Rotational viscosity measurement

Figure 78 shows a plot of the soft mode and Goldstone mode rotational viscosities measured on either side of the phase transition between the smectic A and SmC. It can be seen that, except in the vicinity of the phase transition, the viscosity seems to connect fairly well between the two phases. The activation energies of these two processes are, however, different. This result may be compared to results obtained by Pozhidayev et al. [148], referred to in Fig. 67. They performed measurements of y beginning in the chiral nematic phase of a liquid crystal mixture with corresponding measurements in the SmC phase, and have shown the viscosity values on an Arrhenius plot for the N and SmC phases. Despite missing data of y in the smectic A phase they extrapolate the N values of y down to the smectic C phase and get a reasonably smooth fit. Their measurements also show that y is larger than y, and this is universally the case.

Figure 8Z Comparative study of the / rotational viscosity measured by the polarization reversal technique based on Eq. (363) and dielectric relaxation spectroscopy. The material is LCl (from Buivydas [155]).

Strauss and Williamst have studied coil dimensions of derivatives of poly(4-vinylpyridine) by light-scattering and viscosity measurements. The derivatives studied were poly(pyridinium) ions quaternized y% with n-dodecyl groups and (1 - y)% with ethyl groups. Experimental coil dimensions extrapolated to 0 conditions and expressed relative to the length of a freely rotating repeat unit are presented here for the molecules in two different environments ... 

Rheology. Flow properties of latices are important during processing and in many latex appHcations such as dipped goods, paint, inks (qv), and fabric coatings. For dilute, nonionic latices, the relative latex viscosity is a power—law expansion of the particle volume fraction. The terms in the expansion account for flow around the particles and particle—particle interactions. For ionic latices, electrostatic contributions to the flow around the diffuse double layer and enhanced particle—particle interactions must be considered (92). A relative viscosity relationship for concentrated latices was first presented in 1972 (93). A review of empirical relative viscosity models is available (92). In practice, latex viscosity measurements are carried out with rotational viscometers (see Rpleologicalmeasurement). 

Cone—Plate Viscometer. In a cone—plate viscometer (Fig. 28), alow angle (<3°) cone rotates against a flat plate with the fluid sample between them. The cone—plate instmment is a simple, straightforward device that is easy to use and extremely easy to clean. It is well suited to routine work because measurements are rapid and no tedious calculations are necessary. With careful caUbration and good temperature control it can also be used for research. Heated instmments can be used for melt viscosity measurements. 

Controlled Stress Viscometer. Most rotational viscometers operate by controlling the rotational speed and, therefore, the shear rate. The shear stress varies uncontrollably as the viscosity changes. Often, before the stmcture is determined by viscosity measurement, it is destroyed by the shearing action. Yield behavior is difficult to measure. In addition, many flow processes, such as flow under gravity, settling, and film leveling, are stress-driven rather than rate-driven. 

The Nametre Rotary B rotational viscometer measures torque in terms of the current needed to drive the d-c motor at a given speed while a material is under test. The standard sensors are coaxial cylinders or Brookfield disk-type spindles, but a cone—plate system is also available. The viscosity range for the coaxial cylinder sensors is 5 to 5 x 1(T mPa-s, and the maximum shear rate is 200. ... 

In a research and development laboratory at the Dow Chemical Company in Midland, Michigan, rotational viscometry experiments on various dilutions of a test fluid, such as corn syrup, can generate the required data. Once various challenges are overcome, such as obtaining a uniform and constant temperature throughout the fluid and dealing with unusual physical behaviors of the test fluid, accurate viscosity measurements can be made and the project to optimize mixing performance can move forward. 

Rotational viscosity methods measure viscosity by measuring the torque required to rotate a spindle immersed in the fluid sample. 

Common geometries used to make viscosity measurements over a range of shear rates are Couette, concentric cylinder, or cup and bob systems. The gap between the two cylinders is usually small so that a constant shear rate can be assumed at all points in the gap. When the liquid is in laminar flow, any small element of the liquid moves along lines of constant velocity known as streamlines. The translational velocity of the element is the same as that of the streamline at its centre. There is of course a velocity difference across the element equal to the shear rate and this shearing action means that there is a rotational or vorticity component to the flow field which is numerically equal to the shear rate/2. The geometry is shown in Figure 1.7. 

This second method does not lend itself to the development of quantitative correlations which are based solely on true physical properties of the fluids and which, therefore, can be measured in the laboratory. The prediction of heat transfer coefficients for a new suspension, for example, might require pilot-plant-scale turbulent-flow viscosity measurements, which could just as easily be extended to include experimental measurement of the desired heat transfer coefficient directly. These remarks may best be summarized by saying that both types of measurements would have been desirable in some of the research work, in order to compare the results. For a significant number of suspensions (four) this has been done by Miller (M13), who found no difference between laboratory viscosities measured with a rotational viscometer and those obtained from turbulent-flow pressure-drop measurements, assuming, for suspensions, the validity of the conventional friction-factor—Reynolds-number plot.11 It is accordingly concluded here that use of either type of measurement is satisfactory use of a viscometer such as that described by Orr (05) is recommended on the basis that fundamental fluid properties are more readily determined under laminar-flow conditions, and a means is provided whereby heat transfer characteristics of a new suspension may be predicted without pilot-plant-scale studies. 

On the basis of the dipole moment values and viscosity measurements of the two forms of nitroglycerine, de Kreuk [20] considered that the difference between the two forms is produced by rotational isomerism. According to this hypothesis the labile and stable forms would correspond to cis- and trans-isomers respectively. In a non-polar solvent the traits form predominates. In a polar solvent the content of the cis form increases and reaches a maximum in liquid nitroglycerine. 

Emulsion viscosities have been measured as a function of water content (10, 20 and 40S), temperature and shear rate in a thermostatted rotating viscometer. The shear rates were varied between 0.277 and 27.7 s"1 with measurements taken at temperatures between 5 and 20° C. Above 20°C, separation of water from the emulsion occurred, rendering viscosity measurements unreliable. The apparent viscosity of the emulsion below 20° C increases drastically with the watercut in the emulsion and decreases with Increasing shear rate (Fig. 5). Emulsions containing more than 20X water were found to behave as pseudo-plastic fluids. 

Figure HI. 1.1 Common fixtures used for viscosity measurements with rotational rheometers. Cross-sectional views are shown on the left and external views are shown on the right.

Mooney viscosity—measure of the resistance of raw or unvulcanized rubber to deformation, as measured in a Mooney viscometer. A steel disc is embedded in a heated rubber specimen and slowly rotated. The resistance to the shearing action of the disc is measured and expressed as a Mooney viscosity value. Viscosity increases with continued rotation, and the time required to produce a specified rise in Mooney viscosity is known as the Mooney scorch value, which is an indication of the tendency of a rubber mixture to cure, or vulcanize, prematurely during processing. 

To extract concrete predictions for experimental parameters from our calculations is a non-trivial task, because neither the energetic constant B nor the rotational viscosity yi are used for the hydrodynamic description of the smectic A phase (but play an important role in our model). Therefore, we rely here on measurements in the vicinity of the nematic-smectic A phase transition. Measurements on LMW liquid crystals made by Litster [33] in the vicinity of the nematic-smectic A transition indicate that B is approximately one order of magnitude less than Bo. As for j we could not find any measurements which would allow an estimate of its value in the smectic A phase. In the nematic phase y increases drastically towards the nematic-smectic A transition (see, e.g., [51]). Numerical simulations on a molecular scale are also a promising approach to determine these constants [52],... 

Viscosities of liquid epoxy systems are usually measured with a rotating spindle instrument, such as a Brookfield viscometer. Solid resins are usually dissolved in solvent for viscosity measurement by these instruments. Temperature and spindle speed are important... 

The rotational viscosity method described above to measure working life or pot life is a form of rheological measurement of cure. However, cone and plate rheometry is preferred for accurate measurements because the specimen size and geometry are similar to those that occur in an adhesive joint. 

Viscosity Measurements. These were carried out on solutions of the procyanidin polymers using a Ferranti-Shirley cone viscometer at 25.0 0.1 °C with a 3.5-cm-radius cone. This viscosity was measured at several rotation rates to check for shear dependence. The results were constant over the ranges used (20 to 500 revolutions per minute, depending on the viscosity), and the resulting viscosity values were averaged to obtain the results in the test. 

The behavior of melt viscosity of sulfur-dicyclopentadiene solutions is of obvious interest from the point of sprayable coatings. The melt viscosity behavior has been reported recently, but only qualitatively and over a narrow range of compositions (18). The viscosity of sulfur measured by the capillary method by Bacon and Fanelli (19, 20) is considered to be the best (21). Recently, however, the viscosity of sulfur has been measured by an apparatus containing an electric motor and a rotating cylinder (22). Viscosity of the sulfur-DCP solutions are measured here with the help of a Brookfield synchro-lectric viscometer, which is of the later kind. Viscosity measurements have been carried out to follow the copolymerization reaction and to analyze the viscosity behavior. 

The apparatus used for flow rate measurements is described in [28] (Fig. 4.5). The technique employed in surface viscosity measurements involves a rotating disc suspended on... 

Table 3.14 Transition temperatures (°C), elastic constants fk/y, k22 kjj, 10 N), dielectric anisotropy ( e), dielectric constant measured perpendicular to the molecular long axis (e ), birefringence ( n), refractive index measured perpendicular to the director (noJ, rotational viscosity (y. Poise) and bulk viscosity (r, Poise) for tr ns-l-(4-cyanophe-nyl)-4-pentylcyclohexane (41), iTSins-l-(4-cyanophenyl)-4-[(E)-pent-l-enyl]cyclohexane (74) andtra.ns-l-(4-cyanophenyl)-4-[(E)-pent-3-enyI]cyclohexane (78) extrapolated to 100% at 22°...

---