Thixotropic hysteresis loops

Thixotropy is the time-dependent analogue of shear-thinning and plastic behaviour, and arises from somewhat similar causes. If a thixotropic system is allowed to stand and is then sheared at a constant rate, the apparent viscosity decreases with time until a balance between structural breakdown and structure re-formation is reached. If the sheared system is then allowed to stand, it eventually regains its original structure. A thixotropic hysteresis loop (Figure... 

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. 

As substituent uniformity is increased, either by choosing appropriate reaction conditions or by reaction to high degrees of substitution, thixotropic behavior decreases. CMCs of DS >1.0 generally exhibit pseudoplastic rather than thixotropic rheology. Pseudoplastic solutions also decrease in viscosity under shear but recover instantaneously after the shear stress is removed. A plot of shear rate versus shear stress does not show a hysteresis loop. 

Hygrornetry see humidity determination Hysteresis loop, thixotropic and rhcopcotic fluids 114 Hvtor pumps 846... 

The area of the hysteresis loop is a measure of the thixotropy of the tested system. These measurements are easy to carry out and give a quick overview of the thixotropic properties of... 

The second category, time-dependent behaviour, is common but difficult to deal with. The best known type is the thixotropic fluid, the characteristic of which is that when sheared at a constant rate (or at a constant shear stress) the apparent viscosity decreases with the duration of shearing. Figure 1.21 shows the type of flow curve that is found. The apparent viscosity continues to fall during shearing so that if measurements are made for a series of increasing shear rates and then the series is reversed, a hysteresis loop is observed. On repeating the measurements, similar behaviour is seen but at lower values of shear stress because the apparent viscosity continues to fall. 

Since fluid shear rates vary enormously across the radius of a capillary tube, this type of instrument is perhaps not well suited to the quantitative study of thixotropy. For this purpose, rotational instruments with a very small clearance between the cup and bob are usually excellent. They enable the determination of hysteresis loops on a shear-stress-shear-rate diagram, the shapes of which may be taken as quantitative measures of the degree of thixotropy (G3). Since the applicability of such loops to equipment design has not yet been shown, and since even their theoretical value is disputed by other rheologists (L4), they are not discussed here. These factors tend to indicate that the experimental study of flow of thixotropic materials in pipes might constitute the most direct approach to this problem, since theoretical work on thixotropy appears to be reasonably far from application. Preliminary estimates of the experimental approach may be taken from the one paper available on flow of thixotropic fluids in pipes (A4). In addition, a recent contribution by Schultz-Grunow (S6) has presented an empirical procedure for correlation of unsteady state flow phenomena in rotational viscometers which can perhaps be extended to this problem in pipe lines. 

Figure 6.2. Relations between shear stress, deformation rate, and viscosity of several classes of fluids, (a) Distribution of velocities of a fluid between two layers of areas A which are moving relatively to each other at a distance x wider influence of a force F. In the simplest case, F/A = fi(du/dx) with ju constant, (b) Linear plot of shear stress against deformation, (c) Logarithmic plot of shear stress against deformation rate, (d) Viscosity as a function of shear stress, (e) Time-dependent viscosity behavior of a rheopectic fluid (thixotropic behavior is shown by the dashed line). (1) Hysteresis loops of time-dependent fluids (arrows show the chronology of imposed shear stress).

Thixotropic Fluids. Thixotropic fluids are characterized by a decrease in their viscosity with time at a constant shear rate and fixed temperature. When shear rate is steadily increased from 0 to a maximum value and then immediately decreased toward 0, a hysteresis loop is formed, as shown in Figure 3. The shape of the hysteresis loop is also a function of the rate by which the shear rate, 7, is changed. Oil-well drilling muds, greases, and food materials are examples of thixotropic fluids. 

Bheopectic Fluids. Rheopectic fluids are characterized by an increase in their viscosity with time at a constant shear rate and fixed temperature. As for a thixotropic fluid, a hysteresis loop is also formed with a rheopectic fluid if it is sheared from a low to a high shear rate and back to a low shear rate. However, a different rate is usually followed upon lowering the shear rate, as is shown in Figure 3. Bentonite clay suspensions and sols are typical examples of rheopectic fluids (3). 

The thixotropic fluids have an apparent viscosity, dependent not only on the shear rate but also on the shear time. Therefore the flow curve of thixotropic fluids, opposite to the all types of aforementioned fluids, which curves for increasing and decreasing shear rate were superimposed (giving one curve), form a hysteresis loop (Fig. 5.5). As results from these curves the apparent viscosity of tixotropic fluids is decreasing with time (Fig. 5. 6), because of the progressive structure destruction. 

Fig. 5. Flow curves (up and down) for a thixotropic material hysteresis loop.

Thixotropic Liquids - These liquids exhibit lower viscosity as shear rate increases. A practical example is house paint which appears thinner when stirred. See also Hysteresis Loop. 

The presence of thixotropy can be detected either by measuring shear stress at a constant shear rate as a function of time or by studying ascending and descending shear stress-shear rate curves under certain programmed conditions as shown in Fig. 8.80. A thixotropic dispersion under such conditions will produce a hysteresis loop in the shear stress versus shear rate plot. If the program conditions are kept constant, the hysteresis can be related to the degree of thixotropy. ... 

In the case of thixotropic fluids, the shear rate is a function of the magnitude and duration of shear as well as a function possibly of the time lapse between consecutive applications of shear stress. The shear-stress pattern with time for such fluids is shown in Fig. 2.6. If the shear stress is measured against shear rate which is steadily increasing from zero to a maximum value and then immediately decreasing steadily to zero, a hysteresis loop is obtained as shown in Fig. 2.7. 

With time-dependent fluids, the relation between shear stress and shear rate depends on the time and flow history of the fluid. They can be classified as either thixotropic or andthixotropic (or rheopecdc). For thixotropic fluids, the shear stress will decrease with time for a fixed value of shear rate. A simple explanation is that as the liquid is sheared, the structure breaks down. If a cyclic experiment is carried out, a hysteresis loop is formed. Examples include greases, printing inks, jelly, paints, and drilling muds. For andthixotropic (rheopecdc) fluids, the shear stress will increase with dme for a fixed value of shear rate. Examples include clay suspensions and gypsum suspensions. 

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... 

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