THERMAL MECHANICAL FLUCTUATIONS OF FLUID MEMBRANES IN CONFINED GEOMETRIES - THE CASE OF SOFT CONFINEMENT

By definition, membrane or macromolecular assembly is an event of molecular confinement against the configurational entropy of a disordered state. Bilayer membranes under progressive confinement experience a continual damping of undulatory fluctuations, first interpreted as a steric force (Helfrich. Z. Naturforsch. 1978). This paper uses a new, diffusion-equation formalism based on the Feynman-type variational principle to describe how direct interbilayer forces-of hydration, electrostatic double layers, and van der Waals attraction-confine membrane fluctuations. We recover theoretical results to examine measured forces in multilamellar arrays showing that "soft" collisions, through long-range forces, create a mutual enhancement of both the direct forces and the undulatory steric interactions. Thus, there is yet another way to resolve the old, but false, dilemma to choose between steric and direct forces driving membrane assembly. One may develop a systematic connection between bilayer charge, hydration, and flexibility and the action of configurational entropic forces. The results make clear that one should measure forces between membranes or macromolecules in a way that allows them to express their native mechanical freedom.