Jammed particulate systems are inherently nonharmonic

Abstract

Jammed particulate systems, such as granular media, colloids, and foams, interact via one-sided forces that are nonzero only when particles overlap. We find that systems with one-sided repulsive interactions possess no linear response regime in the large system limit ($N\rightarrow \infty$) for all pressures $p$ (or compressions $\Delta \phi$), and for all $N$ near jamming onset $p\rightarrow 0$. We perform simulations on 2D frictionless bidisperse mechanically stable disk packings over a range of packing fractions $\Delta \phi = \phi-\phi_J$ above jamming onset $\phi_J$. We apply perturbations with amplitude $\delta$ to the packings along each eigen-direction from the dynamical matrix and determine whether the response of the system evolving at constant energy remains in the original eigenmode of the perturbation. For $\delta > \delta_c$, which we calculate analytically, a single contact breaks and fluctuations abruptly spread to all harmonic modes. As $\delta$ increases further all discrete harmonic modes disappear into a continuous frequency band. We find that $<\delta_c >\sim \Delta \phi/N^{\lambda}$, where $1 > \lambda > 0.5$, and thus jammed particulate systems are inherently nonharmonic with no linear vibrational response regime as $N\rightarrow \infty$ over the full range of $\Delta \phi$, and as $\Delta \phi \rightarrow 0$ at any $N$.