Mechanical properties viscous fluids

Visco-elastic models have been developed for the nonlinear mechanical properties of fluids and solids. For a viscous fluid in simple shear flow, the shear stress, r y y), is a function of the effective viscosity, rj(-y) and the shear rate, y, as follows ... 

Polymers have found widespread applications because of their mechanical behaviour. They combine the mechanical properties of elastic solids and viscous fluids. Therefore, they are regarded as viscoelastic materials. Viscoelastic... 

Aliphatic polyesters are low-melting (40-80°C) semicrystalline polymers or viscous fluids and present inferior mechanical properties. Notable exceptions are poly (a-hydroxy acid)s and poly (ft -hydroxy acid)s. 

Other uses of thickening agents include pharmaceutical preparations, paper production, and oil well drilling fluids. This latter use is necessary because oil is obtained from rock that is porous. In order to remove the oil without altering the mechanical properties of the porous rock, viscous liquids ( drilling fluids ) are pumped into the rock to replace the oil. Among the substances that can be used for this purpose are thickened aqueous solutions of polymers such as poly(acrylic acid) or poly(acrylonitrile). 

As noted before, thin film lubrication (TFL) is a transition lubrication state between the elastohydrodynamic lubrication (EHL) and the boundary lubrication (BL). It is widely accepted that in addition to piezo-viscous effect and solid elastic deformation, EHL is featured with viscous fluid films and it is based upon a continuum mechanism. Boundary lubrication, however, featured with adsorption films, is either due to physisorption or chemisorption, and it is based on surface physical/chemical properties [14]. It will be of great importance to bridge the gap between EHL and BL regarding the work mechanism and study methods, by considering TFL as a specihc lubrication state. In TFL modeling, the microstructure of the fluids and the surface effects are two major factors to be taken into consideration. 

Mechanical and chemical methods for qualitative and quantitative measurement of polymer structure, properties, and their respective processes during interrelation with their environment on a microscopic scale exist. Bosch et al. [83] briefly discuss these techniques and point out that most conventional techniques are destructive because they require sampling, may lack accuracy, and are generally not suited for in situ testing. However, the process of polymerization, that is, the creation of a rigid structure from the initial viscous fluid, is associated with changes in the microenvironment on a molecular scale and can be observed with free-volume probes [83, 84]. 

To understand the mechanism of the turbulent mixing process occurring in pipe reactors, we have to consider first some of the properties of fluid flow in pipes. Resistance to fluid flow in a pipe has two components, the viscous friction of the fluid itself within the pipe, which increases as the fluid viscosity increases, and the pressure differential caused by a liquid level difference or a pressure difference between the two vessels. 

The vast majority of concentrated dispersions, such as LADDs, exhibit both viscous and elastic properties. These systems are therefore referred to as viscoelastic. The flow properties discussed in the previous section are not sufficient for complete rheological characterization of viscoelastic fluids. Dynamic mechanical properties, characterized by the storage modulus (G ) and loss modulus (G"), are normally... 

It is convenient to describe these properties in terms of the following mechanical models [396] the Hooke body (an elastic spring), the Saint-Venant body modeling dry friction (a bar on a solid surface), and the Newton body (a piston in a vessel filled with a viscous fluid). By using various combinations of these elementary models (connected in parallel and/or in series), one can describe situations which are rather complex from the rheological viewpoint. 

The mechanical properties of the nucleus, the stiffest component of the cell, are important for the overall cellular response. It is, probably, even more significant that forces transmitted from the cell surface and acting on the nucleus can alter gene expression and protein synthesis. Kan et al. (1999a) have modeled the nucleus as a viscous fluid and analyzed the effect of the nucleus on the leukocyte recovery. Guilak et al. [2000] have estimated the linear viscoelastic properties of nuclei of chondrocytes. Caille et al. [2002] used a model of nonlinear elastic material to estimate Young s modulus of endothelial cell nuclei. Recently, Dahl et al. [2004], by using the micropipette technique, have estimated the mechanical properties of the cell s nuclear envelope. 

FIGURE 103 Mechanical resistance and reactance of soft thigh tissue (2 cm in diameter) in vivo from 10 Hz to I MHz. The measured values (open circles—resistance diamonds— reactance) are compared with the calculated resistance and reactance of a 2-cm-diameter sphere vibrating in a viscous, elastic compressible medium with properties similar to soft human tissue (continuous lines, curves A). The resistance is also shown for the sphere vibrating in a frictionless compressible fluid (acoustic compression wave, curve B) and an incompressible viscous fluid (curve Q. (von Gierke el at., 1952.)... 

It must be recalled that, in the Formal Graph approach, the notion of conductor is generalized to any device or material possessing the constitutive property of conductance. This includes the classical concept of conductor of particles (charges, molecules, etc.) but also conductors of entities that are momenta, impulses, volumes, lengths, and so on. Friction, for instance, is a conduction process of mechanical momentum in a viscous fluid or of geometric entities (surfaces, etc.) in a solid (in that case one speaks of internal friction between solid elements). 

The mechanical properties of highly filled elastomers have led to their use as solid propellants in rocketry. Composite propellants consist of elastomers highly filled with inorganic oxidiser. Mixing is effected in an uncrosslinked state in which the polymer still has a low MW and the consistency of a viscous fluid. The compounded mixture is cast or extruded into the desired shape and hardened by polymerisation... 

The rheological and thermal conductivity properties of the polymer matrix determine the heat necessary to melt the material. Relatively shear-sensitive materials become less viscous as they pass through the nozzle. For example, nylons form low viscous fluids when melted while polyethylene can undergo considerable mechanical working, producing heat necessary for plastica-tion. 

As discussed briefly in the next section, polymers have a unique response to mechanical loads and are properly treated as materials which in some instances behave as elastic solids and in some instances as viscous fluids. As such their properties (mechanical, electrical, optical, etc.) are time dependent and cannot be treated mathematically by the laws of either solids or fluids. The study of such materials began long before the macromolecu-lar nature of polymers was understood. Indeed, as will be evident in later chapters on viscoelasticity, James Clerk Maxwell (1831-79), a Scottish physicist and the first professor of experimental physics at Cambridge, developed one of the very first mathematical models to explain such peculiar behavior. Lord Kelvin (Sir William Thomson, (1824-1907)), another Scottish physicist, also developed a similar mathematical model. Undoubtedly, each had observed the creep and/or relaxation behavior of natural materials such as pitch, tar, bread dough, etc. and was intrigued to explain such behavior. Of course, these observations were only a minor portion of their overall contributions to the physics of matter. 

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