Fluids, nature rheology

Fluid nature has been called many things, such as rheological property, flow behavior, or the specific nature displayed by a slip. The practical implication is that the fluid being cast changes in viscosity... 

Any rheometric technique involves the simultaneous assessment of force, and deformation and/or rate as a function of temperature. Through the appropriate rheometrical equations, such basic measurements are converted into quantities of rheological interest, for instance, shear or extensional stress and rate in isothermal condition. The rheometrical equations are established by considering the test geometry and type of flow involved, with respect to several hypotheses dealing with the nature of the fluid and the boundary conditions the fluid is generally assumed to be homogeneous and incompressible, and ideal boundaries are considered, for instance, no wall slip. 

The comparatively less elastic nature of many food products makes the analyses of extensional rheology data easier than has been possible with polymeric fluids. This relative ease of data interpretation provides an opportunity both for learning the extensional behavior of materials and for effective application of extensional rheometry in the food industry. 

The most straightforward rheological behaviour is exhibited on the one hand by Newtonian viscous fluids and on the other by Hookean elastic solids. However, most materials, particularly those of a colloidal nature, exhibit mechanical behaviour which is intermediate between these two extremes, with both viscous and elastic characteristics in evidence. Such materials are termed viscoelastic. 

As velocity of flow increases, a condition is eventually reached at which rectilinear laminar flow is no longer stable, and a transition occurs to an alternate mode of motion that always involves complex particle paths. This motion may be of a multidimensional secondary laminar form, or it may be a chaotic eddy motion called turbulence. The nature of the motion is governed by both the rheological nature of the fluid and the geometry of the flow boundaries. 

Computer simulations of nanoscopic confined fluids have revealed many details of the dynamics under confinement. The nature of the confined fluids - especially in the immediate vicinity of attractive surface - has been shown to be strongly altered by the confining surfaces, and this is manifested by a behavior dramatically different from the bulk fluids in the local relaxation [38a], the mobility [38c] and rheological properties [39] of molecules near adsorbing surfaces. For monomeric systems many computer simulation studies [40] provide a clear enough picture for the dynamics of confined films of small spherical molecules. On the other hand, for confined oligomers and polymers less has been done, especially towards the understanding of the dynamics of nanoscopic films [41]. 

There are three chapters in this volume, two of which address the microscale. Ploehn and Russel address the Interactions Between Colloidal Particles and Soluble Polymers, which is motivated by advances in statistical mechanics and scaling theories, as well as by the importance of numerous polymeric flocculants, dispersants, surfactants, and thickeners. How do polymers thicken ketchup Adler, Nadim, and Brenner address Rheological Models of Suspensions, a closely related subject through fluid mechanics, statistical physics, and continuum theory. Their work is also inspired by industrial processes such as paint, pulp and paper, and concrete and by natural systems such as blood flow and the transportation of sediment in oceans and rivers. Why did doctors in the Middle Ages induce bleeding in their patients in order to thin their blood ... 

Polyethylene is a man-made homopolymer. Its chemical synthesis is well understood. It is a random walk polymer with little secondary or tertiary structure. A batch can largely be characterised by its molecular weight distribution, and its rheology can be related to these parameters by developed rules of polymer behaviour. The action of specific chemicals as plasticisers can be used to modulate these bulk properties in a predictable way, allowing the nature and characterisation of its glass to fluid transition to be predicted. 

Because the performance within the extmder and the nature of the post-die extru-date will be influenced by the properties of the final viscous fluid produced, many workers have attempted to characterise the rheology of this melted mass (e.g., Jao et al. 1978 Akdogan et al. 1997). Since its stmcture is either that of a polymer melt or a dispersion of highly swollen particles within a polymer melt, then complex... 

The most widely used synthetic and natural enhanced oil recovery polymers, such as partially hydrolyzed polyacrylamide, carboxymethyl(ethyl) cellulose, polysaccharides, or xanthan gums, are not suitable for high-temperature reservoirs (> 90 °C) with high-density brine fluid due to excessive hydrolysis and precipitation [277]. The main advantages of polymeric betaines over the mentioned standard polymers are (1) thermostability (up to 120 °C) (2) brine compatibility and (3) viscosification in brine solution [278]. Carbobetaines grafted onto hydroxyethyl cellulose were tested as a drilling-mud additive for clay hydration inhibition and mud rheological control [279]. An increase in the content of carbobetaine moieties resulted in an enhanced inhibitive abiUty, especially for sahne mud. 

Techniques for measuring rheological properties of polymeric materials have been well described previously by others (e.g., Whorlow, 1980 Macosko, 1994). The text by Van Wazer et al. (1963) is still a valuable reference that explains in detail many facets of earlier attempts to measure rheological properties of polymeric materials as well as basic equations of viscometric flows. The unique nature of fluid foods prompted this author to review both the rheological properties of fluid foods and their measurement about 30 years ago (Rao, 1977a, 1977b). Subsequent efforts on rheology of foods include those of Rao (1992, 2005) and Steffe (1996). 

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