Fluids rheopectic

The behaviour of a rheopectic fluid is the reverse of that of a thixotropic fluid and is illustrated by the broken lines in Figures 3.33 and 3.34. 

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). 

It is true, therefore, for Newtonian, Bingham-plastic, pseudoplastic and dilatant fluids. The same relationship can possibly be extended to thixotropic and rheopectic fluids by evaluating the shear stress at the wall over a differential length of tube, i.e., by replacing D P/4L with DdP/idL. This term will vary with distance along the pipe, however, and as no evident means of developing this relationship has been... 

Exclusive of thixotropic and rheopectic fluids, and of the few materials (see Section VI) which may slip at the wall. 

All pipe-line work to date has dealt with fluids which are not thixotropic and rheopectic. To an extent this may be justified because the limiting conditions (at startup—for thixotropic materials, and after long times of shear for rheopectic fluids) in pipe flow and some mixing problems are of primary importance. Design for these conditions would be similar to the techniques discussed herein for other fluids. This is not true of problems in heat transfer, however, and inception of work on the laminar flow of thixotropic fluids in round pipes would appear to be in order as a prerequisite to an understanding of such more complex nonisothermal problems. 

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).

Figure 6.3. Time-dependent rheological behavior of a rheopectic fluid, a 2000 molecular weight polyester [after Steg and Katz, J. Appl. Polym. Sci. 9, 3177 (7965)].

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. 

Viscoelastic fluids can be further subcategorized as (1) thixotropic fluids, which show a reversible decrease in shear stress with time at a constant rate of shear, and (2) rheopectic fluids, which show an opposite effect. In normal reactor operation, these time dependencies usually become important for start-up conditions and for significant system perturbations. 

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. 

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). 

Figure 3. Flow curves for thixotropic and rheopectic fluid in a single continuous experiment.

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

Figure 3.79 shows the viscosity profiles for time-dependent fluids. From Figure 3.79 one may define the following systems. Time-independent fluids obviously undergo no change in viscosity with respect to time. Rheopectic fluids show an increase in viscosity with respect to time. Thixotropic fluids show a decrease in viscosity with respect to time. 

Time-dependent effects influence of residence time in the viscometer for thixotropic and rheopectic fluids. 

Time-dependent fluids are those for which the components of the stress tensor are a function of both the magnitude and the duration of the rate of deformation at constant temperature and pressure [4]. These fluids are usually classified into two groups—thixotropic fluids and rheopectic fluids—depending on whether the shear stress decreases or increases with time at a given shear rate. Thixotropic and rheopectic behavior are common to slurries and suspensions of solids or colloidal aggregates in liquids. Figure 10.2 shows the general behavior of these fluids. 

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