Metastable versus unstable transients at the onset of a shear-induced phase transition.

We measure the transient stress response at the onset of shear of a wormlike micelles solution which shows an inhomogenous banded flow pattern above a characteristic rate (.)gamma(t.). Depending on the magnitude of the excess rate above (.)gamma(t.), we identify two distinct types of stress response that correspond, respectively, to the metastable and to the unstable regimes, for the formation of the banded flow. This feature further underlines the analogy between the spontaneous formation of a banded flow and a first-order phase transition.