Rheology fluidity

Viscosity is equal to the slope of the flow curve, Tf = dr/dj. The quantity r/y is the viscosity Tj for a Newtonian Hquid and the apparent viscosity Tj for a non-Newtonian Hquid. The kinematic viscosity is the viscosity coefficient divided by the density, ly = tj/p. The fluidity is the reciprocal of the viscosity, (j) = 1/rj. The common units for viscosity, dyne seconds per square centimeter ((dyn-s)/cm ) or grams per centimeter second ((g/(cm-s)), called poise, which is usually expressed as centipoise (cP), have been replaced by the SI units of pascal seconds, ie, Pa-s and mPa-s, where 1 mPa-s = 1 cP. In the same manner the shear stress units of dynes per square centimeter, dyn/cmhave been replaced by Pascals, where 10 dyn/cm = 1 Pa, and newtons per square meter, where 1 N/m = 1 Pa. Shear rate is AH/AX, or length /time/length, so that values are given as per second (s ) in both systems. The SI units for kinematic viscosity are square centimeters per second, cm /s, ie, Stokes (St), and square millimeters per second, mm /s, ie, centistokes (cSt). Information is available for the official Society of Rheology nomenclature and units for a wide range of rheological parameters (11). 

This equation is based on the approximation that the penetration is 800 at the softening point, but the approximation fails appreciably when a complex flow is present (80,81). However, the penetration index has been, and continues to be, used for the general characteristics of asphalt for example asphalts with a P/less than —2 are considered to be the pitch type, from —2 to +2, the sol type, and above +2, the gel or blown type (2). Other empirical relations that have been used to express the rheological-temperature relation are fluidity factor a Furol viscosity P, at 135°C and penetration P, at 25°C, relation of (H—P)P/100 and penetration viscosity number PVN again relating the penetration at 25°C and kinematic viscosity at 135 °C (82,83). 

Fliess-barkeit,/. flowing quality, fluidity fusibility. -bereich, m. plastic range, -betrieb, m. — Fliessarbeit. -druck, m. flow pressure hydraulic pressure, -eigenschaf /. rheologic property. 

Rheological determinations are destructive of the structures they measure for this reason they do not portray the actual structure of the dispersion at rest. Accordingly, various methods have been devised for extrapolating to zero the results of measurements at various shear rates. The most useful one has been the conversion of viscosities to fluidities at various shear rates and the extrapolation of the resulting nearly linear relationship to zero shear, as shown in Figure 7. Sometimes a power of the shear rate, D, provides a better distinction between a sol (essentially a liquid) and a gel (essentially a solid), as shown in the figure, but the difference between a finite intercept (sol) and zero fluidity (gel) is largely fictitious because of the dependence of the intercept on the exponent n. 

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). 

Compatibility of PVC with plasticizer (PI) also strongly influences the rheological and processing properties of plastisols 48>. The effects of the diverse plasticizers on PVC varies since some plasticizers (even well compatible with polymer), e.g. dibutylphthalate (DBP), may readily escape from the obtained materials and articles 17,23,43. Plasticizers which are poorly compatible with PVC (the so-called secondary PI ) may be used for the purpose-oriented change of viscosity properties of a system (fluidity increase) and saving the base PI component. 

Like many polymers, the fluidity or flow (rheological) properties of denrimers are of interest (Box 14.1). Dendrimers behave like soft, spherical particles, surrounded by a hard surface shell and hence are often described as core-shell type bodies. Hence the interior of dendrimers can be deformed according to the type of linkage and building blocks and my contain voids suitable for encapsulating guest species.2... 

Frequently used single-point viscosity tests in the starch plant are orifice pipettes,56 orifice funnels,57 the Hot Scott viscometer, and various methods to determine alkaline fluidity.58 For absolute measurements of the rheological properties, rotating viscometers with coaxial cylinders are used.59 The paper industry uses mainly the Brookfield viscometer and the Hercules viscometer for determining shear-dependent viscosity, pseudoplasticity, and thixotropy. Oscillatory and capillary viscometers are used for more detailed viscosity characterization, such as yield value, elastic properties, and viscoelasticity.60... 

Summary Rheological properties of compositions containing oligoethylsiloxane, poly-a-olefin, and dioctylsebacinate were studied. Both energetic and structural factors were shown to affect the fluidity of compositions. 

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. 

Rheological properties of casting slips are controlled by small additions of suitable electrolytes which promote deflocculation of aggregates and thus cause a decrease in yield point and increase in fluidity, which is desirable with respect to reduction of the required water content (refer to the section on slip casting). The stability of the suspension is also improved, i.e. the sedimentation of the solids is suppressed. 

Increased amounts of membrane cholesterol have been associated with abnormalities of cell shapes for example, the erythrocytes of patients with liver disease develop a characteristic spur cell appearance which is associated with an increase in membrane cholesterol. These cells regain their normal shape on removal of the excess cholesterol [94]. Increased osmotic fragility has been observed with cholesterol depletion in both mycoplasma [104] and in erythrocytes [94]. There are few rheological studies on cells and the role of cholesterol in determining normal cell shape is not known. However, it is almost certainly linked to the fluidity and permeabihty of the cell membrane. 

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