Rheological measurement method

Additional complications can occur if the mode of deformation of the material in the process differs from that of the measurement method. Most fluid rheology measurements are made under shear. If the material is extended, broken into droplets, or drawn into filaments, the extensional viscosity may be a more appropriate quantity for correlation with performance. This is the case in the parting nip of a roUer in which filamenting paint can cause roUer spatter if the extensional viscosity exceeds certain limits (109). In a number of cases shear stress is the key factor rather than shear rate, and controlled stress measurements are necessary. 

Rheo-NMR [86] methods have been shown to be well-suited to emulsion rheology studies [28] and could be combined with any of the topics described above. The combination of structural and rheological measurements is a promising area for further research. 

The estimation of degree of dispersion can be made indirectly by measurement of electrical methods or measurement of mechanical properties. Boonstra54 used a coaxial electrode system to estimate dispersion form electrical resistivity whilst Belokur et al55 investigated the possibility of assessing dispersion from rheological measurements. 

The ODT can be located via a number of methods. From rheological measurements, the ODT is identified from a sharp decrease in the low frequency isochronal dynamic elastic moduli obtained during a heating ramp (Bates et al. 1990 Rosedale and Bates 1990) (Fig. 2.4(a)) however, this method does not generally provide a well-defined temperature for the ordered BCC-disordered phase... 

D.F. James and K. Walters, A Critical Appraisal of Available Methods for the Measurement of Extensional Properties of Mobile Systems, in "Techniques in Rheological Measurement", A.A. Collyer (ed.), Chapman and Hall, London, 1993, pp. 33-53. 

There are a number of types of rheological measurement, some are appropriate for Newtonian fluids only, while others may be used for Newtonian or non-Newtonian fluids. Some of the principal types are listed in Table 6.4. Some very useful descriptions of experimental techniques have been given by Whorlow [355] and others [215,352,353,356,357]. The principal methods are discussed in the next several sections. 

In systems with suspended solids, rheologic measurements are difficult to perform owing to settling in the measurement devices. Conventional methods for measuring rheologic properties (cone-and-plate, concentric cylinder, and rotating-bob viscometers) do not produce accurate and reliable data for some solid suspensions. 

The rotational viscosity method described above to measure working life or pot life is a form of rheological measurement of cure. However, cone and plate rheometry is preferred for accurate measurements because the specimen size and geometry are similar to those that occur in an adhesive joint. 

James DF and Walters K, "A Critical Appraisal of Available Methods for the Measurement ofExtensional Properties of Mobile Systems" in Collyer AA (Ed), "Techniques in Rheological Measurement", Chapman Hall, London, 1993. Janeschitz-Kriegl H, "Flow Birefringence of Elastico-Viscous Polymer Systems", Adv Polym Sci 6 (1969) 170-318. 

Another well documented, but less widely used method for formulation development is the measurement of electrokinetic properties [14]. These tests require more disdnc-tive/elaborate sample preparation and are mosdy restricted to use during development. Also widely used and relied on are rheological measurements. The scope of rheological measurements ranges from viscosity measurements to the determination of yield points or oscillatory properties, such as the G and G -modules [15]. Since suspoemulsions are not ideal viscous but mostly viscoelastic or dilatant, a wide range of characterization techniques exists. Instrumentation required for this are simple rotary viscosimeters (Brookfield) or more sophisticated stress or shear controlled rotational viscosimeters. 

The idea of gel point has received much attention in synthetic and biopolymer gels. One can talk about gel point as an instant in time or as a specific temperature. In this chapter, the symbols and Tgei will be used for the gel time and gel temperature (Tobitani and Ross-Murphy, 1997a Foegeding et al., 1998), respectively. One can imagine many qualitative definitions of gel point, especially based on visual observation of individuals. Here, however, methods based on rheological measurements are emphasized. 

While rheological measurements are wonderfully quantitative, they are usually poor qualitative probes of fluid structure. This is because in rheological experiments, the structural changes responsible for the measured relaxation behavior remain hidden. Thus, rheometry is often most useful when supplemented by other experimental methods that characterize fluid structure and flow-induced structural changes. Some of the most useful methods are microscopy, light, x-ray, and neutron scattering, and polarimetry. 

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