Discontinuous change of shear modulus for frictional jammed granular   materials
  The shear modulus of jammed frictional granular materials with the harmonic repulsive interaction under an oscillatory shear is numerically investigated. It is confirmed that the storage modulus, the real part of the shear modulus, for frictional grains with sufficiently small strain amplitude $\gamma_0$ discontinuously emerges at the jamming transition point. The storage modulus for small $\gamma_0$ differs from that of frictionless grains even in the zero friction limit, while they are almost identical with each other for sufficiently large $\gamma_0$, where the transition becomes continuous. The stress-strain curve exhibits a hysteresis loop even for a small strain, which connects a linear region for sufficiently small strain to another linear region for relatively larger strain. We propose a new scaling law to interpolate between the states of small and large $\gamma_0$. 
Michio Otsuki and Hisao Hayakawa
cond-mat.soft