Showing papers by "Robert Evans published in 1990"

Fluids adsorbed in narrow pores: phase equilibria and structure

Abstract: When gases or liquids are adsorbed in narrow pores or capillaries their properties are significantly different from those in a bulk phase. This article reviews recent developments in the statistical theory and computer simulation of simple fluids confined in model pores, emphasizing the microscopic structure and phase equilibria. The structure reflects the packing of atoms or molecules in confining geometries while the phase behavior reflects the presence of surface and bulk contributions to the fluid's free energy. Confinement shifts first-order transitions, such as condensation or freezing, away from their location in bulk; it also alters the location and nature of the bulk critical point reducing the effective dimensionality. Sometimes surface phase transitions such as layering and prewetting compete with shifted bulk transitions giving rise to rich phase diagrams. The extent to which the theorists' results for fluids in single idealized pores might be relevant for solvation force studies probing liquids between crossed mica cylinders and for gas adsorption studies in real mesoporous solids such as VYCOR is mentioned briefly.

Liquids at interfaces: what can a theorist contribute?

Abstract: Reviews some recent theoretical and computer simulation studies of simple atomic fluids adsorbed at structureless substrates. Emphasis is placed on phase transitions, especially the various types of wetting transition. Criticality is associated with capillary-wave-like fluctuations in a continuously growing wetting film. This is of a subtle nature, which is best understood in terms of the pairwise correlation function of the fluid. Other surface phase transitions, such as prewetting and layering, occur out of bulk coexistence. Theory suggests that for sufficiently attractive substrates a sequence of first-order transitions, corresponding to the growth of new adsorbed liquid layers, should occur as the pressure of the bulk gas increases towards saturation at temperatures not too far above the bulk triple point. The extent to which such behaviour is found in adsorption experiments is discussed. The authors also argue that a simple fluid confined between two parallel hard-walls can exhibit surprisingly rich equilibria.

The North Brazil Current retroflection: Seasonal structure and eddy variability

Abstract: We report results from a 1-year (September 1987 to September 1988) moored current meter array spanning the continental margin off French Guiana near 8° N in the western tropical Atlantic. Current profiles were recorded at three sites: at the shelf break, over the mid-continental slope, and at the base of the continental rise. Upper level mean currents showed a northwestward flowing North Brazil Current (NBC) and offshore retroflection of this flow into the North Equatorial Countercurrent from late summer through about January. Generally weak upper level mean flows were observed during the spring (February–June). Persistent northwestward mean flow was observed at 900 m depth over the continental slope, indicating northward transport of Antarctic Intermediate Waters in a subsurface boundary flow at speeds of 10–15 cm s−1. Deep currents over the continental rise showed a strong southeastward Deep Western Boundary Current (DWBC) extending from 2500 m to the bottom, with mean core speeds of nearly 30 cm s−1 at 4300 m depth. Transport estimates based on these data and a few geostrophic sections suggest a DWBC transport of 20–40 × 106 m3 s−1 at this location. Low-frequency current fluctuations were dominated by a well-defined 40- to 60-day oscillation with peak-to-peak meridional velocity amplitudes of > 1 m s−1 during the fall. Analysis of historical coastal zone color scanner imagery suggests that these oscillations are related to quasi-periodic generation and subsequent westward movement of ≈ 400 km diameter eddies from the NBC retroflection. These results contrast sharply with earlier indications of a quasi-permanent “Demerara Eddy” in this region, and suggest that this commonly observed feature is in fact a transient phenomenon associated with the time-dependent behavior of the NBC retroflection.

Surface-ocean color and deep-ocean carbon flux: how close a connection?

Abstract: Seven years of simultaneous, quasi-continuous data collected by the Nimbus-7 Coastal Zone Color Scanner and by a deep-ocean sediment trap in the Sargasso Sea allow the derivation of empirical relationships between remotely sensed ocean color and the sinking of particulate carbon into the deep sea. In agreement with earlier observations, the results indicate a 1.5-month lag between surface-ocean events observed by the satellite and arrival of a record of those events, carried by sinking particles, at a depth of 3200 m. In addition, the results suggest that the sea-surface area most influential on particle-flux characteristics recorded by the sediment trap in the Sargasso Sea lies to the northeast of the trap's mooring site. The results point towards possible ways of quantifying the role of marine biota in the regulation of atmospheric carbon dioxide through use of satellite observations.

Physical and biological processes in the North Atlantic during the first GARP Global Experiment

Abstract: The seasonal cycle of the phytoplankton productivity in the North Atlantic is investigated using monthly mean pigment composites generated by the Nimbus 7 CZCS for the period of December 1978 through December 1979. Data on the temporal and spatial distributions of surface pigments are compared with quantities derived from the First GARP Global Experiment wind data set, Climate Analysis Center SST fields, concurrent hydrogarphic station data, and climatological mixed-layer depths and heat fluxes. Time series at 11 selected sites throughout the North Atlantic basin are used to illustrate the relation of phytoplankton biomass to local physical forcing.

The effect of confinement on the isotropic-nematic transition

Abstract: The Lebwohl-Lasher model of a liquid crystal is examined in a slab geometry, with free top and bottom surfaces, using standard molecular-field and a new Bethe-type approximation that includes some effects of correlations. The results are compared with computer simulations for a range of slab widths L. We find that the approximate treatments, while predicting a nematic-isotropic transition that is too strongly first-order, are in semiquantitative agreement with the simulation results. The results of the approximate treatments show that the Kelvin equation for the shift in transition temperature due to confinement is accurate for L ≥ 64 layers. For large L a wetting film of the disordered, isotropic phase intrudes between each surface and the nematic phase. The film thickness, evaluated at the transition in the slab, increases as ln L in accord with the theory of complete wetting for systems with short-ranged forces.

For fluids adsorbed at walls the MWDA density functional theory is equivalent to an HNC approach

Abstract: The modified weighted density approximation (MWDA) introduced by Denton and Ashcroft in 1989 in a density functional theory of inhomogeneous fluids is applied to the case of fluid adsorption at planar walls. It is shown that the MWDA is completely equivalent to the hypernetted-chain closure of the wall-particle Ornstein-Zernike equation (HNCWP) for such problems. Because of the nature of the uniform fluid higher-order direct correlation functions within the MWDA, this theory of adsorption constitutes a truncation of the functional expansion of the free energy. The MWDA can also be used as the basis of a theory for the radial distribution function of a homogeneous fluid, where it is equivalent to the bulk HNC. For fluids confined in pores, however, the MWDA is not identical to the HNCWP.

Critical amplitudes for critical wetting with short-ranged forces: the approach to d=3-

Abstract: Critical wetting in fluids with short-ranged forces is investigated for dimensions d

Satellite-derived global sea surface temperature fields: 1982-1989

Abstract: The SASWG [SST (Sea Surface Temperature) Archiving Science Working Group], formed in early 1987 by NASA and NOAA, suggested production of two classes of SST products to meet the needs of the scientific community: SST fields addressing the needs of feature-related studies, and SST values accompanied by ancillary spectral and geometric parameters addressing the needs of heat flux related studies. Following the recommendations of the SASWG, NASA identified a smaller group to define the steps required to produce the feature-related product and to begin execution of the first steps. The underlying assumption in beginning this effort was that the required fields could be produced, as suggested by the SASWG, from the global GAC (Global Area Coverage) AVHRR/2 data set that extends from June 1981 to present. In that interests within the terrestrial community for global vegetation maps using the portion of GAC data covering the continents could be met with the same data stream, NASA also suggested integrating the production of the feature-related SST fields and the vegetation index. The perceived benefit is that, in addition to making these fields available to the research scientists in both communities, the integration of the production of these fields will serve as an excellent prototype for the production of interdisciplinary fields that must be undertaken as part of NASA's EOS effort. This manuscript deals with considerations going into defining the feature-related products, the steps outlined for their actual production, and the progress made thus far.