Rheopexy

Colloidal dispersions often display non-Newtonian behaviour, where the proportionality in equation (02.6.2) does not hold. This is particularly important for concentrated dispersions, which tend to be used in practice. Equation (02.6.2) can be used to define an apparent viscosity, happ, at a given shear rate. If q pp decreases witli increasing shear rate, tire dispersion is called shear tliinning (pseudoplastic) if it increases, tliis is known as shear tliickening (dilatant). The latter behaviour is typical of concentrated suspensions. If a finite shear stress has to be applied before tire suspension begins to flow, tliis is known as tire yield stress. The apparent viscosity may also change as a function of time, upon application of a fixed shear rate, related to tire fonnation or breakup of particle networks. Thixotropic dispersions show a decrease in q, pp with time, whereas an increase witli time is called rheopexy. 

Rheology Rheology of foam Rheometer Rheopexy Rheosyst Rheotron Rheovibron Rheumatic fever Rheumatoid arthritis... 

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. 

In general, for shear-thinning pseudoplastic fluids the apparent viscosity will gradually decrease with time if there is a step increase in its rate of shear. This phenomenon is known as thixotropy. Similarly, with a shear-thickening fluid the apparent viscosity increases under these circumstances and the fluid exhibits rheopexy or negative-thixotropy. 

As in PP-based nanocomposite systems, the extended Trouton rule, 3r 0 (y t) = r E (so t), also does not hold for PLANC melts, in contrast to the melt of pure polymers. These results indicate that in the case of P LANC, the flow induced internal structural changes also occur in elongation flow [48], but the changes are quite different in shear flow. The strong rheopexy observed in the shear measurements for the PLA-based nanocomposite at very slow shear rate reflects the fact that the shear-induced structural change involved a process with an extremely long relaxation time. 

Regarding elongation-induced structure development, Figure 9.12(B) shows the Hencky strain rate dependence of the up-rising Hencky strain (r.t) ) — f.0 x ft) recorded for PLANC at 170 °C. The r.t) increases systematically with the eo- The lower the value of s0, the smaller the value ofs,lE. This tendency probably corresponds to the rheopexy of PLANC under slow shear flow. 

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). 

Figure 3.5 Schematic showing the rheopexy and thixotropic time responses for shear viscosity...

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. 

Rheopexy is time-dependent shear-thickening, and is sometimes observed as an acceleration of thixotropic recovery - for example, bentonite clay suspensions often set only slowly on standing but quite rapidly when gently disturbed. 

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. 

Rheopectic (antithixotropic) fluids are shear-thickening fluids whose r a increases with time under constant or low y. Rheopexy is a property of linear... 

---