Helium liquid films

The story gets better. C. H. Anderson and E. S. Sabisky, "The absence of a solid layer of helium on alkaline earth fluoride substrates," J. Low Temp. Phys., 3, 235-8 (1970), reported the thickness of helium liquid condensed from vapor onto ceramic substrates. Van der Waals attraction nicely explains film thickness vs. the chemical potential of helium in the vapor. 

Example 6.3 Diffusion of species from a gas mixture to a falling liquid film In a wetted wall column, the falling liquid film consists of a mixture of acetone (1) and benzene (2). This falling film is in contact with a downward flowing gas mixture of acetone, benzene, and helium (3). The pressure, temperature, and thickness of the film are P = 1 atm, T = 28.1°C, and 8 = 0.044 cm. The gas entering at the top of the column contains 10 mol% acetone and no benzene, and the composition of the vapor flow at the interface is... 

Modem GC uses capillary columns (internal diameter 0.1-0.5 mm) up to 60 m in length. The stationary phase is generally a cross-linked silicone polymer, coated as a thin film on the inner wall of the fused silica (Si02) capillary at normal operating temperatures, this behaves in a similar manner to a liquid film, but is far more robust. Common stationary phases for GC are shown in Fig. 32.4. The mobile phase ( carrier gas ) is usually nitrogen or helium. Selective separation is achieved as a result of the differential partitioning of individual compounds between the carrier gas and silicone polymer phases. The separation of most organic molecules is influenced by... 

Figure 10.8. Ratio /C12A115 which are elements of the zero-flux matrix of mass transfer coefficients [/c], as a function of the gas-phase Reynolds number. Mass transfer between a gaseous mixture of acetone (l)-benzene (2)-helium (3) and a liquid film containing acetone and benzene. Calculations by Krishna (1982) based on the von Karman turbulent film model and the Chilton-Colburn analogy.

Gas-liquid chromatography (GLC) was used routinely to monitor reactions, in a Carlo Erba HRGC 5160 chromatograph, with a FID detector and a silicone DB-1 (bonded methyl silicone, J W Scientific, Inc., Rancho Cordova, California column, 15 m x 0,25 mm x 0,10 pm. Helium N50 was used as carrier (2 ml/min, 60 KPa, split ratio 100 1) and analyses were performed at 260°C (iiy.) oven, 170°C (7 min) 200°C (2 min, 4 C/min) 285°C (15°C/min 20 min). Melting points were determined on a Reichert Thermovar hot bench and are uncorrected. Infra-red spectra were recorded on a Perkin-Elmer 298, as liquid films in CHCI3 solution. Proton and... 

The column can be an open capillary or a packed tube. In the first case the mobile phase is coated as a thin film on the inner wall of the capillary. If the mobile phase has a certain solvating power, as in SFC, it is necessary to cross-link this liquid film, whereas in GC linear polymers are used in many cases because the usual carrier gases, helium and hydrogen, cannot dissolve any stationary phase. (Owing to problems with manufacturing and... 

There are two characteristics that make helium attractive for space applications the first is weight (about 0.125 kg/1) the second is its superfluidity. Helium becomes superfluid at T< 2.17K (p < 37.8 torr). Thanks to superfluiduty, helium forms a film that completely covers the walls of the container and guarantees a homogeneous cooling even if most of the liquid does not have a fixed position inside the container (no gravity). 

Another example of thin films showing metallic transport properties down to liquid helium temperature is 0-(BET-TTF)2Br.3H2O, where bilayers are grown on transparent polycarbonate substrates (Mas-Torrent et al, 2001). The bilayers are... 

Belertser et al (1988) have observed that the electrical resistivity of amorphous chromium films at liquid-helium temperatures jumps from a value (10 3 O cm) characteristic of a poor metal by a factor 103, when the hydrogen content is increased sufficiently to increase the lattice constant by 10%. The transition is not abrupt, and is thought by these authors to be of Anderson type. They claim that it is the first time such a transition has been observed in a solid, and that it is similar to that in expanded mercury vapour (Section 4). 

Figure 3 shows the result from a typical experiment carried out with a helium flow of 300 cc./min., an initial liquid depth of 1.25 cm., and an initial gas-phase diffusion path, b0, of 1.29 cm. Although Equation 15 is quadratic, the initial part of the release curve may be approximated by a straight line, as shown in Figure 3. This arises from the fact that only a small change in gas-film thickness took place initially.

Very recently, Bailey and Richards (23) have shown that a high degree of sensitivity for adsorbed species can be achieved by measuring the absorption of infrared radiation on a thin sample cooled to liquid helium temperature. The optical arrangement used in these studies is shown in Figure 10. The modulated beam produced by the interferometer is introduced into the UHV sample chamber and reflected off a thin slice of monocrystalline alumina covered on one side by a 1000 k film of nickel or copper. Radiation absorbed by the sample is detected by a doped germanium resistance thermometer. The minimum absorbed power detected by this device when operated at liquid helium temperature is 5 x 10 14 W for a 1 Hz band width. With this sensitivity absorbtivities of 10"4 could be measured. 

Tunneling junctions are most often produced in a crossed stripe geometry so that 4-terminal measurements of their current-voltage characteristics can be made. Electrical contacts are made to the films ( often with miniature brass "C" clamps ), the samples are mounted in a Dewar insert, and cooled to liquid helium temperatures ( 4.2 ° K or below ). 

As mentioned above, the new method of cryochemical synthesis of polymer nanocomposite films has been developed based on co-deposition of M/ SC and monomer vapors at temperature 80K and subsequent low-temperature solid-state polymerization of monomer matrix ([2] and works cited herein). It has been established that a number of monomers (acrylonitrile, formaldehyde, /i-xylylene and its derivatives) polymerize in solid state in absence of thermal movement of molecules owing to own specific supra-molecular structure. When reaction is initiated by y- or UV-radiation the formation of a polymer matrix occurs even at the temperatures close to temperature of liquid helium [66-69]. 

Photo- and cathodoluminescence (PL, CL) measurements at liquid helium temperature are sensitive tools for investigation of the optical recombination properties [62] of the PLD ZnO films. In ZnO, the information depth of PL using a 325 nm He-Cd laser is only about 60 nm [63], corresponding to... 

Put into a vessel, liquid helium will not only wet the walls but will also—in a gravity-defying act—form macroscopically thick films on the walls. The polarizability of liquid helium is greater than that of air (em > air) but less than that of the walls (em < Waii). As a result, the liquid tries to put as much of itself as possible near solid walls, to create an ever-thicker film to the extent that van der Waals energy can pay for this mass displacement against gravity. There is not actually any repulsion. Rather, there is attraction to the wall. Its effect is to thicken the helium "medium" and make it move up the attractive wall. 

E. S. Sabisky and C. H. Anderson, "Verification of the Lifshitz theory of the van der Waals potential using liquid-helium films," Phys. Rev. A, 7, 790-806 (1973). 

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