Dynamics of topological defects in the Lβ′ phase of 1,2-dipalmitoylphosphatidycholine bilayers

We use the optical birefringence of 1,2-dipalmitoylphosphatidycholine bilayers (DPPC) in the gel (Lβ′) phase to study recombination dynamics of topological defects. The birefringence of anisotropic thin films, such as the Lβ′ phase of DPPC bilayers, is related to their molecular polarizability, different on the heads and the acyl chains. When the sample is cooled down into the Lβ′ phase, a period of rapid recombination (taking place over a few seconds) is followed by slow dynamics with metastable states existing in excess of several minutes. After this, either another metastable state or a truly stable state remains where no further change is observed, although a spatially non-uniform distribution of the orientation of the birefringence remains. We compare our results with a model for the free energy and the dynamics of the lipid bilayer in the gel state, finding good qualitative agreement.