  We present experimental results on the relaxation dynamics of vesicles
subjected to a time-dependent elongation flow. We observed and characterized a
new instability, which results in the formation of higher order modes of the
vesicle shape (wrinkles), after a switch in the direction of the gradient of
the velocity. This surprising generation of membrane wrinkles can be explained
by the appearance of a negative surface tension during the vesicle deflation,
due to compression in a sign-switching transient. Moreover, the formation of
buds in the vesicle membrane has been observed in the vicinity of the dynamical
transition point.
