Jamming and unjamming of concentrated colloidal dispersions in channel flows†

We investigated the pressure driven flow of concentrated colloidal dispersions in a converging channel geometry. Optical microscopy and image analysis were used to track tracer particles mixed into dispersions of sterically stabilized poly(methyl methacrylate) (PMMA) spheres. The dispersions were drawn into a round 0.5 mm capillary at one of two pump speeds (^ applied pressure): v1 1⁄4 0.245 ml min 1 and v2 1⁄4 0.612 ml min . We observed that the dispersions at particle volume fractions f # 0.50 followed Hagen–Poiseuille flow for a simple fluid; i.e. the mean flow rate hVi is approximately proportional to pressure drop (pump speed) and inversely proportional to viscosity h. Above this concentration (f P 0.505), the dispersions exhibit granular-like jamming behaviour with hVi becoming independent of the pressure drop. However, at the highest applied pressure (v2), the dispersions are able to unjam and switch from granular-like behaviour back to a simple hard-sphere liquid like system, due to the formation of rotating vortices in the spatial flow pattern. This mechanism is consistent with computer simulations of granular systems and supports for example proposed explanations of anomalously low friction in earthquake faults. We also link this behaviour with the concept of fragile matter (M. E. Cates, J. P. Wittmer, J. P. Bouchaud and P. Claudin, Phys. Rev. Lett., 1998, 81, 1841–1844).