Viscosity rheopectic

The coefficient Tj is termed the modulus of rigidity. The viscosities of thixotropic fluids fall with time when subjected to a constant rate of strain, but recover upon standing. This behavior is associated with the reversible breakdown of stmctures within the fluid which are gradually reestabflshed upon cessation of shear. The smooth sprea ding of paint following the intense shear of a bmsh or spray is an example of thixotropic behavior. When viscosity rises with time at constant rate of strain, the fluid is termed rheopectic. This behavior is much less common but is found in some clay suspensions, gypsum suspensions, and certain sols. 

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. 

Newtonian flow, and their viscosity is not constant but changes as a function of shear rate and/or time. The rheological properties of such systems cannot be defined simply in terms of one value. These non-Newtonian phenomena are either time-independent or time-dependent. In the first case, the systems can be classified as pseudoplastic, plastic, or dilatant, in the second case as thixotropic or rheopective. 

If, in contrast, the viscosity increases with time while the material is being sheared and recovers its original viscosity when allowed to rest, the material is called rheopective. In this case the down curve is positioned below the up curve. 

Among other characteristics, non-Newtonian fluids exhibit an apparent viscosity that varies with shear rate. Consequently, the determination of the shear stress-shear rate curve must be an initial consideration. Although the apparent viscosity of a thixotropic or a rheopectic fluid changes with the duration of shearing, meaningful measurements may be made if the change is relatively slow. Viscoelastic fluids also exhibit behaviour that is a function of time but their apparent viscosities can be measured provided conditions of steady shearing are obtained. 

Rheopexy, a reversible time-dependent effect like thixotropy, is a rare phenomenon in pigmented systems. Rheopectic fluids increase in viscosity t with time when sheared at a constant shear rate D or a constant shear stress t until they approach a viscosity maximum (Fig. 53). 

The 3 1 LDAO/SDS mixture becomes viscoelastic and rheo-pectic when a small amount of NaCl Is added. Its viscosity shows a reversible Increase with time of shearing at constant shear rate. The rheopectic behavior Is probably due to long thread-like micelles that are aligned parallel to the flow In weakly bound clusters, as In the case of cetyltrlmethyl ammonium bromide and monosubstituted phenol mixed solutions (21). 

Further non-Newtonian types of behavior appear when time is introduced as a variable. Fluids whose viscosity decreases with time when sheared at a constant rate are called thixotropic, whereas fluids whose viscosity increases with time when sheared at a constant rate are termed rheopectic. Thixotropic behavior is represented schematically in Figure 4.6. Note that the two viscosities (slope of shear stress vs. shear rate) rip and r P2 are different, depending upon the time they were sheared at a given rate. 

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).

Rheopectic fluids have apparent viscosities that increase with time, particularly at high rates of shear as shown on Figure 6.3. Figure 6.2(f) indicates typical hysteresis effects for such materials. Some examples are suspensions of gypsum in water, bentonite sols, vanadium pentoxide sols, and the polyester of Figure 6.3. 

Rheopectic. If certain thixotropic suspensions are rhythmically shaken or tapped, they will set or build up very rapidly, a phenomenon termed rheopexy. Apparent viscosity of a rheopectic substance increases with time (duration of agitation) at any constant shear rate. 

Rheopectic Time-dependent dilatant flow. At constant applied shear rate, viscosity increases. In a flow curve, hysteresis occurs. Clay suspensions. A suspension which sets slowly on standing, but quickly when gently agitated due to time-dependent particle interference under flow. 

Many suspensions, and also some polymer solutions, change in time (Figure C4-15). This is usually because stmctures are broken or formed by shearing. The result can be that the viscosity decreases in time (a thixotropic liquid) or increases (a rheopectic liquid). Yoghurt is a good example of a thixotropic liquid rheopectic fluids are rare. The changes in viscosity are often, but not always, reversible. Note that these time effects are not the same that we saw in viscoelastic liquids. 

Most polymeric substances are lime dependent to some extent and) = r] y, t), where t here refers to the time under shear. If shearing causes a decrease in viscosity the material is said to thixotropic, the opposite behavior characterizes a rheopectic substance. These patterns are sketched in Fig. 11-27. After... 

Figure 13.39 Time dependence of viscosity for different types of fluids A, thixotropic B, Newtonian C, rheopectic.

The viscosity of some fluids (particle solutions or suspensions) measured at a fixed shear rate that places the fluid in the non-Newtonian regime increases with time as schematically shown by curve C of Figure 13.39. This behavior can be explained by assuming that in the Newtonian region the particles pack in an orderly manner, so flow can proceed with minimum interference between particles. However, high shear rates facilitate a more random arrangement for the particles, which leads to interparticle interference and thus to an increase in viscosity. Models that illustrate the thixotropic and rheopectic behavior of structural liquids can be found elsewhere (58,59). 

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). 

Fig. 1.29 Typical behaviour of fluid classes with viscosities depending on time of shear. Left curves thixotropic fluid, right curves rheopectic fluid...

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