Membrane degasification and freeze pump thaw

Both siphon models–atmospheric and cohesion–predict that the maximum height of a siphon is dependent on the ambient barometric pressure. In the case of the atmospheric model, the pressure of the atmosphere is required to hold the column of water together. In the cohesion model, the limit is explained by the pressure at the top of the siphon falling below the vapour pressure of water, at the given temperature, so that cavitation occurs, i.e. the water starts to boil thereby breaking the column.

However, the cohesion model predicts that if cavitation can be prevented, the barometric height limit can be broken. The reason for the cohesion is that surfaces cost energy and the water/air surface is no different. For water, the surface energy is often referred to as surface tension. The surface energy of the water/air interface is 0.072 J/m2. It costs energy to make bubbles in water because of the energy of the bubble surface. For a bubble to be stable it must be supported either by internal pressure of a gas or by the equivalent tension (negative pressure) in the water. For gas in a bubble the pressure (P) is given by (1). This equation11 is exact for an ideal gas, but an approximation for a real gas.

Australia. Correspondence and requests for materials should be addressed to S.H. (email: )

of water and capped with 5 ml of oil, which stands on a small Perspex tray above the turbomolecular pump.

Pressure gauges are marked 1) APG-M-NW16, 2) AIM-S-NW25 and McLeod.

In the next experiment, the cohesive strength of water was tested using a simple inverted U-tube with the base exposed to vacuum, in the manner of a barometer (Fig. 1). Initially the U-tube was set to below the level of the surface of the liquid, while the glass vessel was evacuated, and all gases fully removed from above and within the liquids. When the partial pressure inside the vessel reduced to 7.5 ± 0.05 × 10−1 Pa the U-tube was raised by lifting the apex of the tube to a height of 300 mm above the surface of the oil. With a density marginally lower than that of water, the oil surface was assumed to be close to that of a hypothetical water vacuum interface. It was observed that the water formed a continuous column with no bubbles/cavities forming at the top of the tube (Fig. 2). The inverted U-tube

into position while the base is held under vacuum.

While the flow was initiated independently of atmospheric pressure within the siphon, it was noted that the movement of the reservoirs between the static and flowing conditions exposed surfaces that were previously covered with water. As this happened the pressure in the vacuum region was observed to rise above 103 Pa. Realising that this represented a fundamental flaw, in this, and in previous attempts by others at producing a water siphon under vacuum conditions, it was deemed that a moderate length siphon could not conclusively discount the effects of vapour pressure on supporting the column.

In order to discount the effect of external pressure acting on the liquid column, a second siphon was constructed, operating under atmospheric conditions, with a height above the nominal barometric limit of 10 m, using water degassed using a vacuum desiccator (Fig. 4).

Figure 5.Diagram of a siphon taller than the barometric limit with the reservoirs open to air. Water in the upper reservoir is capped with a 5 mm layer of silicon oil. A pulley is used at the apex to support the length of tube and prevent kinks in the pipe.

To measure the effects of capillary action in making any contribution to lifting the water within the siphon tube, one end of the empty siphon tube was immersed in the degassed water, which was open to air, while the other open end of the tube was held above the level of the liquid. As no differential was observed between the heights of the liquid inside the nylon tube and outside, capillary action was discounted as playing any significant role in the siphon process.

The ability to completely degas water has always represented a significant challenge in performing experiments investigating liquid tensile strength. It is widely known that the great variance observed both within and across different methods investigating the properties of water is due to the unpredict-able nature of the gases dissolved within22. In water free of all dissolved gases, bubbles only form when the energy gained in forming a cavity is greater than the binding energy of the surrounding molecules.

A simple method to overcome water loss is to change the energy barrier at the surface of the water by applying a layer of immiscible liquid above the surface. By floating a liquid with low specific gravity and ultra-low vapour pressure over the water, molecules at the interface are unable to leave the water and migrate through the capping liquid to the surface. Thus evaporative loss that normally occurs below the water vapour pressure is considerably reduced, if not entirely negated.

After initially degassing the water, there was no further evaporative loss or cavitation within the bulk liquid, or at any interface when the ambient pressure was below 10−3 Pa. While it could be argued that the oil was applying a downward force on the water raising the pressure above that of the vapour point, with a capping layer of only 5 mm, the oil would contribute a downward pressure of less than 43 Pa.

The siphon in the experiment described in this paper was clearly operating above the barometric limit, which, at the given barometric pressure was 10.18 ± 0.01 m for the atmospheric model and 9.94 ± 0.01 m for the cohesion model (ignoring the negligible velocity term). Therefore, it is evident that atmospheric

where TSw is the tensile strength of water. So for example if the tensile strength of a sample of water was 1 MPa, the maximum height of a siphon would be about 100 m. In the case of the siphon is this experi-ment we can say that the tensile strength of the water was greater than − 0.15 MPa.

Extrapolating these results from even the most conservative experimental measurements of the ten-sion under which cavitation occurs it is possible that the cohesive strength of fully degassed water is able to support a continuous vertical column greater than several hundred meters. While the experiment per-formed here did not reach anywhere near the absolute limit predicted it does shed light on the stability of flowing water under tensile stress and the possibility of constructing apparatus of suitable dimensions to test such a limit. These experiments also lend support to the cohesion-tension theory of sap ascent in trees. It would be interesting to perform further experiments to see if it is possible to operate a flowing siphon at above 100 m. If tensions as high as the transient tension of several 100 bar can be maintained at the apex of a siphon, then in principle a siphon should work up to a height of several km. However, it would be challenging to verify this experimentally, requiring a helicopter or UAV with a ceiling of several km capable of supporting several kg of water-filled tubing and cable supporting the siphon. It would also be interesting to repeat the experiment with a larger diameter tube. In view of the many anomalies of bulk water23, it would be interesting to explore the physical properties of water in the negative pressure regime of a siphon above 10 m.

To create the siphon above the barometric limit, a 30 m long 6 mm inner diameter flexible nylon tube (RS components) was used. Attached at either end of the tube were two stainless steel vacuum taps. Prior to filling with the previously degassed water, the tube was continually flushed with tap water for 4 hours to remove any deposits from within the pipe. The tube was then connected at one end to the vacuum pumps and evacuated continually for a period of 48 hours to allow all volatile compounds to be removed. Priming of the tube was achieved by placing the closed end of the evacuated tube into the degassed water, which was then opened allowing the water to flow up the tube, while the other end remained open to the vacuum system. Care was taken to prevent the capping oil from entering the tube during this process. Once the tube was entirely filled with degassed water, both ends of the tube were closed ready for the siphon to be set into position.

Before setting the tube in position the siphon was first inverted so that the ends of the tube were at the highest point with the bend at the lowest. This was done to allow additional degassed water to be added as the increased weight caused a slight expansion in the length of the tube. Once the extra water was added the tube was re-inverted with the bend at the apex and the legs hanging straight down into the reservoirs. To prevent kinks occurring in the hose at the apex the siphon the tube was set into a pulley of 12 cm diameter. Once primed, one end of the siphon was set into a reservoir containing more degassed water, with the other venting 30 cm lower inside an empty 1 litre glass beaker with both reservoirs open to the air. Taps at both ends of the tube were then opened so that the liquid could flow freely over the 14.5 m rise into the lower reservoir (video sequence 4). Once the upper reservoir was nearly depleted of liquid, the end was lifted out of the liquid allowing air to flow into the base of the tube.

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How to cite this article: Boatwright, A. et al. The height limit of a siphon. Sci. Rep.5, 16790; doi: 10.1038/srep16790 (2015).