Sheared Amorphous Packings Display Two Separate Particle Transport Mechanisms

Abstract

Shearing granular materials induces nonaffine displacements. Such nonaffine displacements have been studied extensively, and are known to correlate with plasticity and other mechanical features of amorphous packings. A well known example is shear transformation zones as captured by the local deviation from affine deformation, D 2 min , and their relevance to failure and stress fluctuations. We analyze sheared frictional athermal disc packings and show that there exists at least one additional mesoscopic transport mechanism that superimposes itself on top of local diffusive motion. We evidence this second transport mechanism in a homogeneous system via a diffusion tensor analysis and show that the trace of the diffusion tensor equals the classic D 2 min when this second mesoscopic transport is corrected for. The new transport mechanism is consistently observed over a wide range of volume fractions and even for particles with different friction coefficients and is consistently observed also upon shear reversal, hinting at its relevance for memory effects.

Strain dependent vorticity in sheared granular media

“…Interestingly, the φ for which the peak in A is observed is the same for which we observed a substantial decrease in the diffusive particle transport prop- 1 We showed the same data in the supplemental materials to Ref. [11], in which figure legends were wrongly labeled. We have corrected that in this work.…”

“…Apparently the confinement effects above the peak φ suppress both non-affine displacement fields, suggesting a connection between the underlying mechanism that drives both vorticity and diffusion [6]. However, below the peak φ, we do observe thatĀ increases with φ, yet D 1 [11] is constant in this regime, suggesting a disconnect between vortices and non-affine diffusion effects.…”

“…The data originates from numerous published works on a well studied setup that can be described briefly: we apply simple shear to bidisperse granular systems composed of photoelastic discs (Vishay PSM-4, diameters 0.64 cm and 0.80 cm) with inter-particle friction coefficient µ ≈ 0.7. Shear is performed in an apparatus that suppresses shear bands and related shear-induced density fluctuations entirely [11][12][13]. Particles were carried by the separate slats that form the shear box base.…”

“…erties [11]. Apparently the confinement effects above the peak φ suppress both non-affine displacement fields, suggesting a connection between the underlying mechanism that drives both vorticity and diffusion [6].…”

“…Substantial efforts have been dedicated to elucidate the origin and physical meaning of such nonaffine displacement fields. These fields have been studied via metrics, often non-local, like vortices [1][2][3][4][5][6], a two point velocity correlation function [7] and χ 4 , a four point displacement correlation function [8,9], the Okubo-Weiss parameter borrowed from turbulence [10], and a directional particle transport mechanism recently reported by the authors [11]. The benefit of probing the distribution of granular velocity and vorticity, is that they can be observed directly, as the vortex-like structures are embedded in the non-affine displacement fields of the granular materials.…”