Configurations of fluid membranes and vesicles

Vesicles consisting of a bilayer membrane of amphiphilic lipid molecules are remarkably flexible surfaces that show an amazing variety of shapes of different symmetry and topology. Owing to the fluidity of the membrane, shape transitions such as budding can be induced by temperature changes or the action of optical tweezers. Thermally excited shape fluctuations are both strong and slow enough to be visible by video microscopy. Depending on the physical conditions, vesicles adhere to and unbind from each other or a substrate. This article describes the systematic physical theory developed to understand the static and dynamic aspects of membrane and vesicle configurations. The preferred shapes arise from a competition between curvature energy, which derives from the bending elasticity of the membrane, geometrical constraints such as fixed surface area and fixed enclosed volume, and a signature of the bilayer aspect. These shapes of lowest energy are arranged into phase diagrams, which separate regions of different symmetry by continuous or discontinuous transitions. The geometrical constraints affect the fluctuations around these shapes by creating an effective tension. For vesicles of non-spherical topology, the conformal invariance of the curvature energy leads to conformal diffusion, which signifies a one-fold degeneracy of the ground state. Unbinding and adhesion transitions arise from the balance between attractive interactions and entropic repulsion or a cost in bending energy, respectively. Both the dynamics of equilibrium fluctuations and the dynamics of shape transformations are governed not only by viscous damping in the surrounding liquid but also by internal friction if the two monolayers slip over each other. More complex membranes such as that of the red blood cell exhibit a variety of new phenomena because of coupling between internal degrees of freedom and external geometry.