Role of Bilayer Tilt Difference in Equlibrium Membrane Shapes

Lipid bilayer membranes below their main transition have two tilt order parameters, corresponding to the two monolayers. These two tilts may be strongly coupled to membrane shape but only weakly coupled to each other.We discuss some implications of this observation for rippled and saddle phases, spontaneous breaking bilayer tubules, and bicontinuous phases. Tilt difference introduces a lengthscale into the elastic theory of tilted fluid membranes. It can drive an instability of the flat phase; it also provides a simple spontaneous breaking of inversion symmetry seen in some recent experiments.