ALBERT

Description: When a liquid meniscus held at the exit of a metallic capillary tube is charged to a high voltage V, the free surface often takes the form of a cone whose apex emits a steady microjet, and thus injects a certain charge I and liquid volume Q per unit time into the surrounding gas. This work deals with liquids with relatively large conductivities K, for which the jet diameter dj is much smaller than the diameter dn of the capillary tube. In the limit dj/dn → 0, the structure of the jet (dj and I, in particular) becomes independent of electrostatic parameters such as V or the electrode configuration, being governed mostly by the liquid properties and flow rate Q. Furthermore, the measured current is given approximately by I = f(ε) (γQK/ε)½ for a wide variety of liquids and conditions (ε, and γ are, respectively, the dielectric constant of the liquid and the coefficient of interfacial tension; f(ε) is shown in figure 11). The following explanation is proposed for this behaviour. Convection associated with the liquid flow Q transports the net surface charge towards the cone tip. This upsets the electrostatic surface charge distribution slightly at distances r from the apex large compared to a certain charge relaxation length λ, but substantially when r ∼ λ. When the fluid motion is modelled as a sink flow, λ is of the order of r* = (Qεε0/K)[formula omitted] (ε0 is the electrical permittivity of vacuum). If, in addition, the surface charge density is described through Taylor's theory, the corresponding surface current convected towards the apex scales as Is ∼ (γQK/ε)½, as observed for the spray current. The sink flow hypothesis is shown to be realistic for sufficiently small jet Reynolds numbers. In a few photographs of ethylene glycol cone jets, we find the rough scaling dj ∼ 0.4r* for the jet diameter, which shows that the jet forms as soon as charge relaxation effects set in. In the limit ε ≫ 1, an upper bound is found for the convected current at the virtual cone apex, which accounts for only one-quarter of the total measured spray current. The rest of the charge must accordingly reach the head of the jet by conduction through the bulk. © 1994, Cambridge University Press. All rights reserved.

Description: An investigation is carried out on the effects of Brownian agitation in the motion of small particles in a carrier gas in situations far from equilibrium. Although the standard near-equilibrium closure of the hydrodynamic equations is not valid for the heavy particles, the smallness of their speed of thermal agitation allows an alternative systematic hypersonic closure. The hypersonic equations are solved for two known instances where the kinetic Fokker—Planck equation describing the nonequilibrium particle distribution function admits exact solutions. These problems are characterized by a null or a spatially constant value for the gradient of the velocity field in the carrier gas, both being free from boundary surfaces. In the first case, where the background velocity is uniform, a fundamental solution (expressed as an integral) is obtained for the steady flow of particles from a point source; this result has obvious applications for the description of the Brownian broadening of particle streamlines. An asymptotic integration of the fundamental solution yields analytical expressions for the particle hydrodynamic properties valid everywhere except near the source, where a direct integration of the Vlasov equation completes the description. The exact solution for the second example, where the background velocity field gradient is uniform, is taken from the literature. Once these reference solutions have been established, the hypersonic equations are attacked by a variety of methods. In particular, for the uniform steady flow, a boundary-layer analysis yields analytical results identical to those obtained from the asymptotic evaluation of the kinetic fundamental solution. In both problems, the agreement found between kinetic and hydrodynamic solutions is excellent even for values of order one of the inverse particle Mach number, the expansion parameter of the hypersonic theory. © 1990, Cambridge University Press. All rights reserved.

Description: Melt waters cover c. 20% of the McMurdo Ice Shelf during the austral summer. The streams, ponds, and lakes up to 104 m2 in area occur in two types of terrain systems with differing morphological, chemical, and biological characteristics: pinnacled ice (PI) areas with sparse sediment cover, low relief, and little biomass; and ice-cored moraine (ICM) areas with 10–20 cm sediment cover, hummocky topography with up to 20 m relief, occasional mirabilite deposits, and dense benthic cyanobacterial mats. Pond water composition in the two areas is markedly different. PI area melt waters have low salinities,

Description: The first extended baseline studies of total gaseous mercury (TGM) and dimethylmercury (DMM) in Antarctica are reported. Mean TGM concentrations of 0.52, 0.60 and 0.52 ng m−3 were obtained for three consecutive years at the southern tip of Ross Island (77°S). The levels of DMM in Antarctica are less than 10% of the TGM, and frequently fall below the limit of detection. These results represent the lowest TGM concentrations recorded globally and extend into polar regions the observation of a decrease in atmospheric mercury concentration with increasing latitude.

Description: Two shallow ponds at Cape Evans, Ross Island, were sampled at 1–2 week intervals, during winter freezing throughout the winter and during the subsequent melt period, to examine the physical and chemical conditions imposed on the biota during the year. Liquid water was first detected at the base of the ponds in late December. During the main summer melt period conductivities were less than 10 mS cm−1 with maximum daily temperatures around 5°C. The bottom waters became increasingly saline during freezing and water temperatures decreased below 0°C; by June the remaining water overlying the sediments had conductivities 〉150 mS cm−1 and temperatures of −13°C. Calcium carbonate, then sodium sulphate precipitated out of solution during early freezing. The dominant nitrogen species was dissolved organic-N which reached 12 g m−3 in Pond 1 just prior to final freeze up. The organic and inorganic forms of nitrogen and dissolved reactive phosphorus increased with increasing conductivity in the ponds. The behaviour of particulate-N and particulate-P mirrored that of chlorophyll a with a peak in March-April and a second higher peak just before final freeze-up. This study provides clear evidence that organisms which persist throughout the year in Antarctic coastal ponds must be capable of surviving much more severe osmotic, pH, temperature and redox conditions than those measured during the summer melt. Deoxygenation, pH decline and H2S production, however, point to continued respiratory activity well into the dark winter months.

Description: Two geochemical surveys of the major constituents of the Alph River, situated in Walcott Bay, Victoria Land, were undertaken in the austral summer of 1987–88. At the same time, tributaries and the runoff from various glaciers were investigated. The Alph River has an average total dissolved solids (TDS) concentration of 63.5 mgl−1, approximately half that of average world river water. The chemical composition is dominated by Na+ and HCO−3. Glacial melt waters have very low TDS but chemical weathering over the course of a few kilometres causes solute concentrations in the tributaries to exceed those of the Alph River. The composition of the streams is variable, but often Ca2+ is the principal cation. Enrichment factor and mass balance calculations indicate that the salts in the Alph River and its tributaries have a substantial non-marine component. Chemical weathering of calcite, mirabilite, gypsum and halite contribute solutes to the aquatic system. A “Gibbs Plot” [TDS versus Na:(Na+Ca) weight ratio] indicates that water samples from direct glacial runoff fall outside the world water envelope. They have low solute levels but enhanced Ca2+ concentrations, resulting from the aeolian deposition and subsequent dissolution of calcitic material.