Thixotropy and Rheopexy

According to this diagram, the drying process for moist model clays with a solid content of 55 vol% and a water content of 45 vol% can be divided into three stages  

At the end of the drying process, the clay body attains elastic-brittle fracture characteristics, and has shrunk by about 22vol%. The final dry day body consists of about 70vol% solid day and 30vol% pores. 

The Drying Sensitivity Index (DSI) after Bigot can by correlated with a different DSI expressed by the Ratzenberger Index (Ratzenberger and Vogt, 1993 Aungatichart and Wada, 2009). 

One very important property of dry ceramic bodies is their dry flexural strength that is, the fracture strength of the green body at the end of the shrinkage phase. This is dependent in a complex way on many parameters, as follows  

Ackermann, Ch., Gauglitz, R., and Hennicke, H.W. (1964) Das Bourry-Diagramm und die Trockensdiwindung trocken verpresster Massen. Proceedings IX. International Ceramics Conference, Brussels, pp. 115-123. 

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Chawla, B.S. Thixotropy and rheopexy of propyliodone suspensions in arachis oil effect of median particle size. J. Pharm. Pharmacol. 1968, 20, 168S-175S. 

Microstructural theories of suspensions appear to be particularly well suited to solve problems associated with time-dependent flows, thixotropy and rheopexy (anti-thixotropy) [Russel, 1983 Utracki, 1989, 1995]. 

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

The largest contribution to energy loss (viscosity) comes from the hydrodynamic interactions of particles with different velocities, which increases with 4> and the degree of aggregation. The maximum packing volume fraction, (/> , corresponds to a percolation-like threshold for agglomerate formation that causes the viscosity to increase to infinity (oo). Microstructural theories are well suited to treating the time-dependent flows, thixotropy, and rheopexy [2, 43, 44]. 

For some fluids the relation between the shear rate and the shear stress depends on the duration of the flow, or, more precisely, on the kinematic history of the fluid. We can distinguish two categories thixotropy and rheopexy. 

Once again, please bear in mind that it is not imusual for a material to exhibit both thixotropy and rheopexy depending on the shear rate and concentration of solids. 

Thixotropy and rheopexy Now let us consider what happens when time is considered. Some fluids will display a change in viscosity with time under conditions of constant shear rate. Thixotropic fluids show a decrease in viscosity with time when subjected to constant shearing, as indicated in Fig. 6. Rheopexy is just the opposite (Fig. 6). Rheopectic fluids are very rare and rheopexy would be quite detrimental for A and S Thixotropy is frequently observed in materials such as greases, building adhesives, paints and it is done purposely by the formulator so that the adhesive will not sag when applied gently on a vertical surface but by brushing or rolling it will become more fluid and spread easily. Both thixotropy and rheopexy may occur in combination with any of the previously discussed flow behavior, or only at certain shear rates. The time element is variable under constant shear, some fluids will reach their final viscosity in a few seconds, while others may take up to several days. 

Thixotropy and Other Time Effects. In addition to the nonideal behavior described, many fluids exhibit time-dependent effects. Some fluids increase in viscosity (rheopexy) or decrease in viscosity (thixotropy) with time when sheared at a constant shear rate. These effects can occur in fluids with or without yield values. Rheopexy is a rare phenomenon, but thixotropic fluids are common. Examples of thixotropic materials are starch pastes, gelatin, mayoimaise, drilling muds, and latex paints. The thixotropic effect is shown in Figure 5, where the curves are for a specimen exposed first to increasing and then to decreasing shear rates. Because of the decrease in viscosity with time as weU as shear rate, the up-and-down flow curves do not superimpose. Instead, they form a hysteresis loop, often called a thixotropic loop. Because flow curves for thixotropic or rheopectic Hquids depend on the shear history of the sample, different curves for the same material can be obtained, depending on the experimental procedure. 

Two types of flow are recognized thixotropy, defined as a decrease of apparent viscosity under shear stress, followed by a gradual recovery when the stress is removed, and its opposite, anti-thixotropy, or rheopexy. Both are related to molecular or macroscopic changes in interactions. In thixotropic liquids, the aggregate bonding must be weak enough to be broken by flow-induced hydrodynamic forces. If dispersion is fine, even slight interactions may produce thixotropic effects. When... 

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. 

Figure 2. Hysteresis of shear stress curve 1, thixotropy with slow variation of shear rate curve 2, rheopexy of slowly increasing, held steady at y1, and decreasing y.

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