Slow Steady-Shear of Plastic Bead Rafts

Experimental measurements of the response of a two dimensional system of plastic beads subjected to steady shear are reported. The beads float at the surface of a fluid substrate and are subjected to a slow, steady-shear in a Couette geometry. The flow consists of irregular intervals of solid-like, jammed behavior, followed by stress relaxations. We report on statistics that characterize the stress fluctuations as a function of several parameters including shear-rate and packing density. Over a range of densities between the onset of flow to the onset of buckling (overpacking) of the system, the probability distribution for stress fluctuations is essentially independent of the packing density, particle dispersity, and interaction potential (varied by changing the substrate). Finally, we compare the observed stress fluctuations with those observed in other complex fluids.