Oscillatory dynamic tests

Moresi, M., Mancini, M., Bruno, M., Rancini, R. Viscoelastic properties of alginate gels by oscillatory dynamic tests. J. Texture Stud. 32, 375-396 (2001)... 

Rheological studies can provide useful information on the stability and internal microstructure of the multiple emulsions. Some attention was given to this subject in recent years, and the results help clarify certain aspects of stability and release properties of the multiple emulsions (Benichou et al., 2002b). Oscillatory dynamic tests and steady-state analyses, and parameters such as shear or complex modulus (G ), the lag phase between stress and strain (5), the storage modulus (G ), and the loss modulus (G"), provide quantitative characterization of the balance between the viscous and elastic properties of the multiple emulsions. Oscillatory measurements generally indicate that multiple emulsions are predominantly viscous in that the loss modulus... 

The shear modulus of a material can be determined by a static torsion test or by a dynamic test employing a torsional pendulum or an oscillatory rheometer. The maximum short-term shear stress (strength) of a material can also be determined from a punch shear test. 

In a series of experiments we have tested the type and range of entrainment of glycolytic oscillations by a periodic source of substrate realizing domains of entrainment by the fundamental frequency, one-half harmonic and one-third harmonic of a sinusoidal source of substrate. Furthermore, random variation of the substrate input was found to yield sustained oscillations of stable period. The demonstration of the subharmonic entrainment adds to the proof of the nonlinear nature of the glycolytic oscillator, since this behavior is not observed in linear systems. A comparison between the experimental results and computer simulations furthermore showed that the oscillatory dynamics of the glycolytic system can be described by the phosphofructokinase model. 

H3.1 Dynamic or Oscillatory Testing of Complex Fluids H3.2 Measurement of Gel Rheoogy Dynamic Tests H3.3 Creep and Stress Relaxation Step-Change Experiments... 

The most common dynamic method is oscillatory testing, in which the sample is subjected to a sinusoidal oscillatory strain, and the resulting oscillatory stress measured. The more sophisticated rotational viscometers have the additional capability of dynamically testing liquid-like materials using small angle oscillatory shear. A parallel disc viscometer can be set up for testing solid-like materials (e.g., butter), in oscillatory shear. Some UTM-type solids rheometers, in which the moving crosshead can be made to reciprocate sinusoidally, can be used to test solid-like materials in oscillatory deformation in compression, tension or shear. 

The four variables in dynamic oscillatory tests are strain amplitude (or stress amplitude in the case of controlled stress dynamic rheometers), frequency, temperature and time (Gunasekaran and Ak, 2002). Dynamic oscillatory tests can thus take the form of a strain (or stress) amplitude sweep (frequency and temperature held constant), a frequency sweep (strain or stress amplitude and temperature held constant), a temperature sweep (strain or stress amplitude and frequency held constant), or a time sweep (strain or stress amplitude, temperature and frequency held constant). A strain or stress amplitude sweep is normally carried out first to determine the limit of linear viscoelastic behavior. In processing data from both static and dynamic tests it is always necessary to check that measurements were made in the linear region. This is done by calculating viscoelastic properties from the experimental data and determining whether or not they are independent of the magnitude of applied stresses and strains. 

Gunasekaran, S., and Ak, M.M. (2000). Dynamic oscillatory shear testing of foods selected applications. Trends Food Sci. Technol. 11, 115-127. 

The most popular dynamic test procedure for viscoelastic behavior is the application of an oscillatory stress of small amplitude. This shear stress applied produces a corresponding strain in the material. If the material were an ideal Hookean body, the shear stress and shear strain rate waves would be in phase (Fig. 14A), whereas for an ideal Newtonian sample, there would be a phase shift of 90° (Fig. 14B), because for Newtonian bodies the shear strain is at a maximum, when a maximum of stress is present. The shear strain, when assuming an oscillating sine fimction, is at a maximum in the middle of the slope, because there is the steepest increase in shear strain due to the change in direction. For a typical viscoelastic material, the phase shift will have a value between >0° and <90° (Fig. 14C). 

Small-Amplitude Oscillatory Motion ( Dynamic Testing )... 

Dynamic Testing. Gels can be characterized by dynamic test in which an oscillatory stress is applied to the gel and the phase... 

Shear modulus can be determined by a static torsion test or by a dynamic test using primarily a torsional pendulum (ASTM D 2236). Also used is an oscillatory rheometer test. The torsional pendulum is applicable to virtually all plastics and uses a simple specimen readily fabricated by all commercial fabricating processes or easily cut from fabricated part. The moduli of elasticity, G for shear and E for tension, are ratios of stress to strain as measured within the proportional limits of the material. Thus the modulus is really a measure of the rigidity for shear of a material or its stifihess in tension and compression. For shear or torsion, the modulus analogous to that for tension is called the shear modulus or the modulus of rigidity, or sometimes the transverse modulus. 

The viscoelastic behavior is evaluated by means of two types of methods static tests and dynamic tests. In the first calegtuy a step change of stress or strain is applied and the stress or strain response is recorded as a function of time. Stress relaxation, creep compliance, and creep recovery are static methods. The dynamic tests involve the imposition of an oscillatory strain or stress. Every technique is described in the following sections. 

There are a number of commercial machines available for dynamic testing under varying temperature conditions. An actuator imposes an oscillatory (linear or angular) displacement, and typically a strain-gauge load cell measures force. Testing is frequently in bending mode, producing results that may be difficult to... 

As is the case for any linear system in the frequency domain, the results of an oscillatory shear test can be represented in terms of an amplitude ratio G = 0(JYq and a phase shift 5 which are functions of frequency. While the function S(0)) is sometimes used to characterize the linear behavior of a melt, dynamic test results are usually reported in terms of the storr e and loss moduli G and G" as functions of frequency ... 

Figure 4 shows comparisons of the capillary flow curves tested at three temperatures, the oscillatory dynamic viscosity, and the rotational steady shear viscosities tested at 210°C... 

As an extension of the dynamic test in the linear regime, a large-amplitude oscillatory shear (LAOS), y(t) = /o sin or, is often applied to a material to measure the oscillating shear stress (Ts (and sometimes the first normal stress difference, too). In the stationary state, cTs under LAOS is contributed not only from the fundamental harmonics (oscillating at the angular frequency of strain, cu) but also from the higher order (mostly odd) harmonics and can be expressed as... 

Rheometric Scientific markets several devices designed for characterizing viscoelastic fluids. These instmments measure the response of a Hquid to sinusoidal oscillatory motion to determine dynamic viscosity as well as storage and loss moduH. The Rheometric Scientific line includes a fluids spectrometer (RFS-II), a dynamic spectrometer (RDS-7700 series II), and a mechanical spectrometer (RMS-800). The fluids spectrometer is designed for fairly low viscosity materials. The dynamic spectrometer can be used to test soHds, melts, and Hquids at frequencies from 10 to 500 rad/s and as a function of strain ampHtude and temperature. It is a stripped down version of the extremely versatile mechanical spectrometer, which is both a dynamic viscometer and a dynamic mechanical testing device. The RMS-800 can carry out measurements under rotational shear, oscillatory shear, torsional motion, and tension compression, as well as normal stress measurements. Step strain, creep, and creep recovery modes are also available. It is used on a wide range of materials, including adhesives, pastes, mbber, and plastics. 

Torsion property As noted, the shear modulus is usually obtained by using pendulum and oscillatory rheometer techniques. The torsional pendulum (ASTM D 2236 Dynamic Mechanical Properties of Plastics by Means of a Torsional Pendulum Test Procedure) is a popular test, since it is applicable to virtually all plastics and uses a simple specimen readily fabricated by all commercial processes or easily cut from fabricated products. 

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