Direct observation of stringlike collective motion in a two-dimensional driven granular fluid

Dense ensembles of monodisperse hard spheres spontaneously assemble into regions of local crystalline order (grains) that are separated by relatively disordered grain-boundary (GB) regions. It is well known that grains and grain boundaries (GB) profoundly affect the material properties of polycrystalline materials and plastic deformation and transport in this ubiquitous class of materials can be particularly influenced by GB structure and dynamics. We experimentally investigate the nature of particle motion in the GB regions of a quasi-two-dimensional driven system of hard spheres and find, in accord with recent molecular-dynamics simulations of metallic polycrystalline materials, that particle motion in the GB regions involves a highly cooperative stringlike motion as in glass-forming liquids. Moreover, we show that the addition of small particles to a granular fluid allows the scale of this collective motion to be modulated, a phenomenon that is also observed in simulations and measurements on glass-forming fluids.