Quasi-Elastic Light Scattering Study of Highly Swollen Lamellar and “Sponge" Phases

We investigate experimentally, using quasi–elastic light scattering the dynamical properties of highly swollen lamellar and “sponge" phases. In our system the structural sizes – smectic period for lamellar phases or characteristic cell size for “sponge" phases – may be changed continuously by adding solvent up to scales of the order of optical wavelengths, i.e. several thousand Å. We observe the usual hydrodynamic behaviour, with monoexponential relaxation in time and q2 scaling in reciprocal space when working with moderately dilute systems. At higher dilutions, the behaviour is no longer hydrodynamic: a q3 universal scaling is observed for both lamellar and “sponge" samples, with stretched–exponential relaxation in time and universal exponents for unoriented lamellar or “sponge" samples. The undulation mode is identified in our data on oriented lamellar phases, both in its hydrodynamic limit and beyond, which leads to a measurement of the bilayer bending modulus κ. A tentative interpretation is given for our data on “sponge" phases, also leading to an estimate for κ.