STEADY AND DYNAMIC SHEAR RHEOLOGY OF DISPERSIONS OF LYOTROPIC LIQUID-CRYSTALS

Biphasic dispersions of aqueous solution and lamellar liquid crystal of sodium bis(2-ethyl-hexyl)sulfosuccinate (AOT) were examined at 25-degrees-C from 7 to 20 wt% AOT by cone-and-plate or parallel-plate rheometry. In this concentration range the liquid crystalline phase is continuous. The materials were found to be viscoelastic. Their behavior was linear for strain amplitudes gamma-o less-than-or-equal-to 1% at oscillatory shear frequencies omega = 10(-2) to 10(2) rad/s. Measurement of the dynamic elastic modulus spectrum G'(omega) at gamma-o = 1% provided a good probe of the material structure without perturbing the structure. After oscillatory shear above gamma-o approximately equal-to 2%, or after steady shear, the dispersions become less elastic and more viscous. There is a slow trend (hours and days) for a partial recovery of the initial structure, although no unique reference state is obtainable The results help explain large increases with time of the apparent viscosity (rheopexy) observed with a capillary viscometer. The steady shear viscosities decrease with shear and depend strongly on the shear history.