1. Introduction to colloid science and rheology 2. Hydrodynamic effects 3. Brownian hard spheres 4. Stable colloidal suspensions 5. Non-spherical particles 6. Weakly flocculated suspensions 7. Thixotropy 8. Shear thickening 9. Rheometry of suspensions 10. Suspensions in viscoelastic media 11. Advanced topics.

Colloidal Suspension Rheology

https://cyberleninka.org/article/n/397502.pdf

Melt rheology of organoclay and fumed silica nanocomposites

We report a number of experiments, mainly rheological measurements, examining the yielding behavior of colloidal glasses with hard-sphere interaction plus a short-range attraction The system is a suspension of nearly hard-sphere colloidal particles and non-adsorbing linear polymer which induces an adjustable depletion attraction between the particles The pure hard-sphere glass shows a simple yielding process at a strain corresponding to the maximum distortion of the cage of nearest neighbors—the structural arrest length scale However, the attraction-dominated glasses show a two-step yielding process at different ranges of strain We suggest that the first step at low yield strain corresponds to the breaking of attractive bonds between particles The other at larger strain corresponds to the cage breaking process

Yielding behavior of repulsion- and attraction-dominated colloidal glasses

Rheological measurements are used to compare clay nanocomposites prepared through melt mixing using two different polypropylene matrices Steady state and transient nonlinear rheological experiments are employed to separate the contributions of flow induced orientation of the tactoids and particulate network build-up The conditions under which the rheological properties are dominated by the aggregate network are subsequently identified Under these conditions, the low frequency linear viscoelastic behavior is analyzed using scaling concepts for fractal networks to determine the degree of network formation by exfoliation Moreover, the high frequency behavior of the moduli can be used to quickly assess the dispersion quality The results from the analysis of the linear viscoelastic data are compared to structural features extracted from electron microscopy and small angle X-ray scattering data

Quantifying dispersion of layered nanocomposites via melt rheology

A general structural kinetics model is presented to describe the flow behaviour of thixotropic systems based on inelastic suspending media. The total stress is divided in structure-dependent elastic and viscous contributions. The kinetic equation for the structure parameter takes into account the effect of shear on structure breakdown and build-up, as well as the effect of Brownian motion on build-up. The relaxation and deformation of the flocs is also included. Both the kinetic and the relaxation equations contain a distribution of time constants. The predictions of this model, as well as those of two representative models from the literature, are compared with experimental data using an objective method for parameter estimation. Model validation is based on stress transients resulting from sudden changes in shear rate, including both structure build-up and breakdown. The predictions of the viscous and elastic components are evaluated separately using stress jump experiments with steady and non-steady state starting conditions.

A structural kinetics model for thixotropy

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.

Thixotropy : Build-up and breakdown curves during flow

Stress jumps on weakly flocculated dispersions: steady state and transient results.

Many constitutive equations have been proposed for thixotropic materials but the supporting experimental evidence has usually been rather fragmentary. To a large extent this is due to the difficulties involved in measuring thixotropic systems. These systems normally display various phenomena that result in poor accuracy and poor reproducibility of the measurements. Therefore, it has been attempted here to formulate a robust thixotropic system that allows accurate measurements and that would be suitable for detailed studies of thixotropy. The system that has been developed is based on a matrix liquid that consists of a high boiling paraffin oil and a low molecular weight poly(isobutylene). A suitable type of fumed silica is used as the dispersed phase. The various rheological parameters of the material can be altered by varying the concentration of the components and, if necessary, the molecular weight of the polymer. The relative humidity around the sample turns out to be an important factor in controlling the yield stress, and its effect is shown to be reversible. The selected system can be measured accurately over a wide range of measurement conditions. These include stress jump measurements, which can be used to separate viscous and elastic contributions. The highest possible shear rate is limited by the occurrence of a peculiar phenomenon that shows up in the normal stresses.

Konraad Dullaert

Papers

A structural kinetics model for thixotropy

Thixotropy : Build-up and breakdown curves during flow

Stress jumps on weakly flocculated dispersions: steady state and transient results.

A model system for thixotropy studies