Viscosity dependence rheological measurement

To demonstrate the associativity of the network chains, viscosity and rheological measurements were performed on HM PAAms dissolved in 0.7 % SDS solutions. These measurements also show a strong enhancement of the associativity with increasing Nh (i.e., with increasing length of the hydrophobic blocks) [34]. Figure 6a shows the dependence of the viscosity of 0.5 % (w/v) solutions of HM and... 

Viscous Hquids are classified based on their rheological behavior characterized by the relationship of shear stress with shear rate. Eor Newtonian Hquids, the viscosity represented by the ratio of shear stress to shear rate is independent of shear rate, whereas non-Newtonian Hquid viscosity changes with shear rate. Non-Newtonian Hquids are further divided into three categories time-independent, time-dependent, and viscoelastic. A detailed discussion of these rheologically complex Hquids is given elsewhere (see Rheological measurements). 

Rheological measurements are of central importance in the processing of siloxane polymers. Typical studies would include determination of the dependence of the bulk viscosity of the material on the average molecular weight, molecular weight distribution, and rate of shear. Characterization of the effects of any branched chains or reinforcing fillers present is also of great importance.16... 

Edwards and Wasan [17] and Prud homme and Khan [18] have indicated that many of the reported rheological measurement do not truly or uniquely represent the rheological character of foams. For example, the effective viscosity is proved to depend on the geometry parameters of the devices, often the slip of a foam along the wall is not considered (or is... 

This section draws heavily from two good books Colloidal Dispersions by Russel, Seville, and Schowalter [31] and Colloidal Hydrodynamics by Van de Ven [32] and a review paper by Jeffiey and Acrivos [33]. Concentrated suspensions exhibit rheological behavior which are time dependent. Time dependent rheological behavior is called thixotropy. This is because a particular shear rate creates a dynamic structure that is different than the structure of a suspension at rest. If a particular shear rate is imposed for a long period of time, a steady state stress can be measured, as shown in Figure 12.10 [34]. The time constant for structure reorganization is several times the shear rate, y, in flow reversal experiments [34] and depends on the volume fraction of solids. The viscosities discussed in Sections 12.42.2 to 12.42.9 are always the steady shear viscosity and not the transient ones. 

However, they may not indicate the true bulk viscosity of a suspension that forms a thin layer of the continuous phase (e.g., serum of tomato juice) around the immersed probe or when the probe is covered by a higher viscosity gel due to fouling. Vibrational viscometers are suitable for measuring viscosities of Newtonian fluids, but not the shear-dependent rheological behavior of a non-Newtonian fluid (e.g., to calculate values of the power law parameters). 

We could use a simplified form of rj z) = ri f z). Here is the bulk viscosity, and /(z) can be experimentally determined. Here, z is the distance normal to the solid surface. A partial justification for the above functional form can be drawn from the temperature dependence of the surface diffusion coefficient and the bulk viscosity,or the fly stiction correlation with the bulk viscosity. To develop a rigorous hydro-dynamic model, we need better rheological data and more details are given in the following section on Rheology Measurement. 

The response (a decrease of viscosity) is a direct consequence of the action of the enzyme on its substrate, since the splitting of the glycosidic bonds gives a decrease in the viscosimetric average molecular weight and hydrodynamic volume of the hyaluronan chains and hence a decrease in the intrinsic and relative viscosities. With this method the rate at different concentrations cannot be compared because the initial viscosities are different and rheological measurements do not coincide. To eliminate these problems, a kinetic dilution methodology for the viscosimetric study of the substrate concentration dependence of the action of hyaluronidase was proposed [135,136]. We were able to determine the rate of reaction, expressed as the number of moles of bonds broken per unit of time, from viscosimetric data [136]. 

Even the measurement of the steady-state characteristics of shear-dependent fluids is more complex than the determination of viscosities for Newtonian fluids. In simple geometries, such as capillary tubes, the shear stress and shear rate vary over the cross-section and consequently, at a given operating condition, the apparent viscosity will vary with location. Rheological measurements are therefore usually made with instraments in which the sample to be sheared is subjected to the same rate of shear throughout its whole mass. This condition is achieved in concentric cylinder geometry (Fi re 3.37) where the fluid is sheared in the annular space between a fixed and a rotating cylinder if the gap is small compared with the dimneters of the cylinders, the shear rate is approximately... 

Consequently, the rheological measurements of MPSs should be carried out such that the dimension of the flow channel is significantly larger than the size of the flow element. For example, the relative viscosity, jjr, of diluted spherical suspensions measured in a capillary instrument depends on the (d/D) factor, where 7) is the sphere diameter and d that of the capillary—for d 107), the error is around 1% [Happel and Brenner, 1983]. Thus, if 1% error is acceptable, the size of the dispersion should be at least 10 times smaller than the characteristic dimension of the measuring device (e.g., diameter of a capillary in capillary viscometers, distance between stationary and rotating cylinders or plates). Following this recommendation is not always possible, which lead to the decline and fall of continuum mechanics [Tanner, 2009]. 

Viscosity, as it is observed for a given system, depends largely on the structure, i.e., the type of aggregates that are present, on their interactions, and on the concentration of the system. In principle, rheological measurements contain information regarding these parameters, and dynamic rheological experiments can also yield information on the dynamics of the given system. 

It is instructive to compare rheological behavior of isotropic molten polymer phases to the simplest LC phases, that is nematic ones. In an isotropic phase molecular orientations are completely random the flow process can only introduce some order. In a nematic PLC a certain degree of order (as measured by the parameter s, see Section 41.3.1) already exists. Therefore, a flow process can either enhance or reduce the existing order. This problem has been analyzed by Mamicci and Maffettone [80]. If instead of the order parameter we consider viscosity, then—as defined for MLCs already in 1946 by Miesowicz [81]—one has to distinguish three viscosities dependent on the direction parallel to the flow direction parallel to the gradient of viscosity and perpendicular to both directions just named. 

Rheological measurements are most often used to characterize the bulk properties of a solution (e.g., the viscosity), but the dependence of the rheological propaties on the concentration, molecular weight, and shear rate can be used to infer structural information. 

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