On the equivalence of local and global area-constraint formulations for lipid bilayer vesicles

Abstract

Lipid bilayer membranes are commonly modeled as area-preserving fluid surfaces that resist bending. There appear to be two schools of thought in the literature concerning the actual area constraint. In some works the total or global area of the vesicle is a prescribed constant, while in others the local area ratio is assigned to unity. In this work we demonstrate the equivalence of these ostensibly distinct approaches in the specific case when the equilibrium configuration is a smooth, closed surface of genus zero. We accomplish this in the context of the Euler–Lagrange equilibrium equations, constraint equations and the second variation with admissibility conditions, for a broad class of models—including the phase-field type. In particular, the two formulations have equivalent equilibria characterized by the same local-energy properties.