Couette flow of two-dimensional foams

Abstract

We experimentally investigate flow of quasi two-dimensional disordered foams\nin Couette geometries, both for foams squeezed below a top plate and for freely\nfloating foams. With the top-plate, the flows are strongly localized and rate\ndependent. For the freely floating foams the flow profiles become essentially\nrate-independent, the local and global rheology do not match, and in particular\nthe foam flows in regions where the stress is below the global yield stress. We\nattribute this to nonlocal effects and show that the "fluidity" model recently\nintroduced by Goyon {\\em et al.} ({\\em Nature}, {\\bf 454} (2008)) captures the\nessential features of flow both with and without a top plate.\n