Mechanical properties of jammed packings of frictionless spheres under applied shear stress

TL;DR

This study explores how shear stress influences the mechanical properties of jammed packings of frictionless spheres, revealing a discontinuous unjamming transition and stress-dependent force network anisotropy.

Contribution

It introduces a thermodynamic-like potential sampling method to analyze jammed states under shear stress, highlighting the discontinuous nature of unjamming and force network changes.

Findings

Shear modulus decreases with shear stress but remains finite at yielding.

Discontinuous pressure jump indicates a first-order unjamming transition.

Force network anisotropy increases with shear stress near point J.

Abstract

By minimizing a thermodynamic-like potential, we unbiasedly sample the potential energy landscape of soft and frictionless spheres under constant shear stress. We obtain zero-temperature jammed states under desired shear stresses and investigate their mechanical properties as a function of the shear stress. As a comparison, we also obtain jammed states from the quasistatic-shear sampling in which the shear stress is not well-controlled. Although the yield stresses determined by both samplings show the same power-law scaling with the compression from point $J$, i.e.~the jamming transition point at zero temperature and shear stress, for finite size systems, the quasistatic-shear sampling leads to a lower yield stress and a higher critical volume fraction of point $J$. The shear modulus of jammed solids decreases when increasing the shear stress. However, the shear modulus does not decay to zero at yielding. This discontinuous change of the shear modulus implies the discontinuous nature of the unjamming transition under nonzero shear stress, which is further verified by the observation of a discontinuous jump of the pressure from jammed solids to shear flows. The pressure jump decreases upon decompression and approaches zero at the critical-like point $J$, in analogy with well-known phase transitions under external field. The analysis of force networks in jammed solids reveals that the force distribution is more sensitive to the increase of the shear stress near point $J$. The force network anisotropy increases with the shear stress. Weak particle contacts near the average force and under large shear stresses exhibit asymmetric angle distribution.