The controlled stress rheometer

Since the mid 1980s and the advent of rehable second generation controlled-stress rheometers, the controlled-stress technique has become widely established. The facility which most of this type of instrument offers, i.e. of performing three different types of test (steady shear, oscillation and creep), makes them particularly cost effective. 

The instrument referred to here for illustration is a TA Instruments CSL 100 controlled-stress rheometer (TA Instruments, UK). The rheometer (typically operated imder the control of a microcomputer) and ancillary equipment required for its operation, consist of the following main components (see Figme 2.7). 

The non-rotating lower platen of the measming assembly is fixed to a height-adjustable pneumatic ram which may be raised to provide the desired gap setting, with micrometer-fine adjustment. A temperatme control unit is 

Due in part to its ability to produce extremely low shear rates, the controlled stress technique has been formd to be highly suited to the determiiration of apparent yield stress, and in this respect the controlled-stress instrument is widely claimed to be more successful than its controlled-shear rate-cormterparts. This is usually attributed to the fact that, for suitably low stresses, the structure of the material may be preserved imder the couditions of test, ludeed, the introduction of the second generation of controlled-stress instruments can be said to have provoked considerable interest in, and debate siuroimding, the field of yield stress determination, with some early advocates of the controlled-stress technique advancing the controversial notion of the yield stress myth [Barnes and Walters, 1985]. 

Apart from the range of instnunents described in the preceding sections, several of the inexpensive viscometers used for quality control in industry give rise to comphcated flow and stress flelds (which may be neither known nor 

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Problem 3-23. The Controlled Stress Rheometer. A commonly used viscosity measurement tool is the controlled stress rheometer, in which the applied stress is controlled and the resulting motion of a plate is used to calculate the viscosity. A simplified version of such a system is depicted in the figure. 

Figure 6 (p. 32) shows the high viscosity capability of the controlled stress rheometer. The test was a simple stress ramp up to 6,100 Pa... 

Recently, the use of parallel plate rheometers has increased as the methods and performance have improved. The correlation of the polymer melt data from these tests to the controlled stress rheometers is also discussed. 

A. 1 mm die (L/D=30) to see if the apparent viscosity obtained would correlate to the controlled stress rheometer. All samples were tested under this condition. 

The same samples were then tested on the parallel plate rheometer to see if the correlation would improve to the controlled stress rheometer. 

All of the polyethylene samples were tested at 190 C using a controlled stress and controlled rate capillary rheometers. The controlled stress rheometer was used... 

Fig. 29. Measurements of yield stress with a vane device and Rheometrics controlled stress rheometer. The torque required to cause yielding is between 1.88...

Viscoelasticity can also be determined by a controlled stress rheometer. The shape of a creep curve can show that a fluid is viscoelastic, and the amount of recovery after the stress is removed gives a measure of elasticity. 

Gel formation was monitored using a controlled-stress rheometer (Carri-Med CS 50, TA Instruments, Guyancourt, France) with cone-and-plate geometry (cone diameter 4 cm, angle 3°58 ). The bottom plate was fitted with a Peltier temperature controller that... 

Controlled-rate operation is also available for most controlled-stress rheometers. The stress range measured with this protocol can subsequently be used to set up exact stresses of interest. If controlled-rate operation is not available, a start point ofOPa and an end point of 100 Pa should be programmed, These values will not be suitable for all samples (e.g., an end point of <1 Pa should be used for samples like water, and 100 Pa should be used for samples like ketchup). 

The major types of fluid flow behavior can be described by means of basic shear diagram of shear rate versus shear stress, such as Figures 1-2 and 1-3. In Figure 1-2, the shear stresses are plotted against the shear rates (independent variable) which is the conventional method. However, some authors plot shear rates against the shear stresses (independent variable) as shown in Figure 1-3. With the introduction of controlled-stress rheometers, the use of shear stress as the independent variable is often desirable. 

While the dynamic experiments described above are to be conducted in the linear viscoelastic range, another experiment can be conducted in which the results obtained in the non-linear range are useful. With a controlled-stress rheometer, one can conduct an experiment in which the stress is increased continuously at a constant oscillatory frequency, say 1 Hz. Results obtained in such an experiment are shown schematically in Figure 3-40. As the stress is increased continuously, initially, G and G" remain relatively constant until at a critical value of stress, Oc, the magnitude of G decreases sharply and that of G" also decreases not as sharply after a slight inerease. One may also use the value of the applied stress at which the curves of G and G" intersect... 

Abstract Grease lubrication is a complex mixture of science and engineering, requires an interdisciplinary approach, and is applied to the majority of bearings worldwide. Grease can be more than a lubricant it is often expected to perform as a seal, corrosion inhibitor, shock absorber and a noise suppressant. It is a viscoelastic plastic solid, therefore, a liquid or solid, dependent upon the applied physical conditions of stress and/or temperature, with a yield value, ao- It has a coarse structure of filaments within a matrix. The suitability of flow properties of a grease for an application is best determined using a controlled stress rheometer for the frequency response of parameters such as yield, a, complex shear modulus, G phase angle, 5, and the complex viscosity, rj. ... 

Nolan, S. J. (2003) The Use of a Controlled Stress Rheometer to Evaluate the Rheological Properties of Grease, NLGI Spokesman 67 (3), 18-21. 

More sophisticated techniques use a controlled stress rheometer to evaluate the minimum stress necessary to obtain flow (the yield stress) or to calculate, by appropriate modeling, the zero shear viscosity of the system. Whichever technique... 

The storage modulus (G ) and loss modulus (G") were measured as a function of temperature using a Carri-Med CSL2 500 (TA Instruments) controlled stress rheometer fitted with a 2° steel cone with a diameter of 6 cm (gap width 55 /mi). Carrageenan solutions (1% w/w in a solution containing 35 mM KC1 and 20 mM NaCl) for low deformation oscillatory measurements were prepared from dried material, stored overnight at 4°C and then heated to 80 °C for 10 min. The sample was applied between the cone and plate of the rheometer and covered with paraffin oil to prevent evaporation of the sample. After equilibration for 30 min at 50 °C, measurements were made approximately at 0.2 °C intervals between 50 and 5°C with a cooling rate of 0.5°C min-1. Measurements were performed at a frequency of 1 Hz and a strain of 10%. 

Several researchers reported viscoelastic behavior of yeast suspensions. Labuza et al. [9] reported shear-thinning behavior of baker s yeast (S. cerevisiae) in the range of 1 to 100 reciprocal seconds at yeast concentrations above 10.5% (w/w). The power law model was successfully applied. More recently, Mancini and Moresi [10] also measured the rheological properties of baker s yeast using different rheometers in the concentration range of 25 to 200 g dm. While the Haake rotational viscometer confirmed Labuza s results on the pseudoplastic character of yeast suspension, the dynamic stress rheometer revealed definitive Newtonian behavior. This discrepancy was attributed to the lower sensitivity of Haake viscometer in the range of viscosity tested (1.5 to 12 mPa s). Speers et al. [11] used a controlled shear-rate rheometer with a cone-and-plate system to measure viscosity of... 

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