Rheopectic behavior

Rheopectic behavior is the opposite of thixotropy. Shear stress increases with time at constant shear rate. Rheopeclic behavior has been obsei ved in bentonite sols, vanadium pentoxide sols, and gypsum suspensions in water (Bauer and Colhns, ibid.) as well as in some... 

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). 

The causes for thixotropic and rheopectic behavior are possibly very similar to those for pseudoplasticity and dilatancy, respectively. The proposed causes of pseudoplasticity, i.e., the alignment of asymmetrical molecules and particles or the breakdown of solvated masses, could not always be expected to be instantaneous with respect to time. Therefore it seems that pseudoplastic behavior may simply be that form of thixotropy which has too small a time element to be measurable on most instruments in current use. Exactly the same argument may be applied... 

In rotational viscometers thixotropy shows up as a progressive decrease in torque as time increases (until the infinite-time properties are reached) at constant rotational speed. Care must be taken, however, not to confuse the initial oscillation of a viscometer cup or bob, which is due to inertial forces, with thixotropy. Conversely, rheopectic behavior shows progressively increasing torques at constant rotational speeds. 

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). 

Several conditioning shampoos exhibit rheopectic behavior at low shear rates and examples for two commercial products are provided in Figure 4.29 at 0.05 and 0.1 sec-1. The shear rates are applied sequentially for a time interval of 120 sec. Within this timeframe an equilibrium steady state shear stress is not reached. For product A, the shear rate is extended to 5 sec-1 with similar results. 

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. 

The opposite situation is encountered when the viseosity increases as a function of time and shear rate. This behavior, albeit less common, is termed antithixotropic or rheopectic behavior. 

At higher concentrations of SDS (2%), when more micelles are formed and bound to the polymer, the tendency for polymer unwrapping under shear is less this is seen in Figure 5 where a lower differential viscosity and only a slight degree of rheopectic behavior is evident. We should mention that rheopectic behavior is very unusual for homogeneous systems. It can only occur when shear sensitive structures are present. 

In practice, dilatant and rheopectic behavior are often undesirable because at high shear rates the suspension becomes too stiff to flow smoothly. Plastic behavior is desirable for many ceramic forming methods because the suspension will flow under high stress but will retain its shape when the stress is removed after forming. Pseudoplastic (shear thinning) behavior is often an acceptable compromise. 

There are several terms that apply to time-dependent behavior. Thixotropic fluids possess a structure that breaks down as a function of time and shear rate. Tlius the viscosity is lowered. A famous example of this reversible phenomenon is the ubiquitous catsup bottle, yielding its contents only after sharp blows. The opposite effect, although rarely observed, is called antithixotropic, or rheopectic behavior, where materials set up as a function of time and shear rate that is, the viscosity increases or the material gels. 

Thixotropic/rheopectic behavior can be also investigated using a time-dependent step oscillatory test with three intervals methods (reference interval, high-shear interval, and regeneration interval). For practical users, the decisive factor to evaluate structural regeneration is the behavior in the time frame which is related to practice. 

Polymer melts and solutions may present thixotropic or rheopectic behavior if they degrade or crosshnk during shear, especially at elevated temperatures. Polymers that do not experience reactions during shear traditionally are considered as time-independent fluids. However, recent research shows the disentanglement of polymer chains during shear may cause time-dependent effects. Compared with most traditional thixotropic and rheopectic fluids, the time-de-pendent effects caused by polymer chain disentanglement are relatively small. Hence, in this book, all polymer melts and solutions are treated as time-independent shear-thinning fluids. 

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