VISCOELASTICITY OF LYOTROPIC SMECTICS

We study theoretically the contribution of thermal fluctuations to the linear theological spectrum of lyotropic smectic A phases at low freq uencies. The fluctuations are treated at the Gaussian level; their hyd rodynamics is controlled by the undulation/baroclinic mode. In the lay er-over-layer sliding geometry, we find an anomalous elasticity varyin g as /omega/; the corresponding viscous drag is enhanced logarithmical ly. For other shear geometries we find contributions to the dissipatio n and elasticity that are in practice small (though formally divergent when the layer spacing is taken to zero). The connection between thes e results and those given previously for thermotropic smectics, and fo r block copolymers, is discussed. We outline the possible experimental significance of our findings for the rheology of both aligned and pol ydomain smectics.