at imposed stress but instead of a sharp decrease of the volume flow rate observed at a critical stress, this transposition predicts a progressive decrease in flow rate. The Wyart-Cates theory is used to reproduce the stress-shear rate curve obtained in rotational geometry and then applied to predict the volume flow rate at imposed pressure. The theoretical curve predicts a total stop of the flow at high stress, whereas experimentally it remains constant. We propose a modification of the theory which, by taking into account the relaxation of the frictional contacts in the absence of shear rate, well predicts the high stress behavior. We also hypothesized that the DST transition propagates immediately inside the capillary, once the wall shear stress has reached its critical value: τR = τc, even if the internal shear stress τ(r < R) is below the critical one. In this way the whole experimental curve can be well reproduced by the modified W–C model. [ABSTRACT FROM AUTHOR]

We show that a suspension of non-Brownian calcite particles in glycerol-water mixtures can be tuned continuously from being a yield-stress suspension to a shear-thickening suspension—without a measurable yield stress—by the addition of various surfactants. We interpret our results within a recent theoretical framework that models the rheological effects of stress-dependent constraints on inter-particle motion. Bare calcite particle suspensions are found to have finite yield stresses. In these suspensions, frictional contacts that constrain inter-particle sliding form at an infinitesimal applied stress and remain thereafter, while adhesive bonds that constrain inter-particle rotation are broken as the applied stress increases. Adding surfactants reduces the yield stress of such suspensions. We show that, contrary to the case of surfactant added to colloidal suspensions, this effect in non-Brownian suspensions is attributable to the emergence of a finite onset stress for the formation of frictional contacts. Our data suggest that the magnitude of this onset stress is set by the strength of surfactant adsorption to the particle surfaces, which therefore constitutes a new design principle for using surfactants to tune the rheology of formulations consisting of suspensions of adhesive non-Brownian particles. [ABSTRACT FROM AUTHOR]