Rheopexy hysteresis

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. 

Rheopexy refers to dilatant flow which is time dependent. At a constant applied shear rate viscosity increases, as shown in Figure 6.15. In a flow curve, hysteresis occurs (but opposite to the thixotropic case). An example of a rheopectic system is a bentonite clay gel system which sets slowly on standing, but sets quickly when gently agitated. 

Rheopexy Dilatant flow that is time-dependent. At constant applied shear rate, viscosity increases, and a flow-curve hysteresis occurs (opposite of the thixotropic case). 

Time-Dependent Rheology. The rheological properties of suspensions are often time-dependent. If the apparent viscosity continuously decreases with time under shear with a subsequent recovery of the viscosity when the flow is ceased, the system is called thixotropic. The opposite behavior is called antithixopy or rheopexy. Figure 2 shows the time-dependent behaviors of suspensions. Curve 1 in Figure 2 illustrates a hysteresis produced by a thixotropic suspension, where con-... 

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.

Thixotropy is a reversible process and in an immobile state of fluid the continuous, progressive ordering of structure occurs. The one flow curve without hysteresis can be obtained, but the share must continue until the equihbrium is attained. The structure of ordinary tixotropic fluids is totally destroyed at high shear stress. After the shear stress elimination they behave like normal fluids until the stracture is rebuilt. There are also the tixotropic plastic fluids (Fig. 5.5) which do not lose totally the features of plastic fluids which is evident from the stable, however, even low yield stress value. Some suspensions show outstanding features the stracture is formed under the shear stress exclusively and without shear it collapses these fluids show a rheopexy. These properties are appear at moderate shear rate only. 

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. 

Colloidal suspensions and polymer solutions have interesting mechanical properties. In general these materials have both viscosity and elasticity and hence are called viscoelastic. Colloidal suspensions show curious nonlinear hysteresis effects called thixotropy, rheopexy, and dUatancy. These unusual flow behaviours are the central problems of rheology. 

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