On the behaviour of thixotropic fluids with a distribution of structure

Abstract

This letter discusses the criticism - based on the fact that real fluids are multistructured - that has been made of theoretical studies on thixotropy that employ only one structural parameter. The kinds of experimental observations that could be obtained on multistructured fluids are suggested, but a review of recent results indicates that there is no evidence that multistructured behaviour is significant. There are, of course, many experimental difficulties associated with the study of thixotropy, resulting in much scatter in data. It is concluded that at the present state of knowledge the first approximation in the description of thixotropy in the form of the single-structured theory is adequate, particularly for engineering and technological purposes. In case multistructured behaviour should become apparent, the way in which a second approximation can be set up is summarized, with reference being made to the report in which details are given.

A structural kinetics model for thixotropy

“…The dependency on the initial shear rate implies a "fading memory" effect of the initial microstructure on the resulting build-up and breakdown kinetics. As discussed in the introduction, the structural kinetics models that fit within the general framework postulated by Cheng [6] have a kinetic equation that depends only on the instantaneous values of the structure parameter and the shear rate [7]. Also in the present model the characteristic time (τ λ ), resulting from the kinetic equation for the structure parameter (Eq.…”

Some Aspects over Thixotropic Fluid Behavior

“…One more restriction on is is postulated. In fact, this restriction is associated to the mathematical definition of thixotropy, and in this sense for thixotropic fluids ( [24,25,26]) one has (11) The inequality (5) follows from thermodynamics requirements (Clausius-Duheim inequality). The inequality ( 6) is due to experimental observation that no real substance under deformation without structural change has the following response .…”

“…In this sense, one takes into account a simple shear test with controlled shear strain rate, i.e. (24) Advanced Materials Research Vol. 629 Where is a selected constant positive strain rate, is the instant related to get the constant strain rate by rheometer and 0, f t t   ∈   is the time variable ( is the final time of the analysis).…”

On the need for multi-parameter theories in the analysis of thixotropic behaviour

“…However, there is no conceptual difficulty in extending the framework in order to encompass multistructured models. The corresponding modification has been discussed recently (Cheng 1974).…”

“…The need for such a theory has been discussed repeatedly. Cheng (1974) has reviewed the available evidence in this journal and concluded that there is very little, if any, experimental evidence of deviations from single-parameter model predictions. The discussion above of the multi-structure model points in the same direction.…”