Franco Tapia

We present particle-resolved simulations of dense frictional suspensions undergoing the viscous-inertial transition using pressure-imposed rheology. By varying the fluid viscosity, shear rate, and granular pressure, we demonstrate that the transition occurs at a Stokes number of ≈8 (close to the value of 10 found in the experiments of Tapia et al. [Viscous to inertial transition in dense granular suspension, Phys. Rev. Lett. 129, 078001 (2022).PRLTAO0031-900710.1103/PhysRevLett.129.078001]) and also that the shear stress exhibits a slower transition than the particle pressure. Using our highly resolved numerical simulations, we further demonstrate that this transitional behavior is related to the combined effect of tangential contact and lubrication forces, as the frictional particles concurrently shift from rolling to sliding contacts. This shift is controlled not only by the Stokes number, but also by the distance from jamming. Additionally, we examine the role of increasing interparticle friction on the viscous-inertial transition.

Coated granular materials are involved in numerous industrial processes, including powder handling in pharmaceuticals, additive manufacturing, cement production, and food processing, where surface treatments control flowability, prevent agglomeration, and improve product consistency. Despite their widespread use, the influence of coatings on the collective behavior of granular materials remains poorly understood. While dry granular flows are well described by the μ(I) rheology for frictional, noncohesive particles, many real-world systems involve additional interparticle interactions that fall outside this framework. Here, we investigate how polymer coatings on silica grains modify dry granular rheology by introducing non-Coulombic frictional behavior at the particle scale. Pressure-imposed rheological experiments reveal that coatings activate a low-friction regime in which the bulk friction coefficient and packing fraction approach values typical of frictionless grains. The transition from this lubricated state to a conventional frictional regime depends on both normal stress and shear rate, indicating stress- and velocity-dependent contact mechanics. Tribological measurements show that interparticle friction decreases with coating thickness and sliding velocity, but increases with normal load. Building on these findings, we develop a mean-field rheological model that extends the classical μ(I) framework to include coating-dependent, non-Coulombic friction. Discrete Element Method simulations incorporating the measured friction law capture the key qualitative features observed experimentally. These results demonstrate that controlled surface coatings provide a powerful route to engineer granular rheology and enhance flowability across various industrial applications.

The rheology of suspensions of non-Brownian soft spheres is studied across jamming but also across the viscous and inertial regimes using a custom pressure- and volume-imposed rheometer. The study shows that the granular rheology found for suspensions of hard spheres can be extended to a soft granular rheology (SGranR) by renormalizing the critical volume fraction and friction coefficient to pressure-dependent values and using the addition of the viscous and inertial stress scales. This SGranR encompasses rheological behaviors on both sides of the jamming transition, resulting in an approximate collapse of the rheological data into two branches when scaled with the distance to jamming, as observed for soft colloids. This research suggests that suspensions of soft particles across flow regimes can be described by a unified SGranR framework around the jamming transition.

DOI: https://doi.org/10.1103/PhysRevLett.131.109901

Granular suspensions present a transition from a Newtonian rheology in the Stokes limit to a Bagnoldian rheology when inertia is increased. A custom rheometer that can be run in a pressure- or a volume-imposed mode is used to examine this transition in the dense regime close to jamming. By varying systematically the interstitial fluid, shear rate, and packing fraction in volume-imposed measurements, we show that the transition takes place at a Stokes number of 10 independent of the packing fraction. Using pressure-imposed rheometry, we investigate whether the inertial and viscous regimes can be unified as a function of a single dimensionless number based on stress additivity.

Pressure-imposed rheometry is used to examine the influence of surface roughness on the rheology of immersed and dry frictional spheres in the dense regime. The quasistatic value of the effective friction coefficient is not significantly affected by particle roughness while the critical volume fraction at jamming decreases with increasing roughness. These values are found to be similar in immersed and dry conditions. Rescaling the volume fraction by the maximum volume fraction leads to collapses of rheological data on master curves. The asymptotic behaviors are examined close to the jamming transition.

Pressure- and volume-imposed rheology is used to study suspensions of non-colloidal, rigid fibres in the concentrated regime for aspect ratios ranging from 3 to 15. The suspensions exhibit yield stresses. Subtracting these apparent yield stresses reveals a viscous scaling for both the shear and normal stresses. The variation in aspect ratio does not affect the friction coefficient (ratio of shear and normal stresses), but increasing the aspect ratio lowers the maximum volume fraction at which the suspension flows. Constitutive laws are proposed for the viscosities and the friction coefficient close to the jamming transition.