MODELING THIXOTROPY USING A NOVEL STRUCTURAL KINETICS APPROACH - BASIS AND APPLICATION TO A SOLUTION OF IOTA-CARRAGEENAN

A new methodology for modelling the thixotropy of inelastic fluids is described and its application demonstrated. In order to validate the m odel, data from two measurement protocols are compared and shown to gi ve coherent results. The structural kinetics approach uses a reduced s tructural variable, lambda, which has values between zero, when the st ructure is totally broken down, and one, when the structure is complet e. The new approach is applied to the thixotropy of a solution of iota carrageenan, a well known food hydrocolloid. Solutions were sheared b efore testing in order to eliminate their viscoelasticity. The measure ment protocols used were: (1) stress jumps from above and below each s hear stress tested, and (2) triangular ramp-up/ramp down stress profil es, often called hysteresis loops. The first series of measurements wa s used to determine the equilibrium structure and kinetics of structur al build-up and breakdown at each shear stress. This data completely d etermines the parameters in the thixotropic model and allows predictio n of the sample's response to any stress/time profile. Without using a ny additional parameters, this model was then used to predict the shap e of the hysteresis loops in the second series of measurements. The ag reement between the predictions and measurements was very good, provid ed that the experimental data was first corrected for the effects of i nertia and the finite gap-width of the rheometer.