Capillary-flow technique

As seen in Table 6.1, capillary viscometers are the most extensively used instruments for the measurement of viscosity of aqueous electrolyte solutions at high temperature and pressure. They have the advantage of simplicity of construction and operation. 

The correction factors n and m reflect the fact that in a practical viscometer two chambers must be placed at either end of the capillary in order to measure the pressure drop. Thus, for example, the parabolic velocity distribution characteristic of most of the flow can only be realized some distance downstream from the inlet of the capillary. 

Other corrections - capillary shape and slip, non-uniform cross-sections (Barr, 1931), elliptical cross-section (Ito, 1951a), coiled capillaries (Ito, 1951b Dawe and Smith, 1970), wall roughness (Kawata, 1961), fluid properties effects, compressibility, non-Newtonian (Van Wazer et al, 1963), and surface tension correction (Van Wazer et al, 1963 Goncalves et al, 1991 Kawata et al, 1991) - can be found in the respective literature. 

The working capillary-1 with ID of 0.3 mm and length of 216 mm is made from stainless steel. The capillary-1 is soldered to the extension tube-6. The fluid under study flows to a cold zone through the extension tube-6. Capillary-1 with extension tube-6 located in the high temperature and high pressure autoclave-4. The extension tube-6 is connected to a movable cylinder-9, which in turn is connected to the fixed cylinder-11 by means of the flexible tube-10. Both cylinders (9 and 11) are supplied with identical expanded bottles, employed to stabilize the fluid efflux through the capillary. The input and output sections of the capillary have conical extensions. All parts of the experimental installation in contact with the sample are made out of stainless steel. 

The capillary tube-1 is filled with the fluid and when the movable cylinder is moved vertically at constant speed, the fluid flows through the capillary. To create and also measure accurately the pressure, the autoclave is connected to a dead-weight pressure gauge by means of separating vessel-13. Merciuy is used as the separating liquid. 

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The yield stress can be obtained by the rotational viscometry technique as shown in Chapter 4. Thus from two sets of rheological determinations, one the capillary flow technique and the other rotational viscometry, enough information can be obtained to treat the problems associated with the flow of grease in pipes. 

Finally, Rivkin et al. (1986) also used the capillary flow technique to measure the viscosity of aqueous boron solutions, at temperatures up to 623 K and at pressures up to 30 MPa. In this case they employed a platinum capillary of 500 nun length and 0.3 nun ID, placed in a liquid thermostat in which temperature was controlled with an uncertainty of 0.03 K. A prunp-flowmeter was used to measure the volume of the fluid flowing through the capillary tube at each given temperatme and pressure. The pressure drop across the capillary ends was measmed with a compensation-type differential mercmy pressure gauge with a movable elbow. The details of the experimental apparatus and measurement procedme are given in Rivkin et al. (1979). 

In this section the three techniques employed for the measurement of aqueous electrolyte solutions were discussed. These were the capillary-flow technique, the oscillating-disc technique and the falling-body method. From the discussion of the techniques it is apparent that measurements performed with the capillary-flow viscometer and the oscil-lating-disc viscometer enjoy a low degree of uncertainty. Hence, measurements would be expected to attain an uncertainty of better than 2%. Measurements performed with the... 

The flow profiles of electrodriven and pressure driven separations are illustrated in Figure 9.2. Electroosmotic flow, since it originates near the capillary walls, is characterized by a flat flow profile. A laminar profile is observed in pressure-driven systems. In pressure-driven flow systems, the highest velocities are reached in the center of the flow channels, while the lowest velocities are attained near the column walls. Since a zone of analyte-distributing events across the flow conduit has different velocities across a laminar profile, band broadening results as the analyte zone is transferred through the conduit. The flat electroosmotic flow profile created in electrodriven separations is a principal advantage of capillary electrophoretic techniques and results in extremely efficient separations. 

In reduced-flow LC-MS systems, the solvent flow into the spectrometer is reduced to a level where the pumping system can cope. Essentially, three such systems have been developed direct-liquid-introduction (DLI), flowing FAB [531] and electrospray [532]. An alternative approach to belt transport interfacing is to deliver the column eluate directly into the MS source and use Cl techniques. Methods based on this principle are called direct-liquid-injection systems, which are comprised of capillary flow restrictors, diaphragms,... 

Another technique, capillary flow porometry has been developed by Porous Materials Inc. ° to characterize battery separators.The instrument can measure a number of characteristics of battery separators such as size of the pore at its most constricted part, the largest pore size, pore size distribution, permeability, and envelope surface... 

A variety of microscale separation methods, performed in capillary format, employ a pool of techniqnes based on the differential migration velocities of analytes under the action of an electric field, which is referred to as capillary electromigration techniques. These separation techniques may depend on electrophoresis, the transport of charged species through a medium by an applied electric field, or may rely on electrically driven mobile phases to provide a true chromatographic separation system. Therefore, the electric field may either cause the separation mechanism or just promote the flow of a solution throughout the capillary tube, in which the separation takes place, or both. 

New concepts presented in this edition include monolithic columns, bonded stationary phases, micro-HPLC, two-dimensional comprehensive liquid chromatography, gradient elution mode, and capillary electromigration techniques. The book also discusses LC-MS interfaces, nonlinear chromatography, displacement chromatography of peptides and proteins, field-flow fractionation, retention models for ions, and polymer HPLC. 

The first section of the book explores emerging novel aspects of HPLC and related separation methods based on the differential velocity of analytes in a liquid medium under the action of either an electric field (capillary electromigration techniques) or a gravitational field (field-flow fractionation). The section focusing on applications highlights four significant areas in which HPLC is successfully employed chiral pharmaceutical, environmental analysis, food analysis, and forensic science. 

Polypropylene compositions containing magnesium hydroxide, with and without magnesium stearate surface treatment, were characterised at low and high shear rates using dynamic and capillary measurement techniques [36]. A significant reduction in viscosity was observed when surface treatment was present, particularly at low shear rates. In addition, with this system, the yield stress for the onset of flow was markedly reduced (Compare magnesium hydroxide variants A and E in Fig. 9). 

Capillary-in-capillary mixers were used for electrospray ionization mass spectrometry (ESI-MS), which allows one to perform on-line kinetic studies for a wide range of applications in chemistry, bioorganic chemistry, isotope exchange experiments and enzymology, just to name a few [133], ESI-MS is a method alternative to the traditionally employed quench-flow techniques with off-line analysis. 

Scaffold porosity and information on the pore size distribution can be obtained from intrusion techniques. The most commonly used methods are mercury porosimetry and capillary flow porometry. In mercury porosimetry the pressure required to fill a tissue scaffold with non-wetting mercury is monitored over a set period of time. Higher pressures are required to fill small pores than large pores a fact that can be exploited using the Washburn equation13 to extract structural information where D is the diameter of the pore at a particular differential... 

Figure 5 compares the pore size distributions of the scaffold computed from the intrusive techniques of capillary flow porosimetry and mercury porometry. From this figure it is apparent that the range of pore sizes derived from capillary flow porometry occurs over a smaller length scale than those based on mercury porometry data. This difference is expected since underlying physics of the... 

Figure 5. Comparison between scaffolds pore distributions obtained via different measurement techniques (cap = capillary flow porometry, Hg = mercury porosimetry, = median value, = median value). The error bars represent the span of the distribution.

The distribution of pore sizes can be obtained from both mercury porosimetry and capillary flow porometry. These distributions are only representations of the actual scaffold structure reflecting the limitations of the underlying physics behind each technique. For this reason it is very difficult to compare pore size distributions for complex structures, such as particulate-leached tissue scaffolds. 

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