MAPPING VESICLE SHAPES INTO THE PHASE-DIAGRAM - A COMPARISON OF EXPERIMENT AND THEORY

Phase-contrast microscopy is used to monitor the shapes of micron-scal e fluid-phase phospholipid-bilayer vesicles in an aqueous solution. At fixed temperature, each vesicle undergoes thermal shape fluctuations. We are able, experimentally, to characterize the thermal shape ensemb le by digitizing the vesicle outline in real time and storing the time sequence of Images. Analysis of this ensemble using the area-differen ce-elasticity (ADE) model of vesicle shapes allows us to associate (ma p) each time sequence to a point in the zero-temperature (shape) phase diagram. Changing the laboratory temperature modifies the control par ameters (area, volume, etc.) of each vesicle, so it sweeps out a traje ctory across the theoretical phase diagram. It is a nontrivial test of the ADE model to check that these trajectories remain confined to reg ions of the phase diagram where the corresponding shapes are locally s table. In particular, we study the thermal trajectories of three prola te vesicles which, upon heating, experienced a mechanical instability leading to budding. We verify that the position of the observed instab ility and the geometry of the budded shape are in reasonable accord wi th the theoretical predictions. The inability of previous experiments to detect the ''hidden'' control parameters (relaxed area difference a nd spontaneous cunvature) make this the first direct quantitative conf rontation between vesicle-shape theory and experiment.