Equilibrium "wetting" of surfaces by membrane-covered vesicles.

"Wetting" of surfaces by thin-membrane capsules is the central process involved in coagulation of suspensions of surfactant membrane vesicles and other-related phases of microemulsions. Although equilibrium "wetting" by membrane capsules appears similar to the macroscopic wetting by liquid droplets, there are important differences: i.e. membrane tension (in place of liquid-interfacial tension) and "effective" contact angle are not fixed constants. Since membranes are condensed-cohesive materials with limited permeability, geometric restrictions (surface area and enclosed volume) are the principal determinants of the extent of contact with a substrate (and "effective contact angle) for "strong" adhesion energies. Deviation from this universal feature arises when membrane-membrane attraction becomes sufficient to compete with membrane-substrate attraction; a transition is predicted from maximal contact with the substrate to partial substrate contact plus membrane lamination by self adhesion. Following the transition, the "effective" contact angle is determined by the ratio of self adhesion: substrate adhesion energies. A critical condition is established for the transition by the area excess (over a sphere of equivalent volume). Because of the exceptional flexibility of fluid membranes, thermal excitations produce long-wavelength collective motions and significant shape fluctuations. Restriction of fluctuations (by area: volume constraints and adhesion) lead to unconventional elastic response and crossover from "strong" to "weak" regimes of equilibrium "wetting" for membrane capsules.