Tubeless siphon

The simplest way to use the tubeless siphon is to record the maximum height rise that can be achieved when one applies full vacuum. However, strain rates and tensile stress can be obtained with the same analysis used for fiber spinning except that the sign of the gravity term in eq. 7.5.8 is now positive. This tends to destabilize the flow and reduce the fiber length. Another problem with the 

Tracer line pattern obtained for a tubeless siphon experiment. The lines were drawn from a high spe movie of 2% polyisobutylene in mineral oil. Time interval between line positions, 2.5 ms siphon length, 10 mm. From Mathys (1988). 

Stretch rate as a function of axial distance for a 3% hydrolyzed polyacrylamide solution (3.4 X 10 g/cm Mu, = 8 X 10 ). From Moan and Magueur (1988). 

Bubble of radius R on the end of a capillary tube in test fluid po is the pressure inside the bubble, poo in the surrounding fluid. 

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Another type of experiment involves a fluid filament being drawn upward against gravity from a reservoir of the fluid (101,213,214), a phenomenon often called the tubeless siphon. The maximum height of the siphon is a measure of the spinnabiUty and extensional viscosity of the fluid. Mote quantitative measures of stress, strain, and strain rate can be determined from the pressure difference and filament diameter. A more recent filament stretching device ia which the specimen is held between two disks that move apart allows measurements ia low viscosity Hquids (215). AH of these methods are limited to spinnable fluids under small total strains and strain rates. High strain rates tend to break the column or filament. 

Many materials of practical interest (such as polymer solutions and melts, foodstuffs, and biological fluids) exhibit viscoelastic characteristics they have some ability to store and recover shear energy and therefore show some of the properties of both a solid and a liquid. Thus a solid may be subject to creep and a fluid may exhibit elastic properties. Several phenomena ascribed to fluid elasticity including die swell, rod climbing (Weissenberg effect), the tubeless siphon, bouncing of a sphere, and the development of secondary flow patterns at low Reynolds numbers, have recently been illustrated in an excellent photographic study(18). Two common and easily observable examples of viscoelastic behaviour in a liquid are ... 

Fig. 4.3.1 (a) Photographs of a tubeless siphon formed by dissolving 0.5%w/v poly (ethylene oxide) powder in tap water, where a Fano column can be seen between the tip of the glass pipette at the top and fluid reservoir at the bottom, (b) Excess fluid can be seen just below the fluid entrance, (c) A large amount of excess fluid eventually flows downwards outside and along the Fano column, which can disturb the vertical location of the column. These figures illustrate the fact that there is an optimum volume flow rate for a particular flow system. 

E. F. Matthys 1988, (Measurement of velocity for polymeric fluids by a photo-chromic flow visualization technique the tubeless siphon), J. Rheol. 32 (8), 773-788. 

Tubeless siphon phenomenon, 22 740 Tube precipitators, 26 703 Tuberculosis... 

Explanations of this behavior are difficult. Perhaps we can begin to understand viscoelasticity by considering a dilute solution of PEO in water. If we begin to pour this solution from a raised beaker into a beaker placed below it, a stream of viscous fluid will connect the two beakers. On tipping up the raised beaker, so that fluid would ordinarily no longer flow out, the viscous PEO solution will continue to flow p, out, and down to the lower beaker. This effect is called a tubeless siphon because the liquid is not enclosed in a hose or tube one would use to siphon an ordinary liquid. Cutting the PEO strand with scissors will cause the upper portion to pull itself back into the top beaker. 

While the stress tensor component tfor purely viscous fluids can be determined from the instantaneous values of the rate of deformation tensor 4, the past history of deformation together with the current value of 4, may become an important factor in determining t, for viscoelastic fluids. Constitutive equations to describe stress relaxation and normal stress phenomena are also needed. Unusual effects exhibited by viscoelastic fluids include rod climbing (Weis-senberg effect), die swell, recoil, tubeless siphon, drag, and heat transfer reduction in turbulent flow. 

For evaluation of DUEVs, with less shear viscosity contribution to the total response, the modified stagnation technique is the procedure of preference. For practical applications the vacuum-suction filament-draw technique is probably the more valuable because the deformation rates in many applications are not solely extensional in nature. The comments regarding the velocity gradients in the tubeless siphon (41) are appropriate to the fiber-suction approach flow in a tubeless siphon approximates extensional flow in a sense that the largest components of the velocity gradient tensors are diagonal ones. ... 

Fig. 7.16 Illustration of tubeless siphon and continuous pouring of polymer melt...

Example of (mainly) longitudinal shear flows (a) conveigent duct, (b) the tubeless siphon. With dilute (10 polymer solutions one can often arrive at a column 10 cm high. 

A nearly i al situation of this type is found with the tubeless siphon of Fig. VI. 10b. (This ascending column is easily obtained with polymer solutions because of their ability to thread.) The interest of this geometry, from the present point of view, is to provide us with a simple convergent flow (not perturbed by walls) in the bulk of the fluid behw the siphon. (Chain behavior inside the siphon has been studied in experiments at Naples, but the analysis ignored possible elongations before entry.)... 

This is the main simplification introduced by the tubeless siphon as opposed to the entry of a capillary. However, our discussion remains qualitatively valid for the latter case (provided that the solution is dilute). 

Fig. 4.1. The tubeless siphon a dilute solution of long flexible polymers can be sucked up over large intervals / ( 20cm). This shows the dramatic effects of the polymer-induced stresses in longitudinal shear flows.

Velocimetry. - An analytic model for the velocity field within a tubeless siphon (Fano flow) was presented. The model was based on a simple differential equation in which extensional, shear and gravitational pressure gradient forces are balanced. The role of surface tension in determining boundary conditions for the flow is considered. The analysis is applied to NMR velocimetry data (Xia and Callaghan, J. Magn. Reson., 2003, 16, 365) on a... 

Figure 2.15 Tubeless siphoning can be done for a viscoelastic fluid but not for a Newtonian fluid. (Reprinted from Ref. 23 with kind permission from John Wiley Sons, Inc., New York. USA.)...

A variation on fiber spinning is the ductless or tubeless siphon, which is simply fiber spinning in reverse. A nozzle is dipped in a bath of the test fluid, a vacuum applied, and fluid sucked out of the bath. The nozzle is slowly raised, and a free-standing, rising colunm of fluid develops as shown in Figure 1.4. Sometimes the tubeless siphon is called a Fano flow, after the physician who first reported the technique (1908). 

Fiber spinning (Figure 7.5.2) approximates one end of the axisymmetric stagnation flow. The tubeless siphon is a little closer. But neither has a stagnation point. Ideally we want to confine a fluid to flow within the stream surfaces indicated in Figure 7.7.1. For planar stagnation these surfaces are defined (Winter et al., 1979) by the relation... 

The other important commercial design for extensional measurements on low viscosity fluids is the opposed nozzle device shown in Figure 8.5.2 (Fuller et al., 1987 Mikkelsen et al., 1988). In addition to the opposed-nozzle configuration, if the arm G is turned 90°, the device can also be operated as fiber spinning and tubeless siphon rheometers (Cai et al., 1992). 

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