Controlled strain rheometer

The results of Equation (3.56) are plotted in Figure 3.14. It can be seen that shear thinning will become apparent experimentally at (p > 0.3 and that at values of q> > 0.5 no zero shear viscosity will be accessible. This means that solid-like behaviour should be observed with shear melting of the structure once the yield stress has been exceeded with a stress controlled instrument, or a critical strain if the instrumentation is a controlled strain rheometer. The most recent data24,25 on model systems of nearly hard spheres gives values of maximum packing close to those used in Equation (3.56). 

With a controlled-strain rheometer, instead of using the stress, one may use the applied strain as the independent variable and work with values of G and G". The value of G corresponding to the critical strain, yc, at which G" reached its maximum... 

Rheology Rheological measurements were performed at 25°C with an ARES 2 KFRT controlled strain rheometer (Rheometric Scientific). For the measurements parallel plates of 50 mm diameter were used. The gels were loaded between the plates (2-mm gap) and allowed to rest for 3 min. A strain sweep (0.1 to 100%) was performed at 1 Hz frequency to determine the range of viscoelasticity for each sample and a 2% strain was selected for all samples. A frequency sweep test (0.1 to 16 Hz) was then performed. Samples of 30 and 50% s/w concentration could not be analyzed because of the difficulty in obtaining samples of proper and constant geometry. 

In a controlled strain rheometer , by prescribing a controlled strain and measuring the stress it causes. The rheological curve is obtained by carrying out such an experiment for various strain values. 

Torque may also be maintained in a controlled strain rheometer such as shown in Figure 8.2.2 by using feedback from the torque sensor to adjust the motor velocity or position. However, control is typically difficult because of the sample response. Ideally one should include the viscosity of the sample in the control algorithm. Performance can be improved significantly by avoiding the sample and closing the feedback loop around a torque sensor on the motor, such as motor current to a dc motor (Michel, 1988). However, brush fiiction in the motor limits the lowest torque levels to 10 to 10- N-m. 

TABLE 8,2.1 / Comparison of Typical Controlled Stress and Controlled Strain Rheometers... 

Currently, two general rotational rheometer types [29] are available commercially, that can be used for LAOS experiments controlled strain rheometers (also known as separated motor-transducer, SMT) and controlled stress (or combined motor-transducer, CMT) rheometers. Apart from the rotational rheometers, custom built instruments, such as the sliding plate rheometer by Giacomin et al. [13] are also suitable, but will not be discussed here, as they are not available commercially. 

One of the most direct manifestations of the onset of a glassy state is shown by the mechanical response of the suspension, which can be measured with a rheometer (Mason and Weitz 1995, Weitz 2011). Mason and Weitz used a controlled-strain rheometer and applied a known strain at a given frequency and measured the resultant stress at the same frequency. Frequency-dependent measurements of... 

Specific Commercial Rotational Viscometers. Information on selected commercial rotational viscometers can be found ia Table 7. The ATS RheoSystems Stresstech rheometer is an iastmment that combines controlled stress as well as controlled strain (shear rate) and oscillatory measurements. It has a torque range of 10 to 50 mN-m, an angular velocity range of 0 to 300 rad/s, and a frequency range of seven decades. Operation and temperature programming (—30 to 150°C higher temperatures optional) are computer controlled. 

Rotational rheometer (unithi.i e.g., Bohlin Instruments, Chandler Engineering) controlled stress (for applied step shear stress) or controlled strain (for applied step shear strain) with appropriate software for rheometer control, data acquisition, and data analysis... 

Fig. 5. Diagram illustrating an ultrasonic instrument designed to measure the speed of sound in a fluid under known shea conditions. The design is based on a combination of a pulse-echo ultrasonic reflectometer and a controlled-strain concentric cylinder rheometer.

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. 

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%. 

TA Instruments ARES rheometers measure controlled strain and controlled stress by using two technologies combined motor and transducer (CMT) and separate motor and transducer (SMT). The company s rheometer instrument controls using Ochestrator software include... 

Viscosity is measured with rheometers or viscometers. The methods used include rotational deformation, squeezing deformation, extrusion (capillary) flows, and free surface stretching. For rotational instruments, there are two modes of operation controlled strain and controlled stress. Rotational measurements can be further subdivided into different measuring methods (flow, oscillatory, stress relaxation, and creep) and different measurement devices (spindle, cone-and-plate, parallel plate, concentric cyhnder). As for a capillary rheometer, there are two modes of operation controlled flow (strain) and controlled pressure (stress). 

There are two basic designs of drag flow rheometers controlled strain with stress measurement and controlled stress with strain measurement. Below we Hrst discuss strain control and torque measurement (Section 8.2.2) followed by instrument alignment problems (Section 8.2.3) and normal stress measurement (Section 8.2.4). Then we treat special design issues for stress control. Both designs use the same type of environmental control system, as discussed in Section 8.2.6. 

The first drag flow rheometer, Couette s concentric cylinders shown in lugure 5.1.1, was a controlled strain device. Couette fixed the angular velocity of the outer cup and measured the torque on his inner cylinder by the deflection of a suspending wire. 

With proper inertia correction controlled stress rheometers are very versatile and may replace controlled strain instruments for many applications. The fact that stress and strain are measured on the same shaft in controlled stress instruments allows lower cost and simpler temperature control but also is the source of the inertia limitations. Table 8.2.1 summarizes the advantages and disadvantages of each control mode. 

The two types of rotational rheometer, controlled strain and controlled stress, can use the same environmental control. It can be as important to control the temperature, pressure, or humidity of a sample as it is to control the shear stress. Yet these Victors, especially temperature, seem to be less exciting to the engineers who design rheometers. At least there are often laige temperature gradients even in popular commercial instruments. Fortunately, with a few thermocouples it is easy to check for such problems, and often... 

Measuring yield stress of concentrated suspensions can be carried out using various rheological techniques that can be broadly classified under two categories the controlled rate rheometry and the controlled stress rheometry. A controlled rate rheometer deforms a specimen at a constant shear rate and measures the shear stress. On the other hand, a controlled stress rheometer imposes a constant shear stress on a specimen and then measures the corresponding strain. The latter approach involves a more sophisticated control system and is only introduced in the last ten years. These techniques can be further classified as direct (or static) or indirect methods (or dynamic). The indirect determination of yield stress involves the extrapolation of experimental shear stress - shear rate data to obtain a yield stress, which is the shear stress at zero shear rate. This is illustrated in Figure 9. It is evident that the choice of the model or methods yield differing values of yield stress. 

Both controlled-angular displacement (strain-controlled) and controlled-torque (stress-controlled) rotational rheometers are used, with the former giving superior performance at high frequencies and the latter better precision at low frequencies. There have also recently appeared on the market instruments said to be capable of operating in both modes. Controlled torque instruments can also be used to make creep and creep recovery tests, which are described in the next section. In order to obtain a linear viscoelastic characterization that includes the terminal zone, it is sometimes useful to combine data from oscillatory flow with those from a creep experiment, and this is also discussed in the following section. Rheometrical methods are described in some detail in several books [3,8,9]... 

Since it is the long-time behavior that is closely related to molecular structure, this is the information that is most interesting in the present context. For example, the zero-shear viscosity describes behavior in the limit of zero frequency and is very sensitive to molecular weight. However, for a material whose longest relaxation time is quite large, neither step-strain nor oscillatory shear experiments are useful to probe the behavior at very long times or very low frequencies. The main problem is that the stress is so small that it is not possible to measure it precisely. It is in this region that creep measurements are most useful. This is because it is possible to make a very precise measurement of a displacement, and it is also possible to apply a very small controlled stress. Controlled-torque (controlled-stress) rheometers are available from several manufacturers. 

Controlled strain is the preferred mode of operation for nonlinear studies. In step-strain experiments, an important source of experimental error is the deviation of the actual strain history from a perfect step. Laun [96] and Venerus and Kahvand [43] have discussed this problem and how it can be addressed. Gevgilili and Kalyon [ 100] found that the actual strain pattern generated by a popular coimnerdal rheometer in response to a command for a step was, in fact, a rather complex function of time. One approach that is of use in comparing data from any transient test with the predictions of a model is to record the actual, non-ideal, strain history and use this same history to calculate the model predictions. 

Figure 5.4 Apparent shear viscosity as a function of shear stress for fine surfactant-stabilized mineral oil-in-water emulsions (d 2 = 0.55 pm, 0.05 M phosphate buffer, pH 7) as determined at 25 C in a controlled stress rheometer (filled symbols) and a controlled strain-rate rheometer (open symbols) , O, 30 wt% oil, I wt% Tween 20 A, A, 40 wt% oil, 1.33 wt% Tween 20 , O, 50wt% oil, 1.67 wt% Tween 20 , Q 60 wt% oil, 2 wt% Tween 20...

Figure 4. Comparison of Viscosity measured by controlled stress and controlled strain capillary rheometers at 190 C with all samples... 

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