Capillary diameter

Knudseci s very careful experiments on a long uniform capillary show that N L/ Pj -p ) passes through a marked minimum when plotted as a function of (P +P2)/2, at a value of the mean pressure such that the capillary diameter and the mean free path length are comparable. At higher values of the mean pressure, N L/(pj " 2 rises linearly, as in the case of a porous medium. 

Flow processes iaside the spinneret are governed by shear viscosity and shear rate. PET is a non-Newtonian elastic fluid. Spinning filament tension and molecular orientation depend on polymer temperature and viscosity, spinneret capillary diameter and length, spin speed, rate of filament cooling, inertia, and air drag (69,70). These variables combine to attenuate the fiber and orient and sometimes crystallize the molecular chains (71). 

The original Ostwald viscometer has been modified in many ways, and a number of different versions are on the market (Table 4) (21). Most are available with a wide choice of capillary diameters and therefore a number of viscosity ranges. A number of viscometers are described in ASTM D445, which also Hsts detailed recommendations on dimensions and methods of use. 

Monofilament Synthetic Nonabsorbable Sutures. Monofilament synthetic nonabsorbable sutures are made from thermoplastic resins melt-spun to form monofilaments. Spinnarets of different capillary diameter are used to make a range of suture sizes. 

Fig. 10.14 Liquid velocity and meniscus location versus capillary diameter (L = 2xl0 m, g = 9.8 m/s, = 200 W/m ). Reprinted from Yarin et al. (2002) with permission...

Viscosity measurements were carried out using Cannon-Fenske viscometers of appropriate capillary diameter so as to keep the efflux time between 200-300 seconds. Approximate shear rate at the wall was calculated using the equation... 

FIGURE 9.4 Quick ampoule sampling of volatiles. Ninety-five percent ampoule filling time as a function of capillary diameter for 3 compounds. Calculation for filling through consecutive Knudsen diffusion into a vacuum, super sonic flow, and laminar flow. 

For a capillary of circular cross-section, fRe has a value of 16, and dhyd is equal to the capillary diameter. Equation (5.9) then reduces to Poiseuille s equation. 

From Eqs. 17.31a and 17.31b it can be derived that when the amount injected is very small, the detection path length very narrow and the capillary diameter is also very small, only a influences the broadening of a migrating zone. In such a case, afot is approximated by of and Eq. 17.31b can be simplified to... 

The plug flow profile would only be distorted in very narrow bore capillaries with a diameter smaller than the thickness of two double-layers that then overlap. To achieve an undisturbed flow, Knox suggested that the diameter should be 10-40 times larger than 6 [15]. This can easily be achieved in open capillaries. However, once the capillary is packed with a stationary phase, typically small modified silica beads that carry on their own charged functionalities, the distance between adjacent double-layers is only a fraction of the capillary diameter. However, several studies demonstrated that beads with a submicrometer size can be used safely as packings for CEC columns run in dilute buffer solutions [15,35]. 

The effect that the quality of the bed structure has on the chromatographic properties of columns packed with particles has been well known for a long time [1]. Similarly, the efficiency of capillary electrophoretic separations reaches its maximum for a specific capillary diameter, and then decreases steeply for both larger and smaller size [ 117]. Therefore, any improvement in the efficiency of the polymeric monolithic columns for the isocratic separations of small molecules is likely to be achieved through the optimization of their porous structure rather than their chemistry. 

In the relations given earlier, it is assumed that the fluid can be regarded as a continuum and that there is no slip between the wall of the capillary and the fluid layers in contact with it. However, when conditions are such that the mean free path of the molecules of a gas is a significant fraction of the capillary diameter, the flowrate at a given value of the pressure gradient becomes greater than the predicted value. If the mean free path exceeds the capillary diameter, the flowrate becomes independent of the viscosity and the process is one of diffusion. Whereas these considerations apply only at very low pressures in normal tubes, in fine-pored materials the pore diameter and the mean free path may be of the same order of magnitude even at atmospheric pressure. 

Fused silica capillaries are almost universally used in capillary electrophoresis. The inner diameter of fused silica capillaries varies from 20 to 200 pm, and the outer diameter varies from 150 to 360 pm. Selection of the capillary inner diameter is a compromise between resolution, sensitivity, and capacity. Best resolution is achieved by reducing the capillary diameter to maximize heat dissipation. Best sensitivity and sample load capacity are achieved with large internal diameters. A capillary internal diameter of 50 pm is optimal for most applications, but diameters of 75 to 100 pm may be needed for high sensitivity or for micropreparative applications. However, capillary diameters above 75 pm exhibit poor heat dissipation and may require use of low-conductivity buffers and low field strengths to avoid excessive Joule heating. 

Detection If a small capillary diameter is desired for efficiency purposes, the detection part of the capillary can be adapted for better detection sensitivity. Examples are the bubble cell capillary and the Z-cell. In the bubble cell capillary, the capillary diameter is enlarged at the detection window so that better concentration sensitivity is obtained. If you implement a bubble cell capillary in your pharmaceutical analysis method, it is important to test different batches. Test also whether you need a bubble cell capillary or whether you can gain similar sensitivity increase with a proper injection procedure. Also, check the effect of the bubble cell on band broadening. An approximately three-times sensitivity enhancement is possible. 

We should not forget that the capillary plays a key role in the separation. Even if a fused silica capillary is usually used, the choice of diameter and capillary length has a major impact. Decreasing the capillary diameter or increasing the capillary length will lower the current and... 

A theoretical study of factors which affect pneumatic nebulizers (nebulizer geometry, capillary diameter, temperature fluctuations, gas pressure, solution viscosity, etc.] has been published by Heineman (24). Ultrasonic nebulization (which produces both smaller droplets and a narrower droplet size distribution] continues to attract attention (93, 95). 

Since the driving force of the flow is uniformly distributed across the diameter of the capillary, the flow profile is essentially flat. This flat profile contributes to the very high separation efficiency of CZE. Electroosmotic pumping therefore is beneficial, in contrast to laminar flow generated by a HPLC pump, where a parabolic flow profile is established. The electroosmotic flow rate and its flat profile are generally independent of the capillary diameter. However, if the internal diameter of the capillary exceeds 250 pun, the flat profile is increasingly disrupted. 

From this equation it can be seen that the efficiency per unit length is inversely proportional to the capillary diameter. Decreasing the diameter of the capillary will decrease the height equivalent to a theoretical plate. The efficiency per unit length increases. Therefore, smaller-diameter capillaries can be used at shorter lengths, which ultimately decreases the separation time. 

The fractions coilected are each run dropwise into 100 ml of stirred methanol to which two drops of concentrated hydrochloric acid have been added.Turbidity or precipitation will be observed from about the 6th fraction to about the 20th fraction.The precipitated fractions are filtered off, washed with methanol and dried to constant weight in vacuum at 20 °C. For each fraction the viscosity is measured in water at 25 °C using a capillary viscometer (capillary diameter 0.35 mm) and at as high a concentration as possible (10 g/l) in order to minimize errors.The limiting viscosity number, and hence the molecular weight, is estimated (see Sect. 2.3.3.3.1). Adjacent fractions for which there may be insufficient material for a viscosity measurement,can be combined where necessary. 

The oxidative degradation of polyfvinyl alcohol) is followed at 25 °C by viscosity measurements in an Ostwald viscometer (capillary diameter 0.4 mm). One proceeds as follows ... 

Polymeric stationary phases have many advantages when polymerized in capillaries (diameter < 0.5 mm) as rods in the presence of porogens to yield channels for mobile phase transport. They are frequently used in the analysis of peptides, proteins, and nucleic acids when directly coupled to electro spray mass spectrometry. 

According to the type of scale division, a distinction is made between two forms of compression vacuum gauges those with a linear scale (see Fig. 3.7) and those with a square-law scale (see Fig. 3.8). In the case of the compression vacuum gauges of the McLeod linear-scale type, the ratio of the enclosed residual volume Vc to the total volume V must be knovm for each height of the mercury level in the measurement capillary this ratio is shown on the scale provided with the instrument. In the case of compression vacuum gauges with a square-law scale, the total volume and the capillary diameter d must be known. 

Fig. 4. B. stearothermopbilus SOS needles grow mostly along the capillary axis but scmie of them also grow in different directions (same conditions as in Fig. 3 capillary diameter = 0.7 mm)...

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