Steady shear measurements

Fernandez et al. (2007) have characterized the rheological behavior of the mashed potatoes with added biopolymers using steady shear measurements. Fresh and frozen/thawed mashed potatoes present shear thinning with yield stress (Canet et al., 2005a), and dynamic shear data reveal weak gel-like behavior in potato purees (Alvarez et al., 2004). The effects are strongly... 

In order to investigate the influence of chemical structure on the melt viscosity, polyesters A1.A2.B1 and B2 were compared in the steady mode in plate and plate-geometry at 300°C. A2 and B2 should be in the isotropic region, whereas Al, B1 would be in the nematic state. Prior to steady shear measurements a transient step from... 

Both strain- and stress-controlled rotational rheometers are widely employed to study the flow properties of non-Newtonian fluids. Different measuring geometries can be used, but coaxial cylinder, cone-plate and plate-plate are the most common choices. Using rotational rheometers, two experimental modes are mostly used to study the behavior of semi-dilute pectin solutions steady shear measurements and dynamic measurements. In the former, samples are sheared at a constant direction of shear, whereas in the latter, an oscillatory shear is used. 

While dynamic mechanical and steady shear measurements are frequently used in rheology studies of surfactant systems, extensional viscosity measurements are lacking. This can be attributed to the difficulties associated with such measurements and the lack of commercial laboratory instrumentation since the discontinuance of the Rheometric Scientific RFX rheometer. For many detergent compositions, the relatively low viscosity further complicates such measurements. There appear to be very few data on extensional or elongation viscosity for detergent consumer products and actives in the technical literature at this time. 

For dynamic mechanical and steady shear measurements, the Rheometric Scientific RFSII rheometer was used equipped with the sensitive range force rebalance transducer and couette geometry or parallel plate tooling. 

Weakly flocculated systems have been studied by Buscall et al. (165), Heath and Tadros (166), Goodwin et al. (167), Patel and Russel (168, 169), Otsubo (170-172), Buscall et al. (51,173), Woutersen and de Kruif (174), and Nakai et al. (175). The weak flocculation can be obtained by several means, including secondary-minimum flocculation, depletion flocculation, polymer-bridging flocculation, and incipient flocculation. Details of the various mechanisms of interparticle attraction can be found in Russel et al. (27) and Somasundaran and Yu (176). Normally, flocculated systems have a solid volume fraction of no less than 0.2. When 0 < 0.2, an attractive system will settle down quickly. When 0 > 0.2, a flocculated suspension can be maintained easily for a period of time for a steady shear measurement to be completed. 

Concentrated suspensions commonly display viscoelatic behavior. The viscoelastic properties can be measured by oscillatory tests (26). Comparing with steady shear measurements, oscillatory measurements are made under small deformations, at which the suspension structure is only slightly perturbed. Hence, oscillatory measurements are suitable for correlating rheological behavior with structural data and interparticle potentials, even for strongly flocculated systems that show irreversible changes when subjected to large deformations. 

Steady shear measurements were used to determine flow properties and to estimate the degree of structure breakdown with shear (Elliott and Ganz, 1977). The power law equation (Eq. 3) has been used to describe the shear stress-shear rate behavior of salad dressings (Figoni and Shoemaker, 1983 Paredes et al, 1988, 1989). The flow behavior index of five commercial salad dressings at different temperatures and storage times of up to 29 days were all less than one, indicating that they were pseudoplastic fluids. The consistency index (/f) decreased with the increase in product temperature. 

An alternative to a steady-shear measurement is oscillatory shear, described in the linear regime by the following ... 

However, dynamic measurements, such as the oscillatory tests, are preferred in practice over the steady shear measurements for determining the viscoelastic properties of fluids. This is not only because of the difficulty in determining the first normal stress coefficient accurately, but the determination of a fluid s viscosity by steady shear measurement is only a partial characterization of a viscoelastic fluid s properties. 

Figure 28 (a) Schematic representation of supramolecular cross-linked networked formed with bismetallic pincer molecule 54 and P4VP. (b) Rheological analysis demonstrating the exact scaling of measured values relative to the dissociation rate of the metal complexes for the viscosity from steady shear measurements. Reproduced from Yount, W. C. Loveless, D. M. Craig, S. L. J. Am. Chem. Soc. 2005, 127,14488-14496. ... 

Khalkhal and Carreau (2011) examined the linear viscoelastic properties as well as the evolution of the stmcture in multiwall carbon nanotube-epoxy suspensions at different concentration under the influence of flow history and temperature. Initially, based on the frequency sweep measurements, the critical concentration in which the storage and loss moduli shows a transition from liquid-like to solid-like behavior at low angular frequencies was found to be about 2 wt%. This transition indicates the formation of a percolated carbon nanotube network. Consequently, 2 wt% was considered as the rheological percolation threshold. The appearance of an apparent yield stress, at about 2 wt% and higher concentration in the steady shear measurements performed from the low shear of 0.01 s to high shear of 100 s confirmed the formation of a percolated network (Fig. 7.9). The authors used the Herschel-Bulkley model to estimate the apparent yield stress. As a result they showed that the apparent yield stress scales with concentration as Xy (Khalkhal and Carreau 2011). 

Fig. 7.9 (a) Steady shear measurement of MWCNT suspensions at different concentrations, (b) Scaling behavior of the apparent yield stress obtained using the Herschel-Bulkley model with volume concentration of MWCNTs (Khalkhal and Carreau 2011)... 

The initial steady shear measurements identified several features of the phenomena ... 

Capillary bundle models assume the power-law exponent, n, determined from steady shear measurements is identical to the power-law exponent, n, observed during flow in porous rocks. A comparison of n and n is shown in Figure 7. In our experiments, the value of n was greater than n for polymer concentrations above 500 ppm. A review of Chauveteau s data also reveals that the power-law exponent for flow in porous rocks is larger than the power-law index measured from viscometric data. 

The power-law exponent for flow through Berea core material was larger than the power-law exponent determined from steady shear measurements when polymer concentrations exceed 500 ppm. 

Polymer mobilities are underestimated by a factor of two or more when predicted using capillary bundle models and rheological parameters obtained from steady shear measurements on the polymer solutions. 

The cone and piate rheometer is susceptible to irregularities at the iiquid-air interface and to secondary flows. As a result, the shear rate in steady shear measurements has to be quite iow to avoid the above-mentioned problems. In general, the shear rate should not exceed 1 s data above this rate should be regarded with caution. 

This empirical rule seems to hold up quite well for most polymers. Using this rule, it is possible to determine viscosity data up to 500 s with a cone and plate rheometer by applying an oscillatory motion to the cone. This would be impossible if a steady rotational motion was applied to the cone. In steady shear measurements on a cone and plate rheometer, the maximum shear rate that can be measured is around 1 s, which is much too low for applications to extrusion problems. The same is true for measurements in the parallel plate test geometry. Thus, the dynamic measurement extends the shear rate measurement range considerably, while still being able to take advantage of the cone and plate geometry. 

Dynamic viscosity measurements generally are easier to perform over a wide range of frequencies than are steady shear measurements over a wide range of shear rates, while steady shear viscosity is of more value in the analysis of polymer processing. Thus, the Cox-Merz rule is frequently assumed and t data are used in place of steady shear viscosity data. 

The material parameters that are involved in steady shear measurements are the steady shear viscosity and the normal stress difference. These will be treated in separate chapters as the information available on these two parameters is not balanced equally. 

Steady shear measurements were carried out using the Rheometrics Mechanical Spectrometer at a fixed temperature of 180°C using a cone-plate mode. 

Similarly, from systematic rheological steady shear measurements increases in rjao, 0, K and a decrease in n (more pronounced shear thinning) were detected for (i) an increase in disperse phase fraction, (ii) an increase in the viscosity of the disperse fluid phase and (iii) a reduced surfactant content. 

Recent literature has shown that the modified Cox-Merz rule could fit e rimental data very well [20-22], Isayev and Fan [22] of University of Akron investigated both steady and oscillatory shear flow behavior of silicon-polypropylene ceramic compoimd. They reported that steady shear measurement with either parallel plate or capillary rheometers posed a... 

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