  Using x-ray photon correlation spectroscopy, we studied the layer
fluctuations in the lamellar phase of an ionic lyotropic system. We measured
the relaxation rate of in-plane (undulation) fluctuations as a function of the
wave vector. Static and dynamic results obtained during the same experiment
were combined to yield the values of both elastic constants of the lamellar
phase (compression and bending moduli) as well as that of the sliding
viscosity. The results are in very good agreement with dynamic light-scattering
data, validating the use of the technique in ordered phases.
