Rotational viscosity direct measurement

Fig. 9. Rotational relaxation time as a function of molecular weight (viscosity average) for fluorescein conjugates of polyacrylamide in dilute aqueous solutions. Filled circle represents the value of for free dye (recalculated from Fig. 5 of ref. 11, using the directly measured lifetime t)...

A spectroscopic technique that probes membrane fluidity can either directly measure mobility and order parameters for membrane constituents (NMR) or use probes (ESR, fluorescence). Some fluorescent and ESR probes are shown in Fig. 4. The connection between the rotational correlation time of a membrane embedded probe and the membrane fluidity can be illustrated using the example of a simple isotropic liquid, in which fluidity is merely a reciprocal viscosity ri and the rotational correlation time Xc for a molecule with a hydrodynamic volume V is given by the well-known Debye-Stokes-Einstein relation Xc = r VlkT, where k is the Boltzmann constant and T is the... 

The directly measured variables are not the fluid properties such as viscosity, but the forces, torques, and rate of rotation pertaining to a given apparatus (7). In some cases, the measured shear stress, t, and shear rate 7 data can be used to construct a model or use an established model to fit the data. 

Solids content Viscosity pH Gel time and pot life Dr5lng the sample for 2h at 120°C results can be influenced by the test parameters Using a rotation viscometer or Ford cup (DIN cup) Direct measurement using pH electrodes Simplified method to determine the resin s gel time Gel time at 100°C or at 70°C Pot life at 20°C or at 30°C B-time for PF resins at 100 to 140°C ... 

As there is no appropriate method to measure directly the rotational viscosity of ferroelectric liquid crystals, y is generally deduced from the electrooptic response time measurements [ 12,18,44]. The relationship between fio 9o and t is not straightforward and requires the use of a theoretical model for the optical transmission based on the bookshelf geometry briefly summarized in the following. 

Comparison of dimensions of the PVB chains with different hydroxyl contents requires measurement of the chain dimensions under 6 conditions where A2 vanishes and the chain assumes its unperturbed dimensions. The Matsuda and Inagaki calculation of freely rotating chain dimensions assumes that excluded volume effects are absent, and that there is no hindrance to rotation about C-C bonds. The ratio of the experimental unperturbed dimensions to the calculated freely rotating dimensions is the steric factor o, a characteristic parameter for a given polymer which is dependent on the hindrance to rotation. Although we have not directly measured the chain dimensions even in good solvents, estimation of the unperturbed dimensions can be made with several theories developed for use with intrinsic viscosities in good solvents, These include the theory of Berry 13... 

Brookfield viscosimeters also belong to the group of rotational viscosimeters. In contrast to the devices described earlier, this viscosimeter does not generate a defined shear field. The Brookfield viscosimeter consists mainly of a disc or a pin that is rotating with a defined velocity in the sample fluid. The torque that is required to achieve this rotational speed directly yields a viscosity through comparison with a calibration fluid. The range of measurable viscosities can be adjusted by variation of the disc geometry. 

In most cases viscosity is measured by capillary viscometers or rotating viscometers. In a capillary viscometer one measures the pressure drop by means of constant laminar flow in a capillary the constant flow can be achieved by a pump and the pressure drop is obtained by a differential pressure transmitter whose plus and minus sides are connected to the capillary. The pressure drop is then directly proportional to the viscosity according to the Hagen-Poiseuille law [4, 11] [Eq. (30), where p is the viscosity, r is the capillary radius, I is the capillary length, Ap is the pressure drop, and is the mass flow rate]. The capillary viscometer may also be employed in-line for monitoring of molecular weight in polymerizations, as described in Ref. 14. 

The six Leslie coefficients can not be measured directly. They can only be determined with the aid of several experimental methods which ususally lead to combinations of these coefficients. Taking into account the Parodi equation, the six coefficients can be obtained from five linear independent viscosity coefficients. Thus, the four viscosity coefficients rji, rj2 and t/j2 and the rotational viscosity coefficient /] give a, = r)i2... 

An apparent alternative path to determining the solvent viscosity in the presence of polymer coils is to measure the viscosity 7 of the polymer solution in the high-frequency limit. At high frequency, polymer contributions to the solution viscosity are predicted by a range of models to fade toward zero, so that the solution viscosity was historically expected to tend at high frequency to the viscosity ris of the neat solvent. These are the least direct measurements of solvent behavior, because they are fundamentally model-dependent, so they are treated last, though as a historical matter measurements showing rj rjs predate much work on solvent translational and rotational diffusion. 

The Zwetkoff experiment is described in Section 5.4. This remarkable experiment, involving a magnetic field which rotates relative to a liquid crystal sample, allows the direct measurement of the rotational viscosity 71 in nematics. It also allows a straightforward measurement of the magnetic anisotropy Xa if it is positive, via the observation of a critical angular frequency c c-... 

Small molecules in low viscosity solutions have, typically, rotational correlation times of a few tens of picoseconds, which means that the extreme narrowing conditions usually prevail. As a consequence, the interpretation of certain relaxation parameters, such as carbon-13 and NOE for proton-bearing carbons, is very simple. Basically, tlie DCC for a directly bonded CH pair can be assumed to be known and the experiments yield a value of the correlation time, t. One interesting application of the measurement of is to follow its variation with the site in the molecule (motional anisotropy), with temperature (the correlation... 

Viscoelastic fluids that are more concentrated are characteri2ed with devices that are similar to the rotational viscometers described previously. However, instead of constant rotational motion in one direction, a sinusoidal oscillatory motion is provided. Some instmments allow both viscosity and viscoelastic measurements. 

A shearing action grows between the compound and the rotor, and the resulting torque is measured in arbitrary units called Mooney units, which directly relate to torque. Normally, a preheat period is given to the elastomer following which the disk starts to rotate. An initial high viscosity is recorded which decreases to a minimum value. If the viscosity is more, then the Mooney unit (number) is more and viceversa. 

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. 

When the liquid, solution or lyophobic colloidal suspension contains asymmetric particles or when it is too concentrated, other methods must be applied to measure the viscosity. This is for instance the case with clay suspensions. In the past the viscosity of clay suspensions was measured by means of a bucket with a hole in it. The bucket was filled with clay suspension and after the stopper had been removed from the hole, the time required by the volume to drain was measured as a function of e.g. the volume and composition. Later mechanical methods were applied. One of them is based on the principle that a metal cylinder or disc, suspended from a torsion thread, is exposed to a certain resistance when you rotate it in the solution or suspension. Before the measurement the cylinder or disc is turned 360° anti-clockwise and then released. After having revolved over a certain angle, the cylinder or disc will change its direction of rotation. The rotation angle is a measure for the viscosity. 

On a RDE, in the absence of a surface layer, the EHD impedance is a function of a single dimensionless frequency, pSc1/3. This means that if the viscosity of the medium directly above the surface of the electrode and the diffusion coefficient of the species of interest are independent of position away from the electrode, then the EHD impedance measured at different rotation frequencies reduces to a common curve when plotted as a function of p. In other words, there is a characteristic dimensionless diffusional relaxation time for the system, pD, strictly (pSc1/3)D, which is independent of the disc rotation frequency. However, if v or D vary with position (for example, as a consequence of the formation of a viscous boundary layer or the presence of a surface film), then, except under particular circumstances described below, reduction of the measured parameters to a common curve is not possible. Under these conditions pD is dependent upon the disc rotation frequency. The variation of the EHD impedance with as a function of p is therefore the diagnostic for... 

---