Equilibrium shape degeneracy in starfish vesicles.

Phospholipid dispersed in aqueous buffer above their critical micellar concentration form bilayers, which can spontaneously adopt closed metastable shapes with sizes ranging from a few hundred nanometers to few tens of micrometers. The equilibrium shapes of these vesicles are well described by the Canham-Evans-Helfrich curvature elastic energy. Their floppiness allows for thermal fluctuations to be easily detected, but no spontaneous shape transformation is usually observed for vesicles of spherical topological genus (i.e., shapes with no holes) because of strict geometrical constraints and/or energy barriers. This report shows that for a particular class of shapes with spherical topological genus (starfish vesicles), dramatic spontaneous shape transformations can occur due to the degeneracy of the shape solutions, as demonstrated by numerical calculations. These predictions are supported by experimental observations of a three-arm starfish vesicle undergoing spontaneous shape transformations similar to those predicted numerically.