Rheometry, strain-controlled

Mechanical rheometry requires a measurement of both stress and strain (or strain rate) and is thus usually performed in a simple rotating geometry configuration. Typical examples are the cone-and-plate and cylindrical Couette devices [1,14]. In stress-controlled rheometric measurements one applies a known stress and measures the deformational response of the material. In strain-controlled rheometry one applies a deformation flow and measures the stress. Stress-controlled rheometry requires the use of specialized torque transducers in conjunction with low friction air-bearing drive in which the control of torque and the measurement of strain is integrated. By contrast, strain-controlled rheometry is generally performed with a motor drive to rotate one surface of the cell and a separate torque transducer to measure the resultant torque on the other surface. 

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. 

Oscillatory rheometry, a nonsample destructive technique in which an oscillatory stress is applied to the sample over a range of frequencies and the resultant strain determined, usually at a defined (controlled) temperature. 

The study of mechanical yielding of gels, glasses, and pastes represents a very artive field of current research. A number of different experimental means have been irsed to determine the yielding conditions, static and dynamic alike. Both strain- and stress-controlled rheometry have been employed. The interested reader is referred to indicative references, " some of which describe glasses from star polymers or star-like micelles. Below, we address some aspects of the so-called flow curve, that is, shear stress versus shear rate. 

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