Viscosity rheogram

Non-Newtonian fluids include those for which a finite stress 1,. is reqjiired before continuous deformation occurs these are c ailed yield-stress materials. The Bingbam plastic fluid is the simplest yield-stress material its rheogram has a constant slope [L, called the infinite shear viscosity. 

A plot of Tvs. G yields a rheogram or a flow curve. Flow curves are usually plotted on a log-log scale to include the many decades of shear rate and the measured shear stress or viscosity. The higher the viscosity of a liquid, the greater the shearing stress required to produce a certain rate of shear. Dividing the shear stress by the shear rate at each point results in a viscosity curve (or a viscosity profile), which describes the relationship between the viscosity and shear rate. The... 

Figure 3.2 Rheogram for water at 25 °C. The slope gives a value for the viscosity of 8.85 x 0 4 Pas...

Figure 3 (A) Cooling and (B) heating thermal scanning rheograms for a vaginal cream formulation. The dashed vertical line indicates the 38° C point in both plots. This is clearly the initiation temperature of the phase transition leading to increased apparent viscosity during both heating and cooling.

The apparent viscosity is determined from Eq. 4. The value of shear rate that corresponds to this viscosity is obtained from the known viscosity vs shear rate rheogram for the non-Newtonian fluids generated using the cone-and-plate method. The value of k is determined from Eq. 5. 

The relationships between stress, strain, and viscosity are usually depicted in the so-called rheograms. In the pharmaceutical sciences, typical flow curves are presented, i.e., x = /(y). In the engineering sciences, the viscosity is usually drawn as a function of the shear stress [vj = f(x)]. This is sensible as most viscometers control the shear stress applied rather than the shear strain rate. However, the entity of interest is the viscosity as a function of the shear strain rate [i] = /(y)]. 

Flow rheometry, in which a nonoscillatory shearing stress is applied to the sample and the resultant rate of deformation (shear) measured. By plotting the shear stress against the rate of shear, characteristic rheograms are obtained from which information concerning the resistance to flow (viscosity) may be obtained. 

Figure 7. Rheogram of pectate solutions containing glu-cono-lactone and CaEDTA at 25°C, Left, a-c Lissalous figures obtained at different times during the course of the hydrolysis reaction. Right time course of the complex viscosity components, and of the viscous. i, l, and elastic, n",l, components of...

Elastic and viscons modnlus and complex viscosity are the magnitndes determined commonly by temperature scans in oscillatory tests. In general terms, when a PVC-plastidzer mixtnre is heated, these magnitudes undergo at least 3 steps in the rheograms , as shown in Figure 9.1 in a simplified way. 

Figure 3.379. Rheogram for poly(ethylene oxide) at 25.0 0 1°C showing linear relationship over usual range of wall shear-rates, and increase in viscosity with increasing concentration there is thus no evidence of non-Newtonian behaviour, [1080],...

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