Pseudoplastic rheopectic

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. 

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... 

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... 

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

Feed rheology lliquid) Newtonian/pseudoplastic/dilatant/Bingham piastic/thixotropic/rheopectic/viscoelastic. 

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. 

Non-Newtonian Flow Fluid flow that does not obey Newton s law of viscosity. Non-Newtonian fluids may exhibit non-Newtonian flow only in certain shear-rate or shear-stress regimes. A number of categories of non-Newtonian flow are distinguished, including dilatant, pseudoplastic, thixotropic, rheopectic, and rheomalaxic. See also Newtonian Fluid. 

Most fluids exhibit non-Newtonian behavior—blood, household products like toothpaste, mayonnaise, ketchup, paint, and molten polymers. As shown in Figure 7.9, shear stress, t, increases linearly with strain rate, y, for Newtonian fluids. Non-Newtonian fluids may be classified into those that are time dependent or time independent and include viscoelastic fluids. Shear thinning (pseudoplastic) and shear thickening (dilatant) fluids are time independent while rheopectic and thixotropic are time dependent. The shear stress (viscosity) of shear thinning fluids decreases with increasing shear rate and examples include blood and syrup. The viscosity of dilatant fluids increases with shear rate. The viscosity of rheopectic fluids—whipping cream, egg whites—increases with time while thixotropic fluids— paints (other than latex) and drilling muds— decrease their viscosity with the duration of the shear. 

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. 

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. 

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. 

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. 

When that stress is exceeded, the shear rate grows. Further stress leads finally to linear (Newtonian) behaviour. Examples of plastic systems are chocolate, butter, cheese, various spreads and ice cream. In pseudoplastic systems the observed viscosity decreases with an increase in shear stress. An example of a pseudoplastic system is pudding. Dilatant systems resist deformation more than in proportion to the apphed force. The shear rate is growing much faster than that of Newtonian fluids and viscosity increases with an increase in shear stress. At low apphed forces, the system behaves as a Newtonian fluid. Examples of dilatants systems are honey with added dextran and a slurry of wet beach sand. Thixotropic systems become more fluid (they have lower viscosity) with increasing time of an apphed force. If the apphed force ceases to operate, the original viscosity of the system is restored due to a reversible transformation of the sol gel type. Examples of thixotropic systems are mayonnaise, ketchup, whipped and hardened fats, butter and processed cheeses. Rheopectic systems exhibit behaviour opposite to that of thixotropic systems. Their viscosity increases with increasing time of apphed force. An example is whipped egg white. 

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. 

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