Rheopectic liquids

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, mayonnaise, 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 well 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 liquids depend on the shear history of the sample, different curves for the same material can be obtained, depending on the experimental procedure. 

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. 

Rheopectic liquids. These are rare. Here the apparent viscosity gradually builds up with time as the liquid is sheared. 

Liquids that follow Newton s law are called Newtonian liquids. In non-Newtonian liquids, the quantity rj, which can be calculated from the quotient, aij/D, also changes with the velocity gradient, or with the shear stress. Newtonian behavior is usually observed for the limiting case D - 0 or Gij - 0. Melts and macromolecular solutions often exhibit non-Newtonian behavior. Non-Newtonian liquids are classified as dilatant, Bingham body, pseudoplastic, thixotropic, or rheopectic liquids. 

Rheopectic liquid a liquid that exhibits an increase in viscosity as a function of time. 

F) Rheopectic liquid. The viscosity of these liquids increases with increasing time at a constant flow gradient, and in addition, this effect, which is shown again by certain polymer solutions, can be either reversible or irreversible. 

Thixotropic liquids are unanimously pseudoplastic, i.e. the slope coefficient in the shear stress-shear rate diagram decreases with increasing shear rate. Pseudoplastic fluids are, however, not necessarily thixotropic. Dilatant liquids show generally rheopexy, but there are also exceptions to this rule. Rheopectic liquids are, on the other hand, always dilatant. 

Most shear-thinning fluids like paints and certain polymer solutions are also thixotropic, i.e. the viscosity decreases with time (at constant shear rate). Yoghurt is also an example of a thixotropic liquid. The opposite, i.e. rheopectic liquids (viscosity increases with time). 

Feed rheology (liquid) Newtonjan/pseudoplastic/dilatant/Bingham plastic/thixotropic/rheopectic/viscoelastic... 

Naphthali-Sandholm method, 404 dgorithm flowsketch, 411 Nitric acid reactor, 576 Nitrogen fixation, 574,578,588 Nitrotoluene isomers separation, 544 Noncatalytic reactions with solids, 595 Non-Newtonian liquids, 100, 103-109 Bineham. 104.105.107-109 dilatant, 103, 104 laminar flow, 108,109 pressure drop in lines, 106, 109 pseudoplaslic, 103, 104 rheopectic, 104,105 slurries, 71 thixotropic, 104-106 viscoelastic, 105, 106 Notation, 672 NPSH, pumps, 133,146 centrifugal pumps, 146 positive displacement pumps, 134, 135 various pumps, 144 NRTL equation, 475... 

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). 

Feed rheology /liquid) Newtonian/pseudoplastic/dilatant/BIngham pfasttcAhixotropic/rheopectic/viscoelastic. 

If in non-Newtonian liquids the structure of the liquid is destroyed upon increasing y, hysteresis curves are observed as shown in Fig. 1.29. The behaviour of these liquids depends not only on the time of shear but also on the past shear and thermal history. Pseudoplastic liquids of this kind are named thixotropic, and dilatant liquids are referred to as rheopectic. The longer the duration of shear, the stronger is the destruction of the liquid structure, and the longer it takes to restore it. 

Some systems exhibit flow behavior opposite of thixotropic systems, that is, viscosity increases with increasing shear rate. Such fluids are referred to as dilatant or rheopectic. This type of behavior is not common for liquid products containing a low concentration of the dispersed phase. 

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. 

When the shear stress changes in Newtonian, dilatant, or pseudoplastic liquids, as well as in Bingham bodies or fluids above the flow limit, the corresponding shear gradient or the corresponding viscosity is reached almost instantaneously. In some liquids, however, a noticeable induction time is necessary, i.e., the viscosity also depends on time. If, at a constant shear stress or constant shear gradient, the viscosity falls as the time increases, then the liquid is termed thixotropic. Liquids are termed rheopectic or antithixotropic, on the other hand, when the apparent viscosity increases with time. Thixotropy is interpreted as a time-dependent collapse of ordered structures. A clear molecular picture for rheopexy is not available. 

Thixotropic adj. (1) A liquid or dispersion that exhibits a reduction in viscosity with time at constant shear stress, opposite effect is rheopectic (not to be confused with pseudoplastic, reduction of viscosity with shear stress). (2) A term that describes fiill-bodies material which undergoes a reduction in viscosity when shaken, stirred, or otherwise mechanically disturbed and which readily recovers the original full-bodied condition on standing. 

At high particle concentrations, slurries are often non-Newtonian. For non-Newtonian fluids, the relationship between the shear stress and shear rate, which describes the rheology of the slurry, is not linear and/or a certain minimum stress is required before flow begins. The power-law, Bingham plastic and Herschel-Bulkley models are various models used to describe the flow behaviour of slurries in which these other types of relationships between the shear stress and shear rate exist. Although less common, some slurries also display time-dependent flow behaviour. In these cases, the shear stress can decrease with time when the shear rate is maintained constant (thixotropic fluid) or can increase with time when the shear rate is maintained constant (rheopectic fluid). Milk is an example of a non-settling slurry which behaves as a thixotropic liquid. 

Such solutions are expected to be Newtonian liquids with low viscosities. This has also been found in rheological measurements, if the shear rate or the time, during which the shear was applied, remained below a characteristic value. With a shear rate above this critical value, the solutions show a dilatant and rheopectic flow... 

Rheopexy n (1) The inverse of Thixotropy. The viscosity of a rheopectic material increases with time under an applied constant stress, approaching a constant value. When the stress is removed or reduced, the viscosity diminishes toward its original value. (2) A special form of thixotropic gel which possesses the property of solidifying more rapidly when sheared (stirred) very slowly than when at complete rest. It should not be confused with dilatancy. The equilibrium state of the former is a solid gel, while the latter is a liquid. 

---