Rheology torque

The fluidity of the cement paste can be measured in rheological terms by the torque transmitted to a stationary bob inside a revolving outer cylinder placed in a water-cement system as shown in Fig. 1.10. The shear stress measured at the stationary bob is plotted against the rate of applied shear when, for pastes of varying water-cement ratios, the results shown in Fig. 1.11 are obtained for readings taken of the shear stress as the shearing rate is increased (the up curve). 

Hellen L, Yliruusi J. Process variables of instant granulator and spheroniser III. Shape and shape distributions of pellets. Int J Pharm 1993 96 217-223. Chapman SR, Rowe RC, Newton JM. Characterization of the sphericity of particles by the one plane critical stability. J Pharm Pharmacol 1988 40 503-505. Rowe RC, Sadeghnejad GR. The rheology of mcc powder/water mixes-measurement using a mixer torque rheometer. Int J Pharm 1987 38 227-229. O Connor RE, Schwartz JB. Spheronization II Drug release from drug-diluent mixtures. Drug Dev Ind Pharm 1985 11 (9-10) 1837-1857. 

The molecular theory of Doi [63,166] has been successfully applied to the description of many nonlinear rheological phenomena in PLCs. This theory assumes an un-textured monodomain and describes the molecular scale orientation of rigid rod molecules subject to the combined influence of hydrodynamic and Brownian torques, along with a potential of interaction (a Maier-Saupe potential is used) to account for the tendency for nematic alignment of the molecules. This theory is able to predict shear thinning viscosity, as well as predictions of the Leslie viscosity coefficients used in the LE theory. The original calculations by Doi for this model employed a preaveraging approximation that was later... 

To avoid the apparent complications with absolute rheologic measurement techniques, a number of investigators (4,5). have used relative measurement systems to make rheologic measurements. The major difference between the relative and absolute measurement techniques is that the fluid mechanics in the relative systems are complex. The constitutive equations needed to find the fundamental rheologic variables cannot be readily solved. Relative measurement systems require the use of Newtonian and non-Newtonian calibrations fluids with known properties to relate torque and rotational speed to the shear rate and shear stress (6). 

The impeller method is a technique commonly used to determine rheologic properties of fluids subject to particle settling. The impeller method utilizes a viscometer along with Newtonian and non-Newtonian calibration fluids to obtain constants that relate shear stresses and shear rates to experimentally measured values of torque and rotational speed. Newtonian calibration fluids are used to determine the impeller constant, c, and non-Newtonian calibration fluids are used to calculate the shear rate constant, k. These constants are then used to aid in the determination of rheologic properties of a selected non-Newtonian fluid, such as wet grains. 

Two Brookfield viscometers were used to collect the data necessary for rheologic property studies of wet grains a Brookfield RVDV III viscometer with a cone-and-plate spindle and a Brookfield HBDV III viscometer with a double helical ribbon impeller attachment. The Brookfield RVDV III had a full-scale torque of 7187 dyn-cm, and the HBDV III had a full-scale torque of 57,496 dyn-cm. Each viscometer had a maximum rotational speed of 250 rpm. Both viscometers had accuracy limits of 5% full-scale torque. 

Figures 2-4 show that no experimental data were recorded at low impeller shear rates. Experimental data began at y = 8.53 s4 for 21% solids, 5.15 s 1 for 23% solids, and 3.43 s 1 for 25% solids. The reason for the missing data is that the helical impeller viscometer has limitations. Owing to possible viscometer error, data were not recorded until the impeller torque was >10% of the full-scale torque. Therefore, no experimental data were recorded at low impeller rotational speeds. The lack of experimental data at low shear rates made comparison of rheologic models at low shear rates and the prediction of yield stress impossible.

In general, shear stress at one location (e.g., the bob surface in a concentric cylinder viscometer) is calculated from the dimensions of the sample gap and the measured or applied torque. Shear rate is calculated at the same location from sample gap dimensions and rotational speed. By making experimental measurements over a range of speeds or torques, the flow curve (shear stress versus shear rate) of the sample can be established. Suitable mathematical treatment of the flow curve data yields the sample s constitutive equation and rheological properties. 

The existence of a stress-free meniscus introduces a boundary discontinuity on both upper and lower rims. This discontinuity is bound to result in sharply enhanced stress build-up at the upper and lower contact lines. It may produce an effective slip layer on the sample/plate interfaces at the meniscus, yielding an overall torque, which is less than anticipated on the basis of Fig. 7a, since a large portion of the measured torque on the plate arises from the stress contribution at the rim r=R. This correction may increase with lowering the gap distance. Without a proper analytical treatment, it remains unknown how the magnitude of such an apparent slip depends on the rheological properties of the sample and whether the reported slip like behavior [ 19,33] is a manifestation of such a plausible edge effect. 

For pumping ease, the initial Be should be very low, preferably < 30 Be, which can be measured by the consistometer. The rheological behavior can be measured by means of a rotational viscometer, which consists of an outer sleeve and an inner drum, both rotating at different speeds. The outer sleeve is rotated at a constant speed, which causes a torque on the inner dmm that can be measured on a dial. Starting from 600 rpm, the rotor speed is lowered successively at 20 s intervals, and the measurements of the torque are taken at the end of each period. Typically the speed is lowered in steps of 6 rpm, and the results are represented graphically. 

Rheological Experiments. Melt viscosity and low-strain oscillatory experiments were performed on a Rheometrics RDS-7700 dynamic spectrometer equipped with a 0.2-2.0-g-cm torque transducer. The samples were mounted on 25-mm-diameter parallel-plate fixtures with a gap of 0.5 mm. Prior to each scan, samples were heated to 50 °C and then cooled slowly to room temperature. Steady-shear... 

Creep-compliance studies conducted in the linear viscoelastic range also provide valuable information on the viscoelastic behavior of foods (Sherman, 1970 Rao, 1992). The existence of linear viscoelastic range may also be determined from torque-sweep dynamic rheological experiments. The creep-compliance curves obtained at all values of applied stresses in linear viscoelastic range should superimpose on each other. In a creep experiment, an undeformed sample is suddenly subjected to a constant shearing stress, Oc. As shown in Figure 3 1, the strain (y) will increase with time and approach a steady state where the strain rate is constant. The data are analyzed in terms of creep-compliance, defined by the relation ... 

A torque rheometer is a device that provides an off-line measurement of torque required to rotate the blades of the device and this torque can be used to assess rheological properties of the granulation. It has been extensively used for end-point determination.The torque values thus obtained were termed as measure of wet mass consistency. ° ... 

Rowe, R.C. Sadeghnejad, G.R. The rheological properties of microcrystalline cellulose powder/water mixes-measurement using a mixer torque rheometer. Int. J. Pharm. 1987, 38, 227-229. 

Soh JLP, Liew CV, Heng PWS. Torque rheological parameters to predict pellet quality on extrusion/spheronization. Int J Pharm 2006 315 99-109. 

Luukkonen P, Schaefer T. Hellen L. et al. Rheological characterization of microcrystalline cellulose and silicified microcrystalline cellulose wet masses using a mixer torque rheometer. Im J Pharm 1999 188 181-92. 

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