Rheology viscous flow

A third definition of surface mobility is essentially a rheological one it represents the extension to films of the criteria we use for bulk phases and, of course, it is the basis for distinguishing states of films on liquid substrates. Thus as discussed in Chapter IV, solid films should be ordered and should show elastic and yield point behavior liquid films should be coherent and show viscous flow gaseous films should be in rapid equilibrium with all parts of the surface. 

One simple rheological model that is often used to describe the behavior of foams is that of a Bingham plastic. This appHes for flows over length scales sufficiently large that the foam can be reasonably considered as a continuous medium. The Bingham plastic model combines the properties of a yield stress like that of a soHd with the viscous flow of a Hquid. In simple Newtonian fluids, the shear stress T is proportional to the strain rate y, with the constant of proportionaHty being the fluid viscosity. In Bingham plastics, by contrast, the relation between stress and strain rate is r = where is... 

Rheology. Both PB and PMP melts exhibit strong non-Newtonian behavior thek apparent melt viscosity decreases with an increase in shear stress (27,28). Melt viscosities of both resins depend on temperature (24,27). The activation energy for PB viscous flow is 46 kj /mol (11 kcal/mol) (39), and for PMP, 77 kJ/mol (18.4 kcal/mol) (28). Equipment used for PP processing is usually suitable for PB and PMP processing as well however, adjustments in the processing conditions must be made to account for the differences in melt temperatures and rheology. 

Rheology is the science of the deformation and flow of matter. It is concerned with the response of materials to appHed stress. That response may be irreversible viscous flow, reversible elastic deformation, or a combination of the two. Control of rheology is essential for the manufacture and handling of numerous materials and products, eg, foods, cosmetics, mbber, plastics, paints, inks, and drilling muds. Before control can be achieved, there must be an understanding of rheology and an ability to measure rheological properties. 

The study of flow and elasticity dates to antiquity. Practical rheology existed for centuries before Hooke and Newton proposed the basic laws of elastic response and simple viscous flow, respectively, in the seventeenth century. Further advances in understanding came in the mid-nineteenth century with models for viscous flow in round tubes. The introduction of the first practical rotational viscometer by Couette in 1890 (1,2) was another milestone. 

Caustic Waterflooding. In caustic waterflooding, the interfacial rheologic properties of a model crude oil-water system were studied in the presence of sodium hydroxide. The interfacial viscosity, the non-Newtonian flow behavior, and the activation energy of viscous flow were determined as a function of shear rate, alkali concentration, and aging time. The interfacial viscosity drastically... 

Rheological data are in agreement with this. While a particle flow mechanism was shown to apply below this temperature range, viscous flow occured above it.8... 

Different types of liquid crystals exhibit different rheological properties [16,17]. With an increase in organization of the microstructure of the liquid crystal its consistency increases and the flow behavior becomes more viscous. The coefficient of dynamic viscosity r, although a criterion for the viscosity of ideal viscous flow behavior (Newtonian systems), is high for cubic and hexagonal liquid crystals but fairly low for lamellar ones. However, the flow characteristics are not Newtonian but plastic or pseudoplastic, respectively. 

Mack (58, 59) points out that asphaltenes from different sources in the same petro-lenes give mixtures of approximately the same rheological type, but sols of the same asphaltenes in different petrolenes differ in flow behavior. Those in aromatic petrolenes show viscous behavior and presumably approach true solution. Those in paraffinic media show complex flow and are considered to be true colloidal systems. Pfeiffer and associates (91) consider that degree of peptization of asphaltene micelles determines the flow behavior. Thus, a low concentration of asphaltenes well peptized by aromatic petrolenes leads to purely viscous flow. High concentrations of asphaltenes and petrolenes of low aromatic content result in gel-type asphalts. All shades of flow behavior between these extremes are observed. 

For a nematic polymer in a transition region from LC to isotropic state, maximal viscosity is observed at low shear rates j. For a smectic polymer in the same temperature range only a break in the curve is observed on a lgq — 1/T plot. This difference is apparently determined by the same reasons that control the difference in rheological behaviour of low-molecular nematics and smectics 126). A polymeric character of liquid crystals is revealed in higher values of the activation energy (Ef) of viscous flow in a mesophase, e.g., Ef for a smectic polymer is 103 kJ/mole, for a nematic polymer3 80-140kJ/mole. 

Plastisol properties depend on a number of factors, the most important among which are the nature of the polymer (PVC), the identity of plasticizer and other additives, the composition of formulations, the conditions of preparation and storage, the load-velocity (first of all, stress and shear rate), and temperature conditions of processing. Depending on these parameters, the character of plastisol flow may vary widely. That is why, in spite of a number of papers devoted to the study of the rheological (viscous) properties of plastisols (e.g. 2,5 37,38)), these still can not be considered as well studied and widely accepted. 

Janeschitz-Kriegl H, "Flow-birefringence of elasto-viscous polymer systems", Adv Polym Sci, 6 (1969) 170. Janeschitz-Kriegl H, "Polymer Melt Rheology and Flow Birefringence", Springer Verlag, New York, 1983. 

Plastic solids derive their functionality from their unique plastic nature. Three conditions are essential for plasticity (5) (1) both liquid and solid phases must be present (2) the solid phase must be so finely dispersed that the entire solid-liquid matrix can be effectively bound together by internal cohesive forces and (3) proper proportions must exist between the phases. Incorrect phase ratios adversely influence product rheology. For example, deficient solids content may result in oil separation, whereas excessive solids can cause hardness or brittleness instead of the desired viscous flow. 

Rheological properties of the oligoethylsiloxane-based compositions with additives are of vital importance for the selection of maintenance conditions of various items, but they have not been studied yet. No data on such composition-fluidity relationships (unconfined fluctuating free volume and viscous flow activation energy values) are available. 

Therefore, the study of rheological properties, the determination of the activation energy of viscous flow, composition-fluidity relationships, and key factor effects of the lubricity-promoting additive on the fluidity of oligoethylsiloxane-based compositions were of interest. 

Many important coating processes are of liquids that are not Newtonian, and so the effects of non-Newtonian rheology on flow between rolls is of great interest. The code used here has been applied to the simplest non-Newtonian model, namely the purely viscous, shear-thinning fluid. Viscoelasticity, though also important, is more difficult to treat and is not considered here. 

The diverse biological functions of pectins are reflected in a range of rheological behaviors that will be described in the next section. Aqueous pectin dispersions may exist in the form of a viscous flow or a gel at certain conditions. Accordingly,... 

There are three main rheological properties of materials viscous flow, plastic flow, and elastic deformation. The stress deformation behavior of elastic materials is represented by a straight line through the origin. However, in this case, the... 

Once more, the rheological behavior of many pharmaceutical and biomedical materials is more appropriately described by a number of Voigt units connected in series (18). The model illustrated in Figure 10.4 describes the rheological behavior of a viscoelastic solid as, in this case, the elastic contribution is sufficient to ensure that there is no unlimited, nonrecoverable viscous flow. However, if the spring in one of the units possesses zero elasticity (i.e., G = 0), then nonrecoverable viscous flow will be observed, and the material is better described as a viscoelastic liquid or, alternatively, an elastoviscous system. 

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