A model system for thixotropy studies

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.