Viscosity measurements Newtonian

Gla.ss Ca.pilla.ry Viscometers. The glass capillary viscometer is widely used to measure the viscosity of Newtonian fluids. The driving force is usually the hydrostatic head of the test Hquid. Kinematic viscosity is measured directly, and most of the viscometers are limited to low viscosity fluids, ca 0.4—16,000 mm /s. However, external pressure can be appHed to many glass viscometers to increase the range of measurement and enable the study of non-Newtonian behavior. Glass capillary viscometers are low shear stress instmments 1—15 Pa or 10—150 dyn/cm if operated by gravity only. The rate of shear can be as high as 20,000 based on a 200—800 s efflux time. 

Viscosity can also be determined from the rising rate of an air bubble through a Hquid. This simple technique is widely used for routine viscosity measurements of Newtonian fluids. A bubble tube viscometer consists of a glass tube of a certain size to which Hquid is added until a small air space remains at the top. The tube is then capped. When it is inverted, the air bubble rises through the Hquid. The rise time in seconds may be taken as a measure of viscosity, or an approximate viscosity in mm /s may be calculated from it. In an older method that is commonly used, the rate of rise is matched to that of a member of a series of standards, eg, with that of the Gardner-Holdt bubble tubes. Unfortunately, this technique employs a nonlinear scale of letter designations and may be difficult to interpret. 

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. 

A. G., Goloshevsky, J. H. Walton, M. V. Shutov, J. S. de Ropp, S. D. Collins, M. J. McCarthy 2005, (Nuclear magnetic resonance imaging for viscosity measurements of non-Newtonian fluids using a miniaturized rf coil), Meas. Sci. Technol. 16, 513-518. 

Like the von Karman equation, this equation is implicit in/. Equation (6-46) can be applied to any non-Newtonian fluid if the parameter n is interpreted to be the point slope of the shear stress versus shear rate plot from (laminar) viscosity measurements, at the wall shear stress (or shear rate) corresponding to the conditions of interest in turbulent flow. However, it is not a simple matter to acquire the needed data over the appropriate range or to solve the equation for / for a given flow rate and pipe diameter, in turbulent flow. 

For Molecular weight determination by viscometry we do not need absolute h value, viscosity measurements may be carried out in simple Ostwald Viscometer. Because of (the non-Newtonian behaviour of most macromolecular solutions at high velocity gradients in the capillary, the viscometer dimensions are chosen in such a manner that the viscosity gradient is the smallest possible. 

The dynamic melt viscosity measurements of select star blocks and a similar triblock were carried out on a rheometric mechanical spectrometer, RMS. Circular molded samples of 2 cm diameter and -1.5 mm thickness were subjected to forced sinusoidal oscillations. Dynamic viscosities were recorded in the frequency range of 0.01-100 rad/s at 180 °C. Figure 10 shows the complex viscosities of two select star blocks and a similar linear triblock. The plots showed characteristic behavior of thermoplastic elastomers, i.e., absence of Newtonian behavior even in the low frequency region. The complex viscosity of the star block... 

As pointed out in the paragraph after eq. (3.43), the dimensionless quantity / can be interpreted as a reduced shear stress or also, when the measured ("Newtonian ) zero shear viscosity r]y of the solution is used in eq. (3.43), as a reduced shear rate. 

Capillary viscometers are ideal for measuring the viscosity of Newtonian fluids. However, they are unsuitable for non-Newtonian fluids since variations in hydrostatic pressure during sample efflux results in variations in shear rate and thus viscosity. This unit contains protocols for measuring the viscosity of pure liquids and solutions (see Basic Protocol) and serums from fruit juices and pastes (see Alternate Protocol). 

In the food industry it has often been difficult to obtain true viscosity measurements (unithj.j) of complex fluid foods such as coarse fruit suspensions. These are usually non-Newtonian suspensions. Fruit concentrates are dispersions of solid particles (pulp) in aqueous media (serum). Their rheological properties are of interest in practical applications related to processing, storage stability, and sensory properties. Expensive rheometers are often not available in quality control and product development laboratories. However, viscosity is nonetheless an important quality factor of these products. 

Figure 11.3. (a) Viscosity of PDMS melts swollen with carbon dioxide measured as a function of shear rate and carbon dioxide content at 50 °C. (b) Master viscosity curve produced by applying concentration-dependent scaling factors (see text). A Pure PDMS, 4.84 wt % C02, O 9.03 wt % C02, 14.4 wt % C02 A 20.7 wt % C02. Data denoted by are Newtonian viscosity measurement for pure PDMS. Data from Gerhardt (1994). 

A common method for measuring the viscosity of Newtonian liquids involves the use of gravity force capillary viscometers. A simple capillary viscometer is illustrated in Figure 6.3(a). The viscometer is filled with a sample so that by applying suction one meniscus can be held above point A while the other is below point C. After... 

Viscosity Measurements, Previous studies (2) have shown that dilute solutions of Pluronic F127 exhibit Newtonian flow characteristics and consequently, it is permissible that the viscosity of these solutions is measured by capillary viscometry. A suspended-level viscometer was used and solutions were thermostatted to within... 

Viscometers of relatively complex geometry, for example the Ostwald glass U-tube viscometer, can be used to measure the viscosity of Newtonian liquids, which is independent of shear rate and time, after calibration with a Newtonian liquid of known viscosity. Such instruments cannot be used for Theologically characterizing non-Newtonian liquids, and therefore cannot be classed as rheometers, as geometrical complexity prevents evaluation of shear stress and shear rate at a given location independently of sample rheological behavior. 

Lubricated squeezing flow rheometry (and unlubricated squeezing flow rheometry, in which friction between the sample and discs results in radial shear flow) can be used also to measure Newtonian viscosity and the flow properties of non-Newtonian liquids (Campanella and Peleg, 2002). 

There is Newtonian and Non-Newtonian viscosity. With Newtonian viscosity the ratio of shearing stress to the shearing strain is constant such as, theoretically, water. In non-Newtonian behavior, which is the case for plastics, the ratio varies with the shearing stress. Such ratios are often called the apparent viscosities at the corresponding shearing stresses. Viscosity is measured in terms of flow in Pas (P), with water as the base standard value of 1.0. The higher the number, the less flow. 

The viscosity of Newtonian liquids can be measured simply, by one-point determinations with viscometers, such as rotational, capillary, or falling ball viscometers. For non-Newtonian materials, measurement of... 

Melt Viscosity Equation of Sulfui>-DCP Solutions. An assumption is made that the sulfur-DCP solution is a Newtonian fluid, i.e., the viscosity measured by the Brookfield viscometer is independent of the spindle speed, which is related to shear rate. The linear plots of log (viscosity) vs. time as in Figures 8, 9, and 10 give the following equation for a given sulfur-DCP composition at a given temperature ... 

Here M is the molecular weight and V the partial specific volume of the solute, N the Avogadro number, k the Boltzmann constant, and T the absolute temperature s and D are the sedimentation and translational diffusion coefficients (after extrapolation to infinite dilution). The translational frictional coefficients from both measurements are regarded as identical, i.e., f, = fd. The rotary frictional coefficient, designated as f, can be determined from either flow birefringence or non-Newtonian viscosity measurements. 

For aqueous solutions, say, at 25°C the lower limit of the critical Reynolds number will not be reached as long as V/Rt is well below 30. Thus for all practical purposes this complication will not arise in routine viscosity measurements. On the other hand if the non-Newtonian viscosity measurements are extended to a very high range of the shearing stress, it is desirable to check the possibility of this effect. 

In this review we have briefly discussed the theoretical and experimental aspects of both Newtonian and non-Newtonian viscosities of polymer solutions. To protein chemists one of the interesting developments is no doubt the re-examination of the (Newtonian) viscosity treatments of protein solutions. There are many assumptions involved in the effective use of intrinsic viscosity measurements for evaluating the asymmetry of the protein molecules, however attractive the conventional treatment may have appeared for the past two decades. Carefully interpreted, the intrinsic viscosity (at zero gradient) can still provide a reasonable estimate of the axial ratios of the protein molecules. The concept of equivalent hydrodynamic volume, sound in principle, has put the viscometry of protein solutions in a proper perspective, although the quantitative aspects of this new approach still... 

---