Controlled Viscosity in Dense Granular Materials

Abstract

We experimentally investigate the fluidization of a granular material subject to mechanical vibrations by monitoring the angular velocity of a vane suspended in the medium and driven by an external motor. On increasing the frequency, we observe a reentrant transition, as a jammed system first enters a fluidized state, where the vane rotates with high constant velocity, and then returns to a frictional state, where the vane velocity is much lower. While the fluidization frequency is material independent, the viscosity recovery frequency shows a clear dependence on the material that we rationalize by relating this frequency to the balance between dissipative and inertial forces in the system. Molecular dynamics simulations well reproduce the experimental data, confirming the suggested theoretical picture.

Induced and endogenous acoustic oscillations in granular faults

“…Finally, let us mention that recent studies [30,52] have highlighted the role played by the velocity of the imposed mechanical vibrations on the frictional properties of sheared granular media. The friction reduction phenomenon occurring in a range of vibration frequencies in vertically shaken granular systems has been recently observed in experiments [36]. The setup is shown in Fig.…”

“…The friction reduction phenomenon occurring in a range of vibration frequencies in vertically shaken granular systems has been recently observed in experiments [36]. The set-up is shown in figure 4: The granular particles (steel, glass or delrin spheres) are confined by an aluminium plate, with packing fraction ∼49 ÷ 52%.…”

“…describing the fault as a block subject to a normal and a shear stress, resting on a oscillating plate. This simplified description permits analytical results which can be used to explain the non-monotonic behaviour of the friction as function of the vibration frequency [18,19,24,36].…”

Vibration controlled foam yielding

“…This contrasts the case of flowing granular media, where vibrations induce the motion of particles, which leads to a decrease in the local particle fraction, unjamming the system locally. 33 Here however, the global volume fraction will not change due to volume conservation. Due to the lack of direct evidence, we are inclined to think that since due to geometrical restrictions, other degrees of freedom are suppressed in our system, the effect observed might be due to slight variations in the local fluid concentrations, which lower the yield stress locally.…”

“…Here, we use mechanical vibrations to manipulate the foam behavior, following the footsteps of granular flow [28][29][30][31][32][33][34] and granular temperature. [35][36][37][38] While in granular media the external vibration affects the particles, not the interstitial medium, in foams the bubbles are virtually inertialess, and presumably the vibration affects the interstitial medium between the lightweight bubbles, instead.…”