Thixotropic loop

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. 

Another method for estimating thixotropy involves the hysteresis of the thixotropic loop. The area of the thixotropic loop is calculated or measured, which works well with printing inks (3). In a variation of this method, the up curve on an undisturbed sample is deterrnined. The sample is then sheared at high shear (>2000 ) for 30—60 s, followed by deterrnination of the down curve (22). The data ate plotted as Casson-Asbeck plots, vs 7 / (14), as... 

Results from measurements of time-dependent effects depend on the sample history and experimental conditions and should be considered approximate. For example, the state of an unsheared or undisturbed sample is a function of its previous shear history and the length of time since it underwent shear. The area of a thixotropic loop depends on the shear range covered, the rate of shear acceleration, and the length of time at the highest shear rate. However, measurements of time-dependent behavior can be usehil in evaluating and comparing a number of industrial products and in solving flow problems. 

Since the rheology of many systems depends largely on the temperature, accurate and reproducible measurements require very careful temperature control. A 1°C temperature drop, for instance, increases the apparent viscosity / of an offset printing ink by approximately 15%. To demonstrate the correlation between thixotropy and temperature, Figs. 56 and 57 show the flow curves at different temperatures for two offset printing inks [134], Both materials clearly lose thixotropy-indicated by the area under the thixotropic loop-as the temperature increases. This effect is much more pronounced in the first case (Fig. 56), while the second ink exhibits a very slow decrease thixotropic behavior (Fig. 57). 

The most surprising result is that such simple non-linear relaxation behaviour can give rise to such complex behaviour of the stress with time. In Figure 6.3(b) there is a peak termed a stress overshoot . This illustrates that materials following very simple rules can show very complex behaviour. The sample modelled here, it could be argued, can show both thixotropic and anti-thixotropic behaviour. One of the most frequently made non-linear viscoelastic measurements is the thixotropic loop. This involves increasing the shear rate linearly with time to a given... 

FIG. 4.28 Thixotropic loop measurement results for six commercial U.S. shampoos products. 

Delgado, J. Castillo, R. (2007). Shear-induced structures formed during thixotropic loops in dilute worm-micelle solutions, /. Coll. Int. Sci. 312 481. 

The ascending and descending flow curves show hysteresis that is usually referred to as thixotropic loop . If the same experiment is now repeated over a longer time (say 120 s for the ascending and 120 s for the descending curves), the hysteresis decreases, i.e. the thixotropic loop becomes smaller. 

By repeating the above experiments within various time periods one obtains a series of thixotropic loops (Figure 7.43). 

FIGURE 8.80 Thixotropic flow behavior of a thick-fUm paste illustrating the thixotropic loop. 

Thixotropy is generally determined using the thixotropic loop test, where the shear rate is continuously ramped from zero to a maximum value, depending on the measuring device and on the rheological properties of the adhesive as well and then ramped back to zero. The competition between... 

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