The measurement of viscosity

The concept of viscosity as a physical property of a fluid is considered in Volume 1, first for Newtonian fluids (Section 3.3) and secondly for the non-Newtonian case (Section 3.7). A further discussion of viscosity on the molecular scale in terms of the momentum transfer occurring in a gas flowing over a flat surface appears in Volume 1, Section 12.2.4. Methods for estimating viscosities are described in Volume 6, Section 8.7. 

There are a substantial number of ways in which the viscosity of a fluid can be measured. Not all of these can be adapted easily to on-line process measurement. It is convenient to classify viscometers according to the type of physical measurement made. 

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As should be expected, both (fg ) and r show the same dependence on the degree of polymerization or molecular weight. Since the radius of gyration can be determined experimentally through the measurement of viscosity or light scattering, it is through this quantity that we shall approach the evaluation of 1. 

If a solution tends to be independent of shear, then the measurement of viscosity (q) is based on Poiseuille s law can be made easy by grouping all those terms related to a specific viscometer as a calibration constant A. 

Crisp, S., Lewis, B. G. Wilson, A. D. (1975). Gelation of polyacrylic acid aqueous solutions and the measurement of viscosity. Journal of Dental Research, 54, 1173-5. 

The measurement of viscosity is important for many food products as the flow properties of the material relate directly to how the product will perform or be perceived by the consumer. Measurements of fluid viscosity were based on a correlation between relaxation times and fluid viscosity. The dependence of relaxation times on fluid viscosity was predicted and demonstrated in the late 1940 s [29]. This type of correlation has been found to hold for a large number of simple fluid foods including molten hard candies, concentrated coffee and concentrated milk. Shown in Figure 4.7.6 are the relaxation times measured at 10 MHz for solutions of rehydrated instant coffee compared with measured Newtonian viscosities of the solution. The correlations and the measurement provide an accurate estimate of viscosity at a specific shear rate. 

So far the results have been shown in which the metal alkoxide solutions are reacted in the open system. It has been shown that the metal alkoxide solutions reacted in the closed container never show the spinnability even when the starting solutions are characterized by the low acid content and low water content (4). It has been also shown from the measurements of viscosity behavior that the solution remains Newtonian in the open system, while the solution exhibits structural viscosity (shear-thinning) in the closed system. 

The technical term for the study of and the measurement of viscosity is rheology. Viscosity measurements can be made in a variety of ways. The simplest method is to determine the time it takes for a metal ball to fall through a specified distance of a liquid in a graduated cylinder. The American Society of Testing Materials (ASTM) specifies several methods for the determination of viscosity of specific substances using this method. When the radius of the ball is small compared to the radius of the graduated cylinder, the viscosity will be... 

Most of the DNA in nature has the double helical secondary structure. The hydrogen bonds between the base pairs provide the stability of the double helix. Under certain conditions the hydrogen bonds are broken. During the replication process itself, this happens and parts of the double helix unfold. Under other conditions, the whole molecule unfolds, becomes single stranded, and assumes a random coil conformation. This can happen in denaturation processes aided by heat, extreme acidic or basic conditions, etc. Such a transformation is often referred to as helix-to-coil transition. There are a number of techniques that can monitor such a transition. One of the most sensitive is the measurement of viscosity of DNA solutions. 

FIGURE 20.1 Digital Brookfield viscometer allows the measurement of viscosity and torque. (Photo courtesy of Brookfield Laboratories.)... 

Thus measurements of the viscosity 9 (0,7) over a range of temperature allow determination of f(0,T) as a function of T, provided the value of /(0,T) at a certain temperature T is known from other source. For this purpose we may utilize the measurement of viscosity as a function of diluent concentration at the given T ] the substitution of such data into Eq. (40) may lead to the determination of the required f(0,T ). It is to be expected that, if the free volume theories of viscosity and diffusion developed above are at all correct, the values of /(0,7) thus derived from y data should agree with those obtained from ae data by application of Eq. (40) and also with those from DT data analyzed in terms of Eq. (36). 

Since 1970, two generic types of viscometer have received the greatest attention the first makes use of the torsional oscillations of bodies of revolution and the second is based on the rather simpler concept of laminar flow through capillaries. Both reduce the measurement of viscosity to measurements of mass, length and time. 

For measurements of viscosity of molten salts and glasses at high-temperatures, several methods were proposed. The selection of a particular method depends in general on the viscosity of the liquids to be measured. A broad dispersion of experimental results reflects substantial experimental difficulties connected with viscosity measurement. In general, in the measurement of viscosity of molten salts the method of torsional pendulum is most frequently used, while in the measurement of viscosity of liquids, such as molten glasses, the falling body and the rotational methods are most suitable. Methods for viscosity measurement of liquids with a very high viscosity (above 10 Pa s) will not be described here. 

Two methods of measurement are used. The first one is the classical rotational method employing the original properties of the viscosimeter. Because the measurement of viscosity of molten glasses requires a configuration with a free spindle, the determination of the angular momentum I as the function of a and the shear stress was not taken into account. Direct calibration using the experimental relation t]la =f a) showed to be very simple and relatively accurate. Based on experimental results, the linear function has been chosen in the form... 

Polymers that have been suggested for mobility control in oil reservoirs include polyacrylamides, hydroxy ethyl cellulose, and modified polysaccharides which are produced either by fermentation or by more conventional chemical processes. In this paper the solution properties of these polymers are presented and compared for tertiary oil recovery applications. Among the properties discussed are non-Newtonian character for different environmental conditions (electrolytes and temperature), filterability, and long term stability. The behavior of these water soluble polymers in solution can be correlated with the effective molecular size which can be measured by the intrinsic viscosity technique. A low-shear capillary viscometer with a high precision and a capability of covering low shear rates (such as 10 sec - - for a 10 cp fluid) has been designed to measure the viscosities. The measurement of viscosities at such slow flow conditions is necessitated... 

All the oligo-polyols are used to build the polyurethane high MW structure in a reactive process, as a consequence of the oligo-polyols terminal hydroxyl group reaction with polyisocyanates. The reactivity of oligo-polyols in polyurethane fabrication is a very important practical characteristic. Reactivity is a measure of the reaction rate of an oligo-polyol with an isocyanate in order to make the final polyurethane polymer. One practical method is the measurement of viscosity, in time, by Brookfield Viscosity Test (BVT), especially used to determine the reactivity of ethylene oxide capped polyether polyols. Figure 3.12 shows the effect of the primary hydroxyl content upon the reactivity of ethylene-oxide capped polyether triols of MW of 5,000 daltons. 

The Dean and Davis method required the measurement of viscosities at two temperatures followed by a simple calculation procedure. The simplicity made this method widely appealing, and it remains so today except that the calculation has become more complicated. As developed, when the viscosities at 100°F and 210°F were determined in SUS units, the viscosities at 100°F for the two reference oils with the same viscosities at 210°F as the sample were calculated using equations or tables provided by Dean and Davis and then the VI was calculated from Equation 3.2. Since inception, viscosity measurements have changed from SUS units to centistokes and temperatures from 100°F and 210°F to 40°C and 100°C. 

Capillary Viscometer An instrument used for the measurement of viscosity in which the rate of flow through a capillary under constant applied pressure difference is determined. This method is most suited to the determination of Newtonian viscosities. Various designs are used, such as the Ostwald and Ubbelohde types. 

Viscometer Any instrument employed in the measurement of viscosity. In most cases, the term is applied to instruments capable of measuring only Newtonian viscosity, and not capable of non-Newtonian measurements. See also Rheometer. 

PoiseuilWs Law Poiseuille flow is the steady flow of incompressible fluid parallel to the axis of a circular pipe or capillary. Poiseuille s law is an expression for the flow rate of a liquid in such tubes. It forms the basis for the measurement of viscosities by capillary viscometry. 

PASCAL - The accepted metric unit of measurement or pressure and stress component in the measurement of viscosity. A Pascal is equal to a force of 1 Newton acting an area of 1 square meter. The symbol is Pa. 

Two hundred years after the early contributions of Newton and Hooke, various laws of real fluids emerged as well as a quantitative description of flow and the measurement of viscosity, including the work of Euler, Cauchy, Coulomb, Poiseuille, Hagen, Couette, Reynolds, and Bingham. In 1890, the first rotational rheometer was invented by Couette. In 1929, Reiner and Bingham founded the first rheological society. 

Capillary viscometers are the most commonly used instruments for the measurement of viscosity due, in part, to their relative simplicity, low cost and (in the case of long capillaries) accuracy. However, when pressure drives a fluid through a pipe, the velocity is a maximum at the centre the velocity gradient or shear rate y are a maximum at the wall and zero in the centre of the flow. The flow is therefore non-homogeneous and capillary viscometers are restricted to measuring steady shear functions, i.e. steady shear stress-shear rate behaviour for time independent fluids [Macosko 1994]. Due to their inherent similarity to... 

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