  The rich thermotropic behavior of lipid bilayers is addressed using
phenomenological theory informed by many experiments. The most recent
experiment not yet addressed by theory has shown that the tilt modulus in DMPC
lipid bilayers decreases dramatically as the temperature is lowered toward the
main transition temperature TM . It is shown that this behavior can be
understood by introducing a simple free energy functional for tilt that couples
to the area per molecule. This is combined with a chain melting free energy
functional in which the area is the primary order parameter that is the driver
of the main transition. Satisfactory agreement with experiment is achieved with
values of the model parameters determined by experiments, but the transition is
directly into the gel phase. The theory is then extended to include the
enigmatic ripple phase by making contact with the most recent experimentally
determined ripple structure.
