Oscillatory tests

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... 

Figure H3.1.1 Responses from purely elastic or purely viscous foods under oscillatory testing. The controlled parameters of the oscillatory applied stress or strain are amplitude, frequency, temperature, and duration of measurement. The measured parameters are amplitude and phase shift (8) of the strain or stress response.

The stresses used in a creep test are chosen in two ways. First, a value is chosen from oscillatory tests (specifically stress or strain amplitude sweeps at 1 Hz or 10 rad/sec unit hs.i) to define the linear region. Using two to five different values, the sample is taken from a linear viscoelastic response to the onset of... 

Ritman, E.L., Borah, B., Dufresne, T.E., Phipps, R.J., Sacha, J.P., Jorgensen, S.M., and Turner, R.T. 2002. 3-D Synchrotron pCT allows unique insight of changes in bone quality. The American Society for Bone and Mineral Research Annual Meeting, San Antonio TX, September 20-24. Ross, K.A., Pyrak-Nolte, L.J., and Campanella, O.H. 2004. The use of ultrasound and shear oscillatory tests to characterize the effect of mixing time on the rheological properties of dough. Food Res. Int. 37, 567-577. 

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. 

The inks were examined by using transient, steady flow, and oscillatory tests performed at room temperature, 22 0.5 C. 

The viscosity obtained from the above equation in the linear region of a creep experiment can be used to extend the low-shear rate region of apparent viscosity versus shear rate data obtained in a flow experiment by about two decades (Giboreau et al., 1994 Rayment et al., 1998). The low shear rate region of about 10 -10 is often used for the characterization and differentiation of structures in polysaccharide systems through the use of stress controlled creep and non destructive oscillatory tests. The values of strain (y) from the creep experiment can be converted to shear rate from the expression y t) = y t)/t. 

The viscosity values in oscillatory shear at low frequency, especially for the LM pectin solutions for which the oscillatory tests were extended to lower frequency... 

Using storage modulus (O ) obtained from dynamic oscillatory tests to calculate the number of disulfide bridges per molecule... 

Using plateau storage modulus (G t obtained from small amplitude oscillatory tests to calculate molecular weight between cross-links (Wfc)... 

Me from plateau G obtained from oscillatory tests Me from plateau G obtained from oscillatory tests... 

Details of both continuous shear and dynamic oscillatory testing of pharmaceutical semisolids, including gels, have been addressed in reviews, where the fundamental viscoelastic principles are further developed along with a discussion of data analyses and interpretations. 

Davis SS, Khanderia MS. Rheological characterization of Plastibases and the effect of formulation variables on the consistency of these vehicles part 3 oscillatory testing. Int J Pharm Technol Prod Manuf 1981 2(Apr) 13-18. 

Fig. 4 Dynamic oscillatory test for BDP/clay nanocomposites (BDP series) [G (filled), G (open)]. The BDP 0, BDP 3, BDP 6, and BDP 9 indicate clay contents of 0, 3, 6, and 9%, respectively, in PCN. (View this art in color at www. dekker.com.)...

The structure of Immiscible blends Is seldom at equilibrium. In principle, the coarser the dispersion the less stable It Is. There are two aspects of stability Involved the coalescence In a static system and deformability due to flow. As discussed above the critical parameter for blend deformability Is the total strain In shear y = ty, or In extension, e = te. Provided t Is large enough In steady state the strains and deformations can be quite substantial one starts a test with one material and ends with another. This means that neither the steady state shearing nor elongatlonal flow can be used for characterization of materials with deformable structure. For these systems the only suitable method Is a low strain dynamic oscillatory test. The test Is simple and rapid, and a method of data evaluation leading to unambiguous determination of the state of miscibility is discussed in a later chapter. 

FIGURE 6.3 Principle using a dynamic oscillatory test to determine relaxation time. 

Typically, in dynamic oscillatory testing, a sinusoidal (oscillatory) small-amplitude stress is applied to the sample and the mechanical response measured as functions of both oscillatory frequency and, in some instances, temperature. 

A biconical disk interfacial rheometer is available from Anton Paar, known as the Physica Interfacial Rheology System (IRS). A schematic of the rheometer tool is shown in Figure 4.22. Current specifications of the instrument include a torque range of 0.02 xNm to 150 mNm with temperature control from 5 to 70°C. All rheological test modes are available for the interfacial rheometer including oscillatory testing [59],... 

Given that nonlinearities are ubiquitous, testing with oscillatory excitation is of less practical importance in contact mechanics than in other fields of material science. For instance, stick-slip motion is most easily studied by steadily pulling the object of interest across the supporting substrate. Oscillatory testing will result in complicated trajectories [17]. Sinusoidal excitation mostly... 

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