Rheology effective viscosity

Appearance of rheological effects — yield stress, non-Newtonian viscosity, thixotropy... 

SFA has made a great contribution to the investigations of thin him rheology [41], The measurements on SFA conhrm that there is a signihcant enhancement of the effective viscosity in molecularly thin liquid hlms, and the viscosity grows constantly as the him thickness diminishes. 

An effective viscosity rp has been introduced in the Reynolds equation to describe the non-Newtonian lubricant properties. Ignoring the variation of viscosity across the film thickness, one may evaluate the effective viscosity via the following rheological model that considers a possible shearthinning effect [19],... 

To determine rheological parameters such as the yield stress and effective viscosity of a foam, commercial rheometers are available rotational and conlinuous-lfow-tubc viscometry are most commonly employed (See also Rheology). However, obtaining reproducible results independent of the sample geometry is a diflicull goal which arguably has not been achieved in most of the experiments reported in the scientific lileralure... 

The rheological properties of plastisols generally could not be characterized on the basis of viscosity at some fixed shear rate (effective viscosity). Nevertheless, for practical purposes plastisols are conventionally classified into low-viscous (1-3 Pa s), medium-viscous (10-15 Pa s), and high-viscous (100-1000 Pa s) plastisols. Viscosities are measured at low shear rate y = 1 s l when plastisol flow is Newtonian. 

Electrorheological (ER) fluids are materials whose rheological properties (viscosity, yield stress, shear modulus, etc.) can be readily controlled using an external electric field. For example, in some cases, they can switch from a liquid-like material to a solid-like material within a millisecond with the aid of an electric field, by means of the so-called ER effect.1617 The unique feature of the ER effect is that ER fluids can reversibly and continuously change from a liquid state to a solid state. ER fluid research is focused mainly on the automotive and robotics industry as electrical and mechanical interfaces for applications such as clutches, brakes, damping devices, fuel injection, and hydraulic valves. However, more recently, there is growing... 

Another fractal structure of interest is considered by Adler (1986). A three-dimensional fractal suspension may be constructed from a modified Menger sponge, as shown in Fig. 7(b). A scaling argument permitted calculating the effective viscosity of such a suspension however, this viscosity should be compared with numerical results for the solution of Stokes equations in such a geometry before this rheological result is accepted unequivocally. 

Studies on the rheology of two-phase particulate systems suggest the existence of a deformation threshold that depends on the concentration of particles in the composite. Below this threshold (—68 vol.% for spherical particles), deformation occurs primarily by the flow of the composite matrix. Particles increase the effective viscosity of the matrix by absorbing energy and by forming clusters. 

Viscosity is an important rheological characteristic of a foam. The values of the structural (dissipation) viscosity and the effective viscosity, derived by different authors, range in a wide interval, depending on foam expansion ratio and dispersity, as well as on shear rate. Wenzel et al. [9] have shown that the viscosity falls with the increase in the shear rate, according to the following relation... 

Edwards and Wasan [17] and Prud homme and Khan [18] have indicated that many of the reported rheological measurement do not truly or uniquely represent the rheological character of foams. For example, the effective viscosity is proved to depend on the geometry parameters of the devices, often the slip of a foam along the wall is not considered (or is... 

Like any disperse system, foams produce non-Newtonian systems, and to characterize their rheological properties information must be obtained on the elasticity modulus (the modulus of compressibility and expansion), the shear modulus, yield stress and effective viscosity, and elastic recovery. 

Very often, this medium remains Newtonian from the viewpoint of its rheological properties, but the viscosity of this medium is somewhat larger than that of the continuous phase. This viscosity /ref is called the effective viscosity. In practice, it is convenient to relate the effective viscosity to the viscosity p of the continuous phase and consider the dimensionless effective viscosity p = ef/M-... 

The full rheological curve of such thixotropic system may be presented as a graph showing the effective viscosity, ry- = x/y, as a function of the shear stress, x, (Fig. IX-25). In this Figure r min corresponds to viscosity of the system with completely disintegrated structure. For the above mentioned bentonite... 

Athabasca deposit, the temperature dependence of the viscosity is shown in Figure 6. The Athabasca bitumen rheology is not typical of that in other world locations. For example, the rheology of bitumen in the Utah deposits, which have much higher effective viscosities (48, 49), is more complex and exhibits markedly non-Newtonian behavior (47). 

As outlined earlier (Chapter 3), electrically charged colloidal particles are surrounded by an electrical double layer. This leads to important rheological effects. The primary electroviscous effect is a consequence of the fact that the electrical double layer [Figure 8.10(a)] is deformed from its spherical shape by the shear field. Construction of the double layer ahead of the particle and its disintegration behind the particle take a finite time [Figure 8.10(b)], This causes an increase in the intrinsic viscosity, which for low zeta-potentials ( < 25 mV) is proportional to the square of this potential ... 

Pugh ND (1996) Haemodynamic and rheological effects of contrast media the role of viscosity and osmolality. Eur Radiol 6 [suppl 2] S13-S15. 

Today the rheology of many liquid systems is controlled by hydrophilic or hydrophobic fumed silicas. An interesting example are 2P-epoxy resins. The resin can be thickened effectively with a hydrophilic fumed silica, but when the polyamine hardener is added, the viscosity raises and after a short time it drops tremendously and the application is impaired or even impossible. To overcome this problem surface treated silicas have been developed, which do not show detrimental interactions with the polyamine and which have a good rheological effect [75] (Figure 43.13). 

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