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Title: Note: thixotropic-antithixotropic behavior of surfactant-based lamellar liquid crystals under shear flows
Author: Soltero, J.F.A.
Robles-Vasquez, O.
Puig, J.E.
Manero, O.
Issue Date: 1995
Abstract: Complex materials such as liquid crystals present complicated rheological responses when they are subjected to shear flows [Oswald and Allain (1988); Matsumoto et al. (1989); Paasch et al. (1989)]. In particular, AOT-water lamellar liquid crystals behave as weak gels when they are subjected to oscillatory shear [Robles-Vazquez et al. (1993), (1994)] but when the applied stress surpasses the yield stress, they exhibit viscoelasticity [Soltero (1994)]. Also, for AOT-water biphasic dispersions at concentrations below the phase inversion concentration (≈ 8 wt % AOT) [Alexopoulos et al. (1989)], the formation of shear-induced microstructures at high shear rates causes shear thickening [Valdes et al. (1993)]. Upon decreasing shear rates, the dispersion still preserves much of its structure and consequently its viscosity remains higher than the values measured in the increasing shear rate mode. This behavior is referred as rheopexy or antithixotropy [Mewis (1979)]. Here we examine in detail the rheological response of lamellar liquid crystals under shear flows using a controlled-stress rheometer. We consider two double tail surfactants, one anionic (AOT) and another cationic (DDAB), whose phase diagrams have been studied extensively [Fontell (1973); Franses and Hart (1983); Fontell et al. (1986)]. Particular attention is given to the hysteresis detected when the samples are sheared under increasing-decreasing shear stress modes which result in thixotropic and antithixotropic behavior.
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