Mapping vesicle shapes into the phase diagram: A comparison of experiment and theory

TL;DR

This study combines real-time microscopy and the ADE model to map vesicle shape fluctuations onto a phase diagram, testing theoretical predictions against experimental observations of shape transitions and instabilities.

Contribution

It provides the first direct quantitative comparison between vesicle-shape theory and experiment by mapping thermal shape trajectories onto the phase diagram.

Findings

Trajectories remain within stable regions of the phase diagram.

Observed shape instabilities match theoretical predictions.

Experimental data confirms the ADE model's accuracy in describing vesicle shapes.

Abstract

Phase-contrast microscopy is used to monitor the shapes of micron-scale fluid-phase phospholipid-bilayer vesicles in aqueous solution. At fixed temperature, each vesicle undergoes thermal shape fluctuations. We are able experimentally to characterize the thermal shape ensemble by digitizing the vesicle outline in real time and storing the time-sequence of images. Analysis of this ensemble using the area-difference-elasticity (ADE) model of vesicle shapes allows us to associate (map) each time-sequence to a point in the zero-temperature (shape) phase diagram. Changing the laboratory temperature modifies the control parameters (area, volume, etc.) of each vesicle, so it sweeps out a trajectory across the theoretical phase diagram. It is a nontrivial test of the ADE model to check that these trajectories remain confined to regions of the phase diagram where the corresponding shapes are locally stable. In particular, we study the thermal trajectories of three prolate vesicles which, upon heating, experienced a mechanical instability leading to budding. We verify that the position of the observed instability and the geometry of the budded shape are in reasonable accord with the theoretical predictions. The inability of previous experiments to detect the ``hidden'' control parameters (relaxed area difference and spontaneous curvature) make this the first direct quantitative confrontation between vesicle-shape theory and experiment.