Shear-induced fragmentation of laponite suspensions

Simultaneous rheological and velocity profile measurements are performed in a smooth Couette geometry on laponite suspensions seeded with glass microspheres and undergoing the shear-induced solid-to-fluid (or yielding) transition. Under these slippery boundary conditions, a rich temporal behaviour is uncovered, in which shear localization is observed at short times, which rapidly gives way to a highly heterogeneous flow, characterized by intermittent switching from plug-like flow to linear velocity profiles. Such a temporal behaviour is linked to the fragmentation of the initially solid sample into blocks separated by fluidized regions. These solid pieces get progressively eroded over timescales ranging from a few minutes to several hours depending on the applied shear rate (gamma) over dot. The steady-state is characterized by a homogeneous flow with almost negligible wall slip. The characteristic timescale for erosion is shown to diverge below some critical shear rate (gamma) over dot* and to scale as ((gamma) over dot - (gamma) over dot*)(-n) with n similar or equal to 2 above (gamma) over dot*. A tentative model for erosion is discussed together with open questions raised by the present results.