UNDERSTANDING THIXOTROPIC AND ANTITHIXOTROPIC BEHAVIOR OF VISCOELASTIC MICELLAR SOLUTIONS AND LIQUID-CRYSTALLINE DISPERSIONS - I - THE MODEL

A simple model consisting of the Upper Convected Maxwell constitutive equation and a kinetic equation for destruction and construction of st ructure, first proposed by Fredrickson in 1970, is used here to reprod uce the complex rheological behavior of viscoelastic systems that also exhibit thixotropy and rheopexy under shear flow. The model requires five parameters that have physical significance and that can be estima ted from rheological measurements. Several steady and unsteady flow si tuations were analyzed with the model. The model predicts creep behavi or, stress relaxation and the presence of thixotropic loops when the s ample is subjected to transient stress cycles. Such behavior has been observed with surfactant-based solutions and dispersions. The role of the characteristic time far structure built up, lambda, in the extent and shape of the thixotropic loops is demonstrated. (C) 1999 Elsevier Science B.V. All rights reserved.