  We report on a non trivial dynamics of the interface between shear bands
following a start-up of flow in a semi-dilute wormlike micellar system
investigated using a combination of mechanical and optical measurements. During
the building of the banding structure, we observed the stages of formation,
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 indentified in the time series of the shear stress. The
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
regimes of dynamics are highlighted : a spatially stable oscillating mode
approximately at the middle of the coexistence region flanked by two ranges
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.
