Interface dynamics in shear-banding flow of giant micelles

We report on the non trivial dynamics of the interface between shear-bands following the initiation of flow in a semi-dilute wormlike micellar system investigated using a combination of mechanical and optical measurements under strain-controlled conditions. During build up of the banding structure, we observed the stages of formation and migration of the interface between bands and finally the destabilization of this interface along the vorticity axis. The mechanical signature of these processes has been identified in the time series of the shear stress. Interface instability occurs all along the stress plateau, the asymptotic wavelength of the patterns increasing with the control parameter, typically from a fraction of the gap width to about four times the gap width. Three main dynamics regimes are highlighted: a spatially stable oscillating mode approximately in the middle of the coexistence zone flanked by two regions where the dynamics appears more exotic with propagative and chaotic events respectively at low and high shear rates. The distribution of small particles seeded in the solution strongly suggests that the flow is three-dimensional. Finally, we demonstrate that the shear-banding scenario described in this paper is not specific to our system.