Rheology pseudo-plastic fluids

If the shear rates are constants, the non-Newtonian fluids can also be classified according to their viscosity dependence on time. This classification has been widely applied to describe the rheological characteristics of coatings. For the development of deformation, the time evolution corresponds to the effect of the increase of shear rate. Three typical cases occur with the time evolution the thixotropic fluids exhibit the decrease of viscosity, corresponding to pseudo-plastic fluids the rheopectic fluids exhibit the increase of viscosity, corresponding to dilatant fluids while the viscoelastic fluids exhibit partial recovery of the deformation of pseudo-plastic fluids after the removal of the stress. Since polymers can perform a large scale of elastic deformation, this character appears extremely significant. 

Flow properties See melt-flow index, viscosity, pseudo-plastic fluid, and rheology. 

There are two general types of constitutive equations for fluids Newtonian and non-Newtonian. For Newtonian fluids, the relation between the stress tensor, t, and the rate of deformation tensor or the shear stress is linear. For non-Newtonian fluids the relation between the stress tensor and the rate of deformation tensor is nonlinear. The various Newtonian and non-Newtonian rheologies of fluids are shown in Figure 12.2. There are four types of behavior (1) Newtonian, (2) pseudo-plastic, (3) Bingham plastic, and (4) dilatent. The reasons for these different rheological behaviors will also be discussed in subsequent sections of this chapter. But first it is necessary to relate the stress tensor to the rate of deformation tensor. 

Cross, M.M. Rheology of non-Newtonian fluids a new flow equation for pseudo-plastic systems. J. Colloid Sci. 1965, 20, 417 37. 

Simple classifications of fluids can be made on the basis of their rheological profiles. Figure 3.78 shows the (a) shear stress and (b) viscosity profiles for various systems. From Figure 3.78 one may define the following systems. Newtonian systems have a constant viscosity with respect to shear rate. Dilatant (or shear-thickening) systems have a viscosity that increases with respect to shear rate. Pseudo-plastic (or shear-thinning) systems have a viscosity that decreases with respect to shear rate. Yield-stress materials are materials that have an initial structure that requires a finite stress before deformation can occur. The stress that initiates deformation is defined as the yield stress. 

The given presentation of the mechanism of the interaction of polymer molecules with turbulent flow admits a peculiar theoretical examination. The presence of polymer addition besides the increase of longitudinal viscosity is resulted in the appearance of such rheological solution properties as elastic plasticity, pseudo-plasticity, anisotropy. In [3] the influence of different rheological fluid characteristics on the wall turbulence is theoretically analyzed within the limits of monoharmonic approximation, which affords to take into account turbulent blows-out. Different variants of rheological behaviour were considered. For all that we succeded to show, that the decrease of turbulent friction arose only in mediums, possessing... 

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