Effects of degassing on the long-range attractive force between hydrophobic surfaces in water

The long-ranged attractions between hydrophobic amorphous fluoropolymer surfaces are measured in water with and without dissolved air. An atomic force microscope is used to obtain more than 500 measured jump-in distances, which yields statistically reliable results. It is found that the range of the attraction and its variability is generally significantly decreased in deaerated water as compared to normal, aerated water. However, the range and strength of the attraction in deaerated water remain significantly greater than the van der Waals attraction for this system. The experimental observations are consistent with (1) nanobubbles being primarily responsible for the long-ranged attraction in normal water, (2) nanobubbles not being present in deaerated water when the surfaces are not in contact, and (3) the attraction in the absence of nanobubbles being most probably due to the approach to the separation-induced spinodal cavitation of the type identified by Bérard et al. [J. Chem. Phys. 1993, 98, 7236]. It is argued that the measurements in deaerated water reveal the bare or pristine hydrophobic attraction unobscured by nanobubbles.     

Two-dimensional radiative equilibrium of a gray medium between concentric cylinders

A method for calculating radiative equilibrium temperature and surface heat-flux distributions is developed for an absorbing-emitting medium between concentric cylinders. The cylinder walls are assumed to be gray diffuse absorbers and emitters and have arbitrary temperature distributions along their peripheries. Heat generation may take place within the medium. As a first approximation, the problem is solved for optically-thick systems by using the differential approximation. To obtain accurate results also for the optically-thin and intermediate regimes, the differential approximation is subsequently improved by a number of geometric parameters, as has been discussed in a previous paper. As examples, two cares are presented in detail: (1) a hollow cylinder with uniform internal heat generation and uniform surface temperature and (2) a hollow cylinder with a cosine temperature distribution imposed on the wall, with no internal heat generation. Comparison with some numerical results generated by Hottel's zonal method shows excellent agreement.     

The kinetic isotope effect for carbon and oxygen in the reaction CO + OH

The kinetic isotope effect (KIE) for carbon and oxygen in the reaction CO + OH has been measured over a range of pressures of air and at 0.2 and 1.0 atm of oxygen, argon, and helium. The reaction was carried out with 21–86% conversion under static conditions, utilizing the photolysis of H₂O₂ as a source of OH radicals. The value of the KIE for carbon varies with pressure and the kind of ambient gas; for air the ratio of the reaction rates ¹²k/¹³k has the value 1.007 at 1.00 atm and decreases to 0.997 at 0.2 atm; for oxygen and argon over the same pressure range the values are 1.002–0.994 and 1.000–0.991, respectively. The value of the KIE for the CO oxygen atom is ¹⁶k/¹⁸k = 0.990 over the pressure range 0.2–1.0 atm and is independent of the kind of ambient gas. No exchange of the oxygen atoms in the activated complex, followed by decomposition to the starting molecules, was observed. From the mechanistic standpoint the normal KIE observed for carbon at the high pressure is attributed to the initial formation of the activated HOCO radical, whereas the inverse KIE observed at low pressures is a result of the KIE for the reverse reaction HOCO? → CO + OH being greater than that for the forward reaction HOCO^? → CO₂ + H. The derived isotopic equilibrium constant for HOCO ?CO favors the enrichment of ¹³C in the more strongly bound HOCO.     

Amplitude analysis of hypercharge exchange reactions

We present a smooth parametrization of the amplitudes for meson-baryon hypercharge exchange reactions which describes well all the data available at present for laboratory momenta above 3 GeV/c. The K_V¹ (K_T¹) amplitudes we find are very similar to the ?(ω) amplitudes, respectively, obtained already by other authors. The SU(3) assumption seems to be well supported by the experimental data.     

On the Variability of Demand for Families of Items

For the purposes of inventory control and production planning of multi-product lines, often consisting of hundreds or thousands of items differing only in minor ways from each other, it is often found that the variability of demand for individual items can be expressed as a simple function of the expected demand for each item. Usually this is not Poisson variability. In this paper a simple demand model is proposed which generates variability of the type often observed, and which in fact leads directly to the Quadratic Law of Burgin and Wild. It is noted how, in practice, it may be hard to distinguish between this law and the Power Law (Brown). The difference, however, is by no means unimportant, and this is discussed. The statistical problems involved in estimating either law from practical data are also shown to be non-trivial.     

Absence of magnetism in hcp iron-nickel at 11 K

Synchrotron M?ssbauer spectroscopy (SMS) was performed on an hcp-phase alloy of composition Fe92Ni8 at a pressure of 21 GPa and a temperature of 11 K. Density functional theoretical calculations predict antiferromagnetism in both hcp Fe and hcp Fe-Ni. For hcp Fe, these calculations predict no hyperfine magnetic field, consistent with previous experiments. For hcp Fe-Ni, however, substantial hyperfine magnetic fields are predicted, but these were not observed in the SMS spectra. Two possible explanations are suggested. First, small but significant errors in the generalized gradient approximation density functional may lead to an erroneous prediction of magnetic order or of erroneous hyperfine magnetic fields in antiferromagnetic hcp Fe-Ni. Alternately, quantum fluctuations with periods much shorter than the lifetime of the nuclear excited state would prohibit the detection of moments by SMS.