Viscometers, rheological measurements

Rheological measurements. Routine viscosity measurements were made with a Wells-Brookfield micro-cone and plate viscometer, or a Brookfield LVT(D) viscometer with UL adapter. Viscosity-temperature profiles were obtained using the latter coupled via an insulated heating jacket to a Haake F3C circulator and PG100 temperature programmer or microcomputer and suitable interface. Signals from the viscometer and a suitably placed thermocouple were recorded on an X-Y recorder, or captured directly by an HP laboratory data system. 

Rheological measurements, 21 702-758 techniques in, 21 739-747 viscometers, 21 725-739 Rheological oscillation experiments,... 

In systems with suspended solids, rheologic measurements are difficult to perform owing to settling in the measurement devices. Conventional methods for measuring rheologic properties (cone-and-plate, concentric cylinder, and rotating-bob viscometers) do not produce accurate and reliable data for some solid suspensions. 

Rheologic measurements were performed at 25oC with the cone-and-plate and helical impeller viscometers. Impeller viscometer measurements were performed in a 1000-mL beaker with a diameter of 0.115 m. A liquid height of 0.115 m was used for all tests. 

Factors have been reported in the literature (Johnston and Brower 1966) for the conversion of Brookfield viscometer scale readings to yield value or viscosity. Saravacos (1968) has also used capillary viscometers for rheological measurements of fmit purees. 

The concentric cylinder viscometers are supplied with different inner and outer cylinders such that various gap widths can be formed. For rheological measurements of emulsions and suspensions, care must be taken to ensure a gap width of at least 20 times the suspended particle size in order to avoid wall effects. Moreover, experiments should be conducted with different gap widths to ensure the absence of any wall slip that is usually encountered in emulsion viscosity measurements (J6). However, uniformity of shear rate can be achieved only when the ratio of the gap width to the inner cylinder radius is small. 

Parallel Plate Viscometer, This instrument resembles the cone and plate viscometer, except that it has a flat horizontal rotating plate in place of the cone. The shear rate within the narrow gap of the two plates is not as uniform as for the cone and plate viscometer. The limiting shear rates for the parallel plate viscometer are similar to those of the cone and plate instrument. This type of a viscometer is suitable for rheological measurements of suspensions and emulsions. 

From R D to quality control, rheology measurements for each phase of the product development life cycle involve raw materials, premixes, solutions, dispersions, emulsions, and full formulations. Well-equipped laboratories with stress- and strain-controlled oscillatory/steady shear rheometers and viscometers can generally satisfy most characterization needs. When necessary, customized systems are designed to simulate specific user or process conditions. Rheology measurements are also coupled with optic, thermal, dielectric, and other analytical methods to further probe the internal microstucture of surfactant systems. New commercial and research developments are briefly discussed in the following sections. 

An exception to the generally observed drag reduction in turbulent channel flow of aqueous polymer solutions occurs in the case of aqueous solutions of polyacrylic acid (Carbopol, from B.F. Goodrich Co.). Rheological measurements taken on an oscillatory viscometer clearly demonstrate that such solutions are viscoelastic. This is also supported by the laminar flow behavior shown in Fig. 10.20. Nevertheless, the pressure drop and heat transfer behavior of neutralized aqueous Carbopol solutions in turbulent pipe flow reveals little reduction in either of these quantities. Rather, these solutions behave like clay slurries and they have been often identified as purely viscous nonnewtonian fluids. The measured dimensionless friction factors for the turbulent channel flow of aqueous Carbopol solutions are in agreement with the values found for clay slurries and may be correlated by Eq. 10.65 or 10.66. The turbulent flow heat transfer behavior of Carbopol solutions is also found to be in good agreement with the results found for clay slurries and may be calculated from Eq. 10.67 or 10.68. 

Figure 9. Rheological measurements performed with a coaxial cylinder viscometer. (A) Slurry showing a power law behavior and (B) slurry showing a Bingham plastic behavior.

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 viscosity of the suspension at difierent biomass concentration was measured by Modular Compact Rheometer Physica MCR 300 (Paar-Physica). Controlled shear-stress measurements were done using the concentric cylinder system with a FL 100/6W impeller at temperatures of 30, 40, and 50 °C. Samples were mixed before measurements were taken. Then, an appropriate volume was placed into the viscometer, allowing several minutes for the temperature to stabilize. The rheological measurements were performed three times for each value of biomass concentration using a fresh sample each time. 

The main focus of the measurement programme was the rheological measurements with the high-pressure capillary viscometer. In addition, a number of measurements were performed with the aim of permitting statements about the dispersion state (pyrolysis, microscopic investigations) and determining (conductivity measurements). 

Rheological measurements were carried out using a capillary viscometer and the studies were conducted with and without application of nltrasonic oscillation. The results obtained are shown in Figure 5.3 and indicate the dependence of the effective flow state (p f on the stress shear. The definition of the toughness depends on the arguments about the influence of the ultrasonic oscillations using the expression ... 

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]. 

J Zhang, Y Dafan. Rheological measurements of oil-inwater emidsions concentric cylinder viscometer. Fomth UNITAR International Conference on Heavy Crade and Tar Sands. Edmonton, Canada, 1988. 

For characterization of polymer blends, low strain dynamic rheological measurements are preferred over steady-state shearing (e.g., in a capillary viscometer). Since... 

RhGOlogy. Flow properties of latices are important during processing and in many latex applications such as dipped goods, paint, and fabric coatings. Rheology is used to characterize the stability of latices (45). For dilute, nonionic latices, the relative latex viscosity is a power-law expansion of the particle volume fraction. The terms in the expansion account for flow aroimd the particles and particle-particle interactions. For ionic latices, electrostatic contributions to the flow around the diffuse double layer and enhanced particle-particle interactions must be considered (46). A relative viscosity relationship for concentrated latices was first presented in 1972 (47). A review of empirical relative viscosity models is available (46). In practice, latex viscosity measurements are carried out with rotational viscometers (see Rheological Measurements). 

Rheological measurements were carried out with a computer-controlled Rheometer MCI (Paar Physica) rotational viscometer, with a Z4 concentric cylindrical measurement system at 25 0.1 °C. The complete flow and viscosity curves of the samples were plotted, and the yield value and the size of the thixotropic area were determined. 

The isothermal rheological measurements (at T = 190, 205 and 220/C) were carried out in a capillary viscometer, HKV 7901, at shear rates varying from 10 to 2500 (1/s). The constant-stress viscosities of the component pol)miers and their blends were determined from the primary flow curves t = f( y ). These were used to calculate the constant-stress viscosity ratios and capillarity numbers. 

Dynamic rheological measurements will be briefly described. In dynamic measurements an oscillatory microscopic strain is given and the corresponding stress is measured. This method gives information on viscoelastic behavior even in the region where no viscous flow takes place. The measurements can be carried out by using the rotating cylinder viscometer or the cone and plate viscometer. 

The rheological measurements were carried out using the rate controlled coaxial cylinder viscometer Rotovisko-Haake 20, system M5-Osc., measuring device MVIP with serrated surfaces. The temperature was kept strictly constant at 25 0.1°C. The tests were accomplished under both continuous and oscillatory flow conditions. Flow curves were... 

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