Oscillatory sweep

Figure H3.1.3 For oscillatory (sweep) testing, four control parameters can be varied amplitude, frequency, time, and temperature.

An alternative rheological technique for assessing flocculation involves oscillatory measurements which, as noted above, can include two sets of experiments, namely strain and oscillatory sweep measurements. 

In oscillatory measurements, two sets of experiments are carried out, namely strain sweep measurements and oscillatory sweep measurement. 

In oscillatory measurements one carries out two sets of experiments, strain sweep and oscillatory sweep, which are detailed below. 

In the oscillatory sweep experiment, the strain amplitude is kept constant in the linear viscoelastic region (one usually takes a point far from y, but not too low, i.e. in the midpoint of the linear viscoelastic region) and measurements are carried out as a function of frequency. This is schematically represented in Fig. 3.50 for a viscoelastic liquid system. 

Figure 16 shows two examples of the dynamic behavior of the system as obtained by applying oscillatory sweeps of the same amplitude /U.0 = 0.3 eV but different periods r. 

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... 

The evolution of the dynamic viscosity rp (co, x) or of the dynamic shear complex modulus G (co.x) as a function of conversion, x, can be followed by dynamic mechanical measurements using oscillatory shear deformation between two parallel plates at constant angular frequency, co = 2irf (f = frequency in Hz). In addition, the frequency sweep at certain time intervals during a slow reaction (x constant) allows determination of the frequency dependence of elastic quantities at the particular conversion. During such experiments, storage G (co), and loss G"(co) shear moduli and their ratio, the loss factor tan8(co), are obtained ... 

Formulating appropriate rate laws for CO adsorption, OH adsorption and the reaction between these two surface species, a set of four coupled ordinary differential equations is obtained, whereby the dependent variables are the average coverages of CO and OH, the concentration of CO in the reaction plane and the electrode potential. In accordance with the experiments, the model describes the S-shaped I/U curve and thus also bistability under potentiostatic control. However, neither oscillatory behavior is found for realistic parameter values (see the discussion above) nor can the nearly current-independent, fluctuating potential be reproduced, which is observed for slow galvanodynamic sweeps (c.f. Fig. 30b). As we shall discuss in Section 4.2.2, this feature might again be the result of a spatial instability. 

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. 

Moreover, rotational rheometers can be used in dynamic oscillatory mode, frequency sweep, to assess the elastic G module as well as the viscous G" module and the correlated phase angle 6, as a function of the frequency co. G and G" allow to study the viscoelastic behaviour of HA macromolecules. Fig. (15) shows the frequency sweep curves (G, G", and tg(5) vs. the frequency co) for an HA sample (Mw=1350 kDa, polydispersity index D=1.6, concentration c = 2%) at 20 °C. 

No influence of strain on the viscoelastic properties was found. A strain sweep to 14% oscillatory strain was performed with frequencies ranging from 1 rad/sec to 10 rad/sec. and gave an essentially linear viscoelastic respons. 

Rheology. The rheological properties of the blends and their components were determined on a Rheometrics Mechanical Spectrometer (RMS 800). Three kinds of dynamic oscillatory measurements (i.e. temperature, time, and frequency sweeps) were carried out. All experiments were done by using a parallel plate attachment with a radius of 12.5 mm and a gap setting from 1.2 to 1.8 mm. There was no significant dependence of the experimental results on the gap setting. 

For most oscillatory measurements, one has to control multiple variables, ramp (cover a range of values with a set rate of change) one variable, and measure several more variables. Classical oscillatory measurements included on most systems are stress sweeps, frequency sweeps, time sweeps, and temperature sweeps (if a pettier unit is included). 

Figure 12.7 shows, as ejamples, oscillatory functions obtained through frequency-sweep tests at several weight ratios (R) of amphiphile/CigSE, where, in all cases, a 90 wt% water and a 10 wt% total surfactant -F cosurfactant was maintained for allR. 

Dynamic mechanical properties are measured to evaluate melt rheology of thermoplastics with and without additives which may modify rheological characteristics of these compositions. " Dynamic oscillatory shear rheometers are used for these purposes. Two geometries of test fixtures are used including parallel plates and cone and plate. Instrument use for these measurements must be capable of measuring forces (stress or strain) and frequency. Temperature must be controlled in a broad range and various modes of temperature sweeps should be available. Sample geometry is not specified but it should be suitable for measurement in particular experimental setup. 

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