Physics Colloquium - Thursday, January 14th, 2010, 2:30 P.M.

Mahesh Bandi
Applied Mathematics Lab
Harvard University

When loose granular matter is progressively compressed to higher packing fractions, the material stiffens at a critical packing fraction owing to the development of inter-particle constraints - the system is then said to be jammed. This transition from an athermal, particulate gas to a disordered solid has been related to the behavior of a wide variety of materials including glasses, foams, and colloids in a unifying phase diagram proposed by Liu and Nagel. Most studies (theoretical, numerical, and experimental) concentrate on jamming in the absence of friction since this makes the problem amenable to theoretical analysis. Nonetheless, inter-particle friction plays an important role in fundamental as well as technologically relevant problems, and gives rise to richer complexity in such systems. In this talk I will report a quasi-static experiment performed on a bi-dispersed set of disks where frictional effects are fully manifest. Firstly, I will demonstrate the existence of an intermediate (fragile) state separating the unjammed (unconstrained) state from the jammed (constrained) one. Whereas current jamming theories do not predict this fragile state, I will attempt an explanation via rigidity percolation arguments and propose an experimental test. Secondly, I will demonstrate the existence of hysteresis when the granular system is repeatedly jammed and unjammed. I will present results that trace the source of this hysteresis to inter-particle friction. Time permitting, I will show how this friction-induced hysteresis is related to an open question in geophysical rock measurements and discuss how our experiment can help resolve this question.