Spinodal Fluctuations of Budding Vesicles

We report the first systematic observations of precursor effects in shape transitions of phospholipid-bilayer vesicles in aqueous solution. Vesicles change abruptly, as temperature $T$ is raised, from a prolate ellipsoidal shape to a “budded” shape consisting of two unequal spheres connected by a narrow neck. On the low- $T$ side of this transition, we see large thermal shape fluctuations (quasicritical fluctuations) and long relaxation times (quasicritical slowing down), which may be interpreted, in the context of a ${\phi }^6$ Landau theory, as the fluctuations of a metastable state near its spinodal instability.