Controlled Strain

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. 

Schematic diagram of the Bohlin VOR iheometer. A dc motor with tachometer A drives gear box B, which drives the timing belt to provide steady rotation of the shaft. When the electromagnetic clutch C, is activated, the eccentric arm D can oscillate the shaft sinusoidally. The sample cell is surrounded by a temperature control bath E. An air bearing F centers a torsion bar H, whose rotation is sensed by linear variable differential transformer. Adapted from Bohlin (1988). 

Optical encoders have been used to provide feedback control of both rate and position to direct current motors (Michel, 1988 Amari et al., 1992). In contrast to capacitance transducers, which are typically limited to a 0.5 rad window, encoders can control position over 360 . This permits, for example, the superposition of sinusoidal oscillations on steady rotation. However, encoders are digital devices with a minimum step size around 1 mrad. With interpolation circuits encoders can resolve about 10 fitad. Thus at very low rates and low frequency there may be a long time between pulses, which can make control difficult. At high rates, accurately reading all the pulses sets an upper limit. Amari et al. report a steady rotation range of 0.001 to 3 rad/s with their stepper motor 

Normalized angular displacement versus time for a dc motor with RVCT feedback. The 0.1 rad step is achieved in less than 5 ms but shows some overshoot, From Starita (1980). 

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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. 

Controlled Strain-rate Tests Controlled strain-rate tests were first developed by Parkins (see Ugiansky and Payer ) for the study of stress-corrosion cracking. These took the form of constant strain-rate tests (also known, perhaps more accurately, as constant extension-rate tests). Since then alternative forms of test have been developed to modify the conditions under which the specimen is exposed. 

Since amino adds are used as essential components of the microbial cells and their biosynthesis is regulated to maintain an optimal level, they are normally synthesised in feedback limited amounts and are subjed to negative feedback control. The main problem using control strains is, therefore, the production of minor amounts of amino adds at an early... 

Direct shear test of soils under consolidated drained conditions Consolidated-undrained triaxial compression test on cohesive soils One-dimensional consolidation properties of soils One-dimensional consolidation properties of soils using controlled-strain loading... 

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). 

Additionally, we tested cell extracts in the colorimetric assay both negative control strains E. coli pET28a containing the empty vector and E. coli BFDL476Q (Table 2.2.1.2) did not show any enzyme activity in the TTC assay, whereas E. coli BAL (Table 2.2.1.2) led to the formation of an intense red color (data not shown). 

The BFD mutant library generated by epPCR was expressed in microtiter plates and 8000 clones were subjected to the carboligation assay with benzaldehyde 4 and dimethoxyacetaldehyde 8 as the substrates. The reaction was incubated for 24 h at 30 °C as suggested from experiments using the positive control strain... 

A ,Ga)As buffer layer is grown before epitaxy of (Ga,Mn)As. To control strain in the film, strain-relaxed thick (In,Ga)As ( 1 /zm) with the lattice constant a0 greater than the subsequent (Ga,Mn)As layer can be employed. The Mn composition x in the Gai - Mn As films can be determined from measurements of a0 by x-ray diffraction (XRD), once the dependence a0(x)is calibrated by other means, such as electron probe micro-analysis (EPMA) or secondary ion mass spectroscopy (SIMS). 

If the first data points do not reach equilibrium but exceed the maximum time allowed, then the test may be too demanding for the instrument as it is set up. The run should be repeated with a more sensitive geometry (larger surface area) or a more sensitive transducer (controlled-strain instruments only), which may improve the data. Prolonging each step may or may not improve data. 

To cover the widest range of frequencies in a single test, a controlled-stress device operating in a controlled-strain mode, or the equivalent test for a controlled-strain device, will provide the optimal configuration because the stresses or strains, respectively, are boosted to stay within the dynamic range of the instrument. If the results for LVE behavior have not been generated, then a fixed strain of 0.1% to 1.0% is likely to be adequate. As before, lower stresses should be used if the sample is a dispersion or emulsion, and higher stresses should be used if it is polymeric in form. 

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... 

The critical phase of either a creep test or a stress relaxation test is in the first few seconds. It is here that the transducer can be overloaded or the optical encoder can give a noisy response. Most controlled-strain instruments will give an audible error signal if the transducer is overloaded. For controlled-stress instruments, the noise level is determined by the resolution of the optical encoder. 

Alexander, J., Chang, G. Q., Dourmashkin, J. T. and Leibowitz, S. F. (2006) Distinct phenotypes of obesity-prone AKR/J, DBA2J and C57BL/6 J mice compared to control strains. IntJ Obes (Lond) 30, 50-59. 

TensUe tests, at controlled strain rates, were performed on an Instron tensile tester. Samples were rectangular, 6.35 mm by 3.17 mm in cross-section, or cylindrical, with a diameter of 5.1 mm. Both types had a gauge length of 12.7 mm. Fatigue tests were carried out on similar cylindrical samples, or on rectangular specimens, 5.1 mm by 3.17 nun in cross-section, at various selected stress amplitudes and at frequencies... 

Figure 3-13 Vane Yield Stress Setup at Controlled Strain Qiu and Rao (1988). The Numbers Denote Separate Pieces of Equipment 1—torque unit, 2—vane, 3—constant temperature vessel for sample, 4— constant temperature bath, 5—viscometer drive motor, 6—viscometer control panel, 7—chart recorder for torque output, and 8— voltmeter to monitor vane rpm signal.

F ure 3-14 Torque versus Time Plot with a Vane in Controlled Strain Operation Where the Maximum Torque is Used to Calculate the Yield Stress. 

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... 

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.

In order to clarify the effect of over-produced CA in carboxysomes on photosynthetic performances, growth rate and CO2 fixation rate were evaluated. CA over-expressed transformants when cultured under the CO2 concentration in the air, showed elevated levels of growth rate comparing with wild type or vector-control strains. This effect was enhanced as Ae aeration rate was decreased or under the concentration of 20ppm CO2 in the culture (Fig.4), suggesting that over-expressed CA is especidly important vmder the limited supply of DIC. 

Table 1 shows initial characterization of a small fraction of the isolated transformants. Strain 1 was the control , strains 2-6 were DNA insertional transformants that displayed relatively low Chi fluorescence, and strain 7 was a transformant with a relatively hi i Chi fluorescence. The Chi content of the cells and the Chi a/Chi b ratio were measured following strain cultivation in a small volume of liquid culture. A common feature of these transformants was the significantly lower than the control Chi content of the cells. Interestingly, this was the case for the five low-Chl fluorescence (strains 2-6) as well as the sole high Chi fluorescence strain ( 7). 

Figure 4 (left panels) shows light-induced fluorescence induction kinetics, the variable part of which reflects the photoreduction of QA in the thylakoid membranes of control and the Chi Mess mutant [Melis and Duysens 1979]. The fluorescence induction kinetics of the control strain were faster than that of the Chi Mess mutant, suggesting a larger PSII Chi antenna size for the former. 

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