Rheopexy or negative thixotropy

It is not possible to put forward simple mathematical equations of general validity to describe time-dependent fluid behavioin, and it is usually necessary to make measurements over the range of conditions of interest. The conventional shear stress-shear rate curves are of limited utility imless they relate to the particular history of interest in the application. For example when the material enters a pipe slowly and with a minimum of shearing, as from a storage tank directly into the pipe, the shear stress-shear rate-time curve should be based on tests performed on samples which have been stored imder 

The above brief discussion of time-dependent fluid behaviour provides an introduction to the topic, but Mewis [1979] and Barnes [1997] have given detailed accounts of recent developments in the field. Govier and Aziz [1982], moreover, have focused on the practical aspects of the flow of time-dependent fluids in pipes. 

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In general, for shear-thinning pseudoplastic fluids the apparent viscosity will gradually decrease with time if there is a step increase in its rate of shear. This phenomenon is known as thixotropy. Similarly, with a shear-thickening fluid the apparent viscosity increases under these circumstances and the fluid exhibits rheopexy or negative-thixotropy. 

Time-dependent fluid behaviour may be further sub-divided into two categories thixotropy and rheopexy or negative thixotropy. 

The relatively few fluids for which the apparent viscosity (or the corresponding shear stress) increases with time of shearing are said to display rheopexy or negative thixotropy. Again, hysteresis effects are observed in the flow curve, but in this case it is inverted, as compared with a thixotropic material, as can be seen in Figure 1.11. 

When the rate of structural reformation exceeds the rate of decay under a constant sus tained shear rate, the behavior is classified as rheopexy (or negative thixotropy). 

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