Rheological terms

As the detailed description of rheological terms is not the subject of this volume, only a very brief introduction is given. 

The shear rate y is the rate of deformation, y, of a body. A deformation or a shear rate occurs due to the application of a shear stress a. The apparent viscosity is given by  

If rj is independent of the shear rate y a liquid is called Newtonian. Water and other low molecular weight liquids typically are Newtonian. If rj decreases with increasing y, a liquid is termed shear thinning. Examples for shear thinning liquids are entangled polymer solutions or surfactant solutions with long rod-like micelles. The zero shear viscosity is the value of the viscosity for small shear rates ij0 = lim, o tj y). The inverse case is also sometimes observed rj increases with increasing shear rate. This can be found for suspensions and sometimes for surfactant solutions. In surfactant solutions the viscosity can be a function of time. In this case one speaks of shear induced structures. 

If a sample shows elastic, solid-like deformation below a certain shear stress ay and starts flowing above this value, ay is called a yield stress value. This phenomenon can occur even in solutions with quite low viscosity. A practical indication for the existence of a yield stress value is the trapping of bubbles in the liquid Small air bubbles that are shaken into the sample do not rise for a long time whereas they climb up to the surface sooner or later in a liquid without yield stress even if their viscosity is much higher. A simple model for the description of a liquid with a yield stress is called Bingham s solid  

More detailed information can be found in the relevant literature e.g. [36]. 

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BS 5168 1975 British Standard 5168 (British Standards Institution, London). Glossary of Rheological Terms. 

The fluidity of the cement paste can be measured in rheological terms by the torque transmitted to a stationary bob inside a revolving outer cylinder placed in a water-cement system as shown in Fig. 1.10. The shear stress measured at the stationary bob is plotted against the rate of applied shear when, for pastes of varying water-cement ratios, the results shown in Fig. 1.11 are obtained for readings taken of the shear stress as the shearing rate is increased (the up curve). 

Figures 3.2 and 3.3 show the sub-division of fluids into those with and without yield stress. Fluids with yield stress require a shear stress o0 in order to flow. We are all familiar with this characteristic from tomato ketchup, which requires a certain minimum force before it starts to flow out of the bottle. Below o0 it is still a solid - in rheological terms. This behavior is described in the model by adding a shear stress o0 to Eq. 3.5. We then obtain a Flerschel-Bulkley correlation ...

The discussion in rheological terms of the problems associated with rubber processing is relatively recent and results from theoretical and experimental progress in understanding flow properties of pure polymers, and particularly thermoplastics. However the rheology of elastomers is further complicated by the necessary presence of fillers, plasticizers and other ingredients. These lead to peculiar flow properties associated with heterogeneous matter and therefore not yet well understood. 

The first part of this definition we can understand well enough in rheological terms, and we can use appropriate instruments for measuring with sufficient accuracy. The last part of the definition, however, namely the part about maximum achievable deformation prior to crack formation may be useful in actual practice, but must remain unsatisfactory in theory, because crack formation does not count among the accurately detectable physical phenomena. [13]... 

The viscoelastic behavior of biomaterials is typically measured using DMA. In rheological terms, viscoelastic is the concomitance of viscous (fluid-like) and elastic (solid-like) elements. The proportion of viscous and elastic properties is depending on the used material as well as on the measuring conditions such as the temperature. In DMA measurements, a sinusoidal shear load is applied to the sample while measuring the shear stress (cr ) with a stress transducer. The strain induced... 

Consistency n. The property of a material or composition which is evidenced by its resistance to flow, represented by an undefined composite of properties, each measurable from the complete, force-rate flow curve as plastic viscosity, yield value and thixotropy. The term is applied to a variety of materials. For Newtonian liquids, consistency is simply viscosity. While consistency is an accepted rheological term, it has qualitative... 

Yield value n. A rheological term describing the flow properties of a printing ink, and defined as the minimum force required... 

Mieras " reported work aimed at relating molecular structure of raw rubbers to the processing properties of their black-filled compounds by relating both to the rheological properties of the rubber. However, he noted two limitations firstly, that there is no theoretical or experimental means to characterize a rubber fully in rheological terms, and secondly, that it is virtually impossible to simulate processing... 

In this chapter, we have discussed some basic rheological properties of polymer solution flow in porous media, such as rheological terms, shear rate models, experiment... 

Theoretical treatments of emulsion rheology are less well developed than those devised for particulate dispersions. It therefore seems appropriate to give a brief survey of classical work on particulate dispersion rheology as a prelude to discussing the rheology of dispersions of droplets. A further justification for this approach is the observation that, in practice, many a fine emulsion system can be reliably treated in rheological terms as if it were a dispersion of solid spherical particles. 

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