RHEOLOGICAL RESPONSE OF SURFACTANT CUBIC PHASES

We model the response of the surfactant cubic phase to an oscillatory shear strain. The theology of this ordered structure is determined in terms of the dynamics of the surfactant layer. Above a critical stress, ''slip planes'' form giving rise to a bulk relaxation mechanism, which maintains the topology of the network while being able to capture the liquid nature of the material. The system behaves as a Maxwell liquid at low frequencies and displays Voigt cell characteristics at high frequency. We calculate the storage and loss moduli by assuming a pseudolinear response and by treating the periodic lattice potential perturbatively. The key results which emerge are the following; there exists a single relaxation time for the bulk, even when there is a distribution of slip plane densities and this relaxation time depends on the value of the average slip plane density. We also find that the periodicity of the lattice introduces other nonlinear effects: oscillations appear in the complex modulus components which depend on both the frequency and amplitude of the applied shear rate.