A lipid bilayer is a self-assembled double layer of amphiphilic molecules. Its bending modulus controls thermal undulations and the resulting interaction between nearby membranes.
Nearby lipid bilayers experience attractive van der Waals forces, screened electrostatic repulsion, and fluctuation-induced steric repulsion.
The Hamaker constant sets the strength of pairwise-summed van der Waals attraction between macroscopic bodies across a medium.
The Debye–Hückel screening length is the distance over which mobile ions suppress an electrostatic potential in the linearized electrolyte theory.
Helfrich repulsion is the entropic interaction produced when neighboring membranes suppress each other's thermal undulations. Its free energy per area scales as .
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The lipid bilayer is a fundamental structure of cell membranes, composed primarily of phospholipids. In this bilayer, phospholipids arrange themselves in two layers with their hydrophilic (water-attracting) "heads" facing outward towards the water inside and outside of the cell, and their hydrophobic (water-repelling) "tails" facing inward, away from water.