Experimental methods, rheology

There are several other comparable rheological experimental methods involving linear viscoelastic behavior. Among them are creep tests (constant stress), dynamic mechanical fatigue tests (forced periodic oscillation), and torsion pendulum tests (free oscillation). Viscoelastic data obtained from any of these techniques must be consistent data from the others. 

The title of the book, Optical Rheometry of Complex Fluids, refers to the strong connection of the experimental methods that are presented to the field of rheology. Rheology refers to the study of deformation and orientation as a result of fluid flow, and one principal aim of this discipline is the development of constitutive equations that relate the macroscopic stress and velocity gradient tensors. A successful constitutive equation, however, will recognize the particular microstructure of a complex fluid, and it is here that optical methods have proven to be very important. The emphasis in this book is on the use of in situ measurements where the dynamics and structure are measured in the presence of an external field. In this manner, the connection between the microstructural response and macroscopic observables, such as stress and fluid motion can be effectively established. Although many of the examples used in the book involve the application of flow, the use of these techniques is appropriate whenever an external field is applied. For that reason, examples are also included for the case of electric and magnetic fields. 

Bremer et al. (1989) described a detailed experimental method used to measure the permeability fractal dimension of a fat crystal network. Similarly to the rheology fractal dimension, the permeability fractal dimension, Dp, could be obtained from the nonlinear regression between Q and O as shown in Figure 17.23 (Tang and Marangoni 2005). 

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. 

Possibly the most typical property of a liquid-fluid interface is that it cannot be autonomous it only exists as the boundary between two adjacent bulk fluids. Any movement or flow in an interface will cause some corresponding motion in the adjacent bulk phases and vice versa. To identify interfacial (excess) rheological properties, measured rheological properties of the system have to be divided into two parts, one attributable to the interface and one to the bulk. Such a division is always somewhat arbitrary and may depend on the experimental method used. 

M. Joly, Rheological Properties of Monomolecular Films, in Surface and Colloid Science, E. Matljevic, M., Vol. 5 Wiley (1972), Part I Basic Concepts and Experimental Methods, p. 1 Part II Experimental Results. Theoretical Interpretation, Applications, p. 79. (Extensive review with several illustrations further reading to secs. 3.6 and 3.7e.)... 

Moreover, adsorption isotherms, or equations of state, represent the basis for the evaluation of adsorption kinetics and rheological properties of adsorption layers. Exact equilibrium values of surface or interfacial tensions are necessary to determine adsorption isotherms. For surfactants of low surface activity (for example, sodium octyl or decyl sulphate, hexanol or hexanoic acid) the adsorption reaches its equilibrium state in a time of the order of seconds to minutes. Higher surface activity results in greater times for establishing the equilibrium state of adsorption which sometimes cannot be realised by available experimental methods. To avoid long-time experiments, extrapolations were often carried out in order to get equilibrium values. Different extrapolation procedures as well as criteria of an equilibrium state of adsorption are discussed in the literature (cf Miller Lunkenheimer 1983). 

The present chapter gives also detailed introduction to a large number of experimental methods, suitable for studying dynamic interfacial tensions. The methods are discussed in terms of the available time window. There are methods which complement each other such that a time interval from less than 100 microseconds up to hours and days of adsorption time can be covered (about ten orders of magnitude). The relaxation methods, also suitable for detecting the adsorption mechanism of surfactant s adsorption provide in addition the dilational rheology of interfacial layers. It is discussed that in particular these dilational rheological studies are most informative in respect to adsorption mechanisms, as the data interpretation includes the thermodynamic model as well as the adsorption dynamics. 

Non-Newtonian liquids are used in numerous microfluidics applications, including microscale viscosity and rheology measurements, amplification and sequencing of DNA, fundamental investigations of elastic flows, and development of fluidic memory and control devices. Although these applications span a wide range of flow conditions and non-Newtonian fluid properties, similar experimental methods are used. In this section we summarize some of the experimental... 

Dilute solution viscometry and melt rheology are fundamental techniques for characterizing polymeric materials. Given PLA s growing importance, it is important to understand the fundamental chain properties that are reflected in the data obtained using these experimental methods. 

AU these models require the values of diffusion coefficients and interaction parameters, which are, in most cases, not accurately known. Their measurement needs the implementation of some experimental methods as soft X-ray or neutron scattering, infrared spectroscopy, and application of rheological techniques [28]. In theoretical works, these coeflBcients are often empirically introduced. 

The basic principles of rheology and the various experimental methods that can be applied to investigate these complex systems of food colloids have been discussed in detail in Chapter 7. Only a brief summary is given here. Two main types of measurements are required (1) Steady-state measurements of the shear stress versus shear rate relationship, to distinguish between the various responses Newtonian, plastic, pseudo-plastic and dilatant. Particular attention should be given to time effects during flow (thixotropy and negative thixotropy). (2) Viscoelastic behaviour, stress relaxation, constant stress (creep) and oscillatory measurements. 

The properties of these systems were studied with the experimental methods that we have already mentioned earlier in this chapter (1) the rheological properties of the interfacial layers using the torsion pendulum device (Figure 4.11) (2) the investigation of the compression of two nonpolar droplets immersed in an aqueous surfactant solution and the measurement of the force, Xoai. necessary for their coalescence and (3) the estimation of the free energy of interaction between... 

The present book is composed of the seven chapters that cover the material used as the basis for the lecture course on physical-chemical mechanics and basic material essential for understanding the content from the areas of colloid and surface chemistry, strength of materials, rheology, and tensors. Such coverage makes the book suitable for readers who do not have extensive knowledge and expertise in any of these areas. The book also devotes a lot of attention to the experimental methods used in physical-chemical mechanics and the relevant instruments, many of which were built and developed over the years by collaborators of Professor Shchukin. Where appropriate and... 

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