"Quaking" in flows of tightly packed granular particles (1): Two "types" and shear unjamming

Understanding intermittency, an ubiquitous behavior in flows of packed grains, is pivotal for establishing the rheology of granular matter. A straightforward explanation has been missing despite the long development of theories at various levels of abstraction. Here, we propose the use of a Stribeck-Hertz model that starts with the classic Coulomb friction but also takes into account the tribology between particles, i.e. the reduction of friction coefficient with speed as is commonly observed. Our numerical experiments reveal a state diagram covering a wide range of packing fractions, and produce the quaking intermittency in the mid-range of a dimensionless shear rate defined accordingly, in consistence with our recent laboratory observation [1]. Monitoring the discontinuities in the evolution of mean contact number allows us to distinguish two types of quaking event, which represent different pathways of microstructural change underlying the spontaneous jamming-unjamming transitions along with the intermittency. These studies prompt us to reexamine the prevailing paradigm of shear jamming -- once a more realistic interparticle tribology is considered [2].