Showing papers by "Robert Evans published in 1986"

Fluids in narrow pores: adsorption, capillary condensation, and critical points

Abstract: By means of a density functional approach the phase equilibria of a simple fluid confined by two adsorbing walls have been investigated as a function of wall separation H and chemical potential μ for temperature T corresponding to both partial and complete wetting situations. For large values of H and small undersaturations Δμ ≡ μsat−μ, we recover the macroscopic formulas for the undersaturation at which a first‐ order phase transition (capillary condensation) from dilute ‘‘gas’’ to a dense ‘‘liquid’’ occurs in a single, infinitely long slit. For smaller H we compute the lines of coexistence between gas and liquid in the (Δμ, 1/H) plane at fixed values of T. The adsorption Γ(Δμ), at fixed T and H, is characterized by a loop. At the first order transition Γ jumps discontinuously by a finite amount; however metastable states exist and these could give rise to hysteresis of the adsorption isotherms obtained for the single slit. The loop disappears at a capillary critical point (Δμc, 1/Hc) at each T. For H

Capillary condensation and adsorption in cylindrical and slit-like pores

Abstract: The nature of adsorption of simple fluids confined in model pores is investigated by means of a density functional approach. For temperatures T corresponding to a partial wetting situation a first-order phase transition (capillary condensation) from dilute ‘gas’ to dense ‘liquid’ occurs at relative pressures p/psat close to those predicted by the macroscopic Kelvin equation, even for radii Rc or wall separations H as small as 10 molecular diameters. In a complete wetting situation, where thick films develop, the Kelvin equation is, in general, not accurate. At fixed T the adsorption Γm(p) exhibits a loop; Γm jumps discontinuously at the first-order transition, but the accompanying metastable portions of the loop could produce hysteresis similar to that observed in adsorption measurements on mesoporous solids. Metastable thick films persist to larger p/psat in slits than in cylinders and this has repercussions for the shape of hysteresis loops. For a given pore size the loop in Γm shrinks with increasing T and disappears at a capillary critical temperature Tcapc( Tcapc condensation no longer occurs and hysteresis of Γm will not be observed. Such behaviour is found in experiments. A prewetting (thick–thin film) transition can occur for confined fluids. The transition is shifted to a smaller value of p/psat than that appropriate to prewetting at a single planar wall. Whereas the magnitude of the shift is very small for slits, it is substantial for cylinders and this leads to the possibility of finding a triple point, where ‘liquid’ and thick and thin films coexist, in cylindrical pores whose radii may not be too large for investigation by experiment or computer simulation. Adsorption of supercritical fluids (T > Tc, the bulk critical temperature) in cylinders is mentioned briefly.

Rings of the Agulhas current

Abstract: A combination of satellite-tracked surface drifters, satellite-derived sea surface temperature, and ship-measured isotherm depth fields are used to describe two rings formed from the Agulhas Retroflection south of Africa. The drifter and satellite data are used to calculate ring trajectories. The two rings move westward into the South Atlantic. Their trajectories suggest the rings interact with the bottom topography. Ring motion histories are used to remap hydrographic data and estimate the mean radial shape of the ring thermocline. Thermocline topography as defined by the 10°C isotherm depth is then combined with a two-layer diagnostic model to compute the rings' momentum and energetics. The results indicate these rings are some of the most energetic in the world. Their large horizontal scales, however, make them less nonlinear than rings associated with the Gulf Stream in terms of a Rossby number criterion. Ring translation appears to be influenced by a complicated combination of self-induced propagation and steering by the mean currents. The rates of translation are significantly higher than expected from theoretical computations of self-induced motion. The energy flux an Agulhas ring contributes to the South Atlantic is 7% of the annual wind input over the entire basin. Two rings per year can replace the eddy energy calculated for the basin outside of the direct influence of the Agulhas Retroflection.