Grain-scale origins of ultrastability in cyclically driven shear-jammed granular material

Ultrastable states in granular system can be produced by subjecting a shear-jammed sample to small-amplitude cyclic shear. In such states, all the particle positions and contact forces are reproduced after each shear cycle so that a strobed image of the stresses and particle positions appears static, and the behavior of the system is nearly perfectly elastic. In the present work, we use photoelastic imaging methods to examine the evolution of contact forces within a shear cycle after an ultrastable state is formed in a layer of plastic discs. We find that there are two types of contacts: non-persistent contacts that reversibly open and close; and persistent contacts that never open. We show that the non-persistent contacts make a non-negligible contribution to the emergent shear modulus.