Unified rheology of vibro-fluidized dry granular media: From slow dense flows to fast gas-like regimes

TL;DR

This paper introduces a unified rheological model for vibro-fluidized dry granular media that captures diverse flow behaviors from slow dense to fast gas-like regimes, validated by experiments showing complex phenomena like shear thinning and thickening.

Contribution

It proposes a generalized rheological framework combining frictional and kinetic contributions, extending beyond traditional $(I)$-rheology, to describe vibro-fluidized granular flows.

Findings

The model describes velocity weakening and shear thinning phenomena.

Diffusivity increases exponentially with shear stress, especially near the transition.

Experimental data supports the unified rheological scenario across regimes.

Abstract

Granular media take on great importance in industry and geophysics, posing a severe challenge to materials science. Their response properties elude known soft rheological models, even when the yield-stress discontinuity is blurred by vibro-fluidization. Here we propose a broad rheological scenario where average stress sums up a frictional contribution, generalizing conventional $\mu(I)$-rheology, and a kinetic collisional term dominating at fast fluidization. Our conjecture fairly describes a wide series of experiments in a vibrofluidized vane setup, whose phenomenology includes velocity weakening, shear thinning, a discontinuous thinning transition, and gaseous shear thickening. The employed setup gives access to dynamic fluctuations, which exhibit a broad range of timescales. In the slow dense regime the frequency of cage-opening increases with stress and enhances, with respect to $\mu(I)$-rheology, the decrease of viscosity. Diffusivity is exponential in the shear stress in both thinning and thickening regimes, with a huge growth near the transition.