Amy Graves, Swarthmore College

Granular systems are ubiquitous in daily life.  By “granular” we mean not only hard objects like salt, rice, or sand; but also soft objects like bubbles, living cells and flocks of animals or people.  Dense systems can behave dramatically by suddenly solidifying from a granular liquid to a disordered “jammed” solid.  Much has been learned about jamming in last two decades.  Our research with undergraduates uses computer simulation to study what is new and different about jamming when the mobile particles coexist a framework of “pinned” particles.  (In two dimensions, one might think of pillars in a room full of people, or pins in a Pachinko game.)  Pins influence when the liquid-to-solid transition occurs, and affect its structure and mechanical properties.  They control how elastic is the material when subject to compression or shear stress.  This research is both theoretical and practical.  Pillars are known to channel crowds and make it safer to exit a crowded room.  A lattice of pins permits one to use less material to make a roomier jammed solid; one which is less stiff, with a built-in anisotropy based on the symmetry of the lattice.