Rheological critical shear rate

The stress in viscoelastic liquids at steady-state conditions is defined, in simple shear flow, by the shear rate and two normal stress differences. Chapter 13 reviews the evolution of both the normal stress differences and the viscosity with increasing shear rate for different geometries. Semiquantitative approaches are used in which the critical shear rate at which the viscosity starts to drop in non-Newtonian fluids is estimated. The effects of shear rate, concentration, and temperature on die swell are qualitatively analyzed, and some basic aspects of the elongational flow are discussed. This process is useful to understand, at least qualitatively, the rheological fundamentals of polymer processing. 

Rheology is a powerful method for the characterization of HA properties. In particular, rotational rheometers are particularly suitable in studying the rheological properties of HA. In such rheometers, different geometries (cone/plate, plate/plate, and concentric cylinders) are applied to concentrated, semi-diluted, and diluted solutions. A typical rheometric test performed on a HA solution is the so-called "flow curve". In such a test, the dynamic viscosity (q) is measured as a function of the shear rate (7) at constant strain (shear rate or stress sweep). From the flow curve, the Newtonian dynamic viscosity (qo), first plateau, and the critical shear rate ( 7 c), onset of non-Newtonian flow, could be determined. 

It has often been stated that DR of surfactant solutions is related to their rheological properties. A rise in shear viscosity at a critical shear rate, caused by a shear-induced structure (SIS), viscoelasticity (nonzero first normal stress difference, quick recoil, and stress overshoot), and high extensional viscosity/shear viscosity ratios ( 100) are rheological properties found in many DR surfactant solutions. After reviewing the rheological behavior of many DR surfactant solutions, Qi and Zakin concluded that SIS and viscoelasticity are not always observed in DR surfactant solutions while high extensional/shear viscosity ratios may be a requirement for surfactant solutions to be DR. ... 

Rheology of the molten fluoropolymers is of critical importance in processing these polymers. Fluoropolymers, and generally thermoplastic materials, must be processed below the velocity at which melt fracture occurs, referred to as the critical shear rate. Melt fracture in molten plastics takes place when the velocity of the resin (in flow) exceeds the critical... 

Comparison Between Different Viscometers. To validate their rheological measurements, several authors have tried to compare the results obtained using coaxial cylinder and pipe viscometers. Their findings are not necessarily in agreement. Bannister (15) was able to predict the frictional pressure drops of a cement slurry in a 1.815-in. ID pipe from pipe viscometer data corrected for wall slip. Mannheimer, who tried to reconcile coaxial cylinder and pipe viscometer data, both of them being corrected for wall slip was successful with one cement slurry formulation, but the approach failed with another one (13). Denis et al. (16) showed good agreement between coaxial cylinder and pipe viscometer data above a critical shear rate—or shear stress—that is pipe diameter dependent. 

The critical shear rate, above which equation (11.31) leads to the first physically useful dispersion degree (>0.1%), is in the neighbourhood of what is known to cause melt fracture ( r > 1000 s , T > lO Nm). This leads to the hypothesis that dispersion can only occur and be observed under melt fracture conditions, a widely known rheological instability [122]. ... 

All experiments that undergo edge failure, shear rate discontinuities, and slip should be discarded. However, it is often diflicult to avoid slip flow altogether. In this case, it is recommended to perform rheological evaluations below the critical shear rate or stress that trigger the slip phenomenon (see Fig. 37). If data are needed only for comparison purposes, all samples to be compared should be measured with exactly the same sensor tool under the same measuring conditions. 

It is well known that the rheological properties of partially hydrolyzed polyacrylamide depend on the stresses associated with a given flow field. In a simple shear flow, the apparent viscosity is constant at low shear rates (Newtonian behavior). At a critical shear rate, the apparent viscosity decreases as the shear rate is increased, i.e., a shear thinning behavior [48]. The viscosity shear-rate data of water soluble-polymers are commonly fitted using the Carreau viscosity model [49]. According to this model, the apparent viscosity, p, is a function of the shear rate, Y, as follows ... 

Figure 26 Dependence of zero-shear viscosity and critical shear rate concentration on salt concentration for polyacrylamide aqueous solutions. (From KC Tam and C Tiu, Water-soluble polymers (rheological properties) in Polymeric Materials Encyclopedia, JC Salamone, ed. Boca Raton, FL CRC Press, 1996, p. 8655.)...

Solution Rheology. Solutions of polyacrylamides tend to behave as pseudoplastic fluids in viscometric flows. Dilute solutions are Newtonian (viscosity is independent of shear rate) at low shear rates and transition to pseudoplastic, shear thinning behavior above a critical value of the shear rate. This critical shear rate decreases with the polymer molecular weight, polymer concentration, and the thermodynamic quality of the solvent. A second Newtonian plateau at high shear rates is not readily seen, probably because of mechanical degradation of the chains... 

Moat lubricants which contain polymers will show a constant viscosity Newtonian behavior up to a critical shear rate. Above that shear rate, the viscosity of these polymer solutions decreases with increasing shear rate and as the first approximation this Shear-thinning region can be represented by a power-law rheological model. At very high shear rates, polymer solutions will also characteristically approach an upper Newtonian limit. 

Rheology of the molten fluoropolymers is of critical importance in processing these polymers. Fluoropolymers, and generally thermoplastic materials, must be processed below the velocity at which melt fracture occurs, referred to as the critical shear rate. Melt fracture in molten plastics takes place when the velocity of the resin in flow exceeds the critical velocity, the point where the melt strength of the polymer is surpassed by internal stresses. Critical velocity of most fluoropolymers is usually much lower than most thermoplastics. Parts molded in a process where critical velocity is exceeded exhibit typical symptoms of melt fracture that is, they may have a frosty or cloudy surface. In some cases, a part may have a smooth and shiny surface but is internally fractured. 

Hoffman [64] studied monodisperse suspensions of polyvinyl chloride and styrene-acrylonitrile copolymer particles of diameter 0.4 to 1.25 pm using both rheological and structural characterization techniques as shown in Figure 2.1. The shear viscosities showed striking viscosity increases at critical shear rates. Structurally, the suspensions at low shear rates were found to exhibit a hexagonal crystalline lattice. At a critical shear rate, this lattice structure broke up into less-oriented arrays with a jump increase in shear viscosity. 

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