Transient and steady-state shear banding in aging soft glassy materials

Transient and steady-state shear banding in a Laponite suspension were studied by particle imaging velocimetry in Couette and cone and plate flow geometries on samples with varying age. The time evolution of the velocity profile and shear stress was measured simultaneously as a fixed shear rate was imposed on the sample. By varying the aging time of the rejuvenated samples, we examined the effect of the stress overshoot on the shear banding behavior. Transient banding only occurred for aged samples that showed a stress overshoot. However, steady-state banding occurred when the imposed shear rate was below a critical value, even in the absence of a stress overshoot or transient banding. These results were consistent for independently prepared samples with vastly different overall ages, and in different flow geometries with varying intrinsic stress gradients. Different mechanisms for transient and steady-state shear banding will be discussed. For an aging Laponite soft glass, conditions were determined where transient and steady-state shear banding occur, including a scenario where banding is "trapped" by aging of the low shear band.