Pseudoplastic, viscosity characteristics

Table III shows the viscosity characteristics of these systems as obtained from the Epprecht rotational viscometer. With this instrument, shear rate can be obtained in units of reciprocal seconds (sec-1). These data are plotted in Figure 9. Under these shearing rates (19.2 to 125 sec"1), all three formulations are pseudoplastic.

A typical viscosity characteristic of many non-Newtonian fluids (e.g., polymeric fluids, flocculated suspensions, colloids, foams, gels, etc.) is illustrated by the curves labeled structural viscosity in Figures 5.2 and 5.3. These flnids exhibit Newtonian behavior at very low and very high shear rates, with shear thinning or pseudoplastic behavior at intermediate shear rates. This can often be attributed to a reversible structure or network that forms in the rest or eqnilibrinm state. When the material is sheared, the structure breaks down, resnlting in a shear-dependent (shear thinning) behavior. This type of behavior is exhibited by flnids as diverse as polymer solutions, blood, latex emulsions, paint, mud (sediment), etc. An example of a useful model that represents this type of behavior is the Carreau model ... 

Although pectins are not employed as thickening agents, pectin solutions exhibit the non-Newtonian, pseudoplastic behavior characteristic of most polysaccharides. As with solubility, the viscosity of a pectin solution is related to the molecular weight, DE, and concentration of the preparation and the pH and presence... 

Dilatant Basically a material with the ability to increase the volume when its shape is changed. A rheological flow characteristic evidenced by an increase in viscosity with increasing rate of shear. The dilatant fluid, or inverted pseudoplastic, is one whose apparent viscosity increases simultaneously with increasing rate of shear for example, the act of stirring creates instantly an increase in resistance to stirring. 

Different types of liquid crystals exhibit different rheological properties [16,17]. With an increase in organization of the microstructure of the liquid crystal its consistency increases and the flow behavior becomes more viscous. The coefficient of dynamic viscosity r, although a criterion for the viscosity of ideal viscous flow behavior (Newtonian systems), is high for cubic and hexagonal liquid crystals but fairly low for lamellar ones. However, the flow characteristics are not Newtonian but plastic or pseudoplastic, respectively. 

An understanding of the rheological behaviour is necessary as PVC pastes are classified as non-Newtonian liquids and can be dilatent (shear thickening), pseudoplastic (shear thinning) or thixotropic (viscosity reduces with time under constant shear). Each process requires specific rheological characteristics and this is achieved by formulation of appropriate PVC grades and knowledge of the influence of shear rate and time under constant shear. 

For practical purposes, casting slips are often characterized by so-called apparent viscosity. This characteristic is defined for any point on the rheological curve (Fig. 155) as a ratio of shear stress to the deformation rate at the given point. The inadequacy of this characteristic is demonstrated by Fig. 155 a dilatant and a pseudoplastic mix have the same apparent viscosity at a stress corresponding to the intersection of their rheological curves, despite their quite diverse rheological behaviours. However, determination of apparent viscosity may be useful, for example in routine qualitative inspection. ... 

Most emulsions, unless very dilute, display hoth plastic and pseudoplastic flow behaviour rather than simple Newtonian flow. The flow properties of fluid emulsions should have little influence on their biological behaviour, although the rheological characteristics of semisolid emulsions may affect their performance. The pourability, spreadability and syringeability of an emulsion will, however, be directly determined by its rheological properties. The high viscosity of w/o emulsions leads to problems with intramuscular administration of injectable formulations. Conversion to a multiple emulsion (w/o/w), in which the external oil phase is replaced by an aqueous phase, leads to a dramatic decrease in viscosity and consequent improved ease of injection. 

Rheologically, these formulations are pseudoplastic with a significant yield value. The pseudoplastic characteristic is responsible for the performance of coating and wetting the form in the dipping tank (relatively low viscosity) and for providing a uniform coating of required thickness as form is withdrawn. The yield... 

To further investigate the characteristics of the slags, plots of the logarithm of viscosity versus temperature were made. These indicated straight lines or two line segments. For slags with a sudden increase in viscosity at lower temperatures, two segments were observed. Shear rates were not varied and the various types of non-Newtonian behavior were not explored. A related study indicated pseudoplastic behavior for this type of material (9 ). 

The flow curve. Fig. 2, is a plot of Equation 3 showing experimental results obtained at the 1186°C isotherm. Fig. 3 is a log-log plot of apparent viscosity as a function of shear rate at the same temperature. The apparent viscosity decreases with increasing shear rate, which is characteristic for pseudoplastic systems (n ). The logarithm of the viscosity at unit shear rate, log Pq, is calculated from Equation 3 ... 

As discussed earlier, LADDs are complex, multicomponent mixtures consisting of both organic and inorganic compounds dispersed in a liquid matrix. Such compositions can exhibit a broad range of rheological characteristics from simple Newtonian to complex pseudoplastic flow. Shown in Figure 9.6 and Figure 9.7 are flow and viscosity profiles of Newtonian and non-Newtonian fluids as a function of applied shear rate. A number of mathematical models have been proposed [76] to describe the flow characteristics of various systems. These equations are called constitutive equations and are used to predict flow behavior in complex systems. 

The concentration dependence of the Brookfield viscosity (Figures 9 and 10) indicates a rapid drop in viscosity upon dilution below a critical concentration. Bagley (7) attributed such rheological behavior to a structure of swollen, deformable gel particles closely packed in intimate contact. Davidson (8) later attributed the thickening efficiency of a cross-linked poly(acrylic acid) to the dispersed rather than the continuous phase. In general, pseudoplastic and viscoplastic rheology is characteristic of dispersions with low... 

The hydrolysis of starches by amylases Is most conveniently considered In terms of sugar production (for saccharification) or viscosity reduction (for liquefaction). Starch pastes are characteristically pseudoplastic (shear thinning), and thus satisfy, over an Intermediate range of shear rates, a power law relationship between shear stress, T, and shear rate, y, of the form (2.4) ... 

As there seems to be no single influence that can be associated with non-Newtonian properties it is not realistic to look for explanations which cover all the effects observed. However, it may be useful to consider behaviour that is related to the individual characteristics of pseudoplasticity and viscoelasticity. Pseudoplastic fluids have a lower viscosity in regions of high shear, as near an impeller, and a higher viscosity in regions of low shear, as distant from an impeller. It is possible that this characteristic leads to mixing behaviour different from that observed for Newtonian fluids, in particular for fluid motion to be more than usually confined to the impeller region. 

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