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 wormlik e micelles solution which shows an inhomogenous banded flow pattern above a characteristic rate gamma(t). Depending on the magnitude of the excess rat e above gamma(t), we identify two distinct types of stress response that co rrespond, respectively, to the metastable and to the unstable regimes, for the formation of the the banded flow. This feature further underlines the a nalogy between the spontaneous formation of a banded flow and a first-order phase transition. [S1063-651X(99)11909-3].