Thixotropy: Build-up and breakdown curves during flow

Abstract

Thixotropic systems are usually characterized by measuring the stress transients resulting from sudden changes in shear rate. By using a procedure that takes into account the transfer function of the transducer on a strain-controlled rheometer, the initial part of the transients could be accessed. In this manner complete transient stress curves and accurate constant-structure curves could be recorded on a model system consisting of fumed silica particles dispersed in a Newtonian medium. In agreement with previously obtained stress jump data [Dullaert and Mewis (2005b)], an initial viscoelastic response is detected that precedes the structural time effects in either build-up or breakdown experiments. The effect of flow history on the characteristic times and on the shape of the transient curves is investigated. It is shown that common assumptions in thixotropic models, such as a structure that can be characterized by a single parameter or the existence of exponential stress transients, do not generally hold. Although the material response is quite complex, the time constants of the stress transients obey a simple scaling law. The present data set is very suitable for model evaluation. Here, two representative models, those of Houska and of Coussot, have been tested. The initial parts of the curves are not well described. The time constants for the thixotropic structure build-up are reasonably well predicted, although shear history effects are not properly modeled. For the breakdown curves larger errors are recorded.