Characterizing nonaffinity upon decompression of soft-sphere packings

Abstract: Athermal elastic moduli of soft sphere packings are known to exhibit universal scaling properties near the unjamming point, most notably the vanishing of the shear-to-bulk moduli ratio $G/B$ upon decompression. Interestingly, the smallness of $G/B$ stems from the large nonaffinity of deformation-induced displacements under shear strains, compared to insignificant nonaffinity of displacements under compressive strains. In this work we show using numerical simulations that the relative weights of the affine and nonaffine contributions to the bulk modulus, and their dependence on the proximity to the unjamming point, can qualitatively differ between different models that feature the same generic unjamming phenomenology. In canonical models of unjamming we observe that the ratio of the nonaffine to total bulk moduli $B_{na}/B$ approaches a constant upon decompression, while in other, less well-studied models, it vanishes. We show that the vanishing of $B_{na}/B$ in non-canonical models stems from the emergence of an invariance of net (zero) forces on the constituent particles to compressive strains at the onset of unjamming. We provide a theoretical scaling analysis that fully explains our numerical observations, and allows to predict the scaling behavior of $B_{na}/B$ upon unjamming, given the functional form of the pairwise interaction potential.