Metastability and nucleation in capillary condensation

This paper is devoted to thermally activated dynamics of capillary condensation. On the basis of a simple model we identify the critical nucleus involved in the transition mechanism and calculate the nucleation barrier from which we obtain information on the nucleation time. Close to the condensation point, the theory predicts extremely large energy barriers leading to strong metastabilities, long time dependencies, and large hysteresis in agreement with experimental observations in mesoporous media. The validity of the model is assessed using a numerical simulation of a time-dependent Ginzburg-Landau model for the confined system.     

Kinetics of capillary condensation in nanoscopic sliding friction

The velocity and humidity dependence of nanoscopic sliding friction has been studied on CrN and diamondlike carbon surfaces with an atomic force microscope. The surface wettability is found to be decisive. Partially hydrophilic surfaces show a logarithmic decrease of friction with increasing velocity, the slope of which varies drastically with humidity, whereas on partially hydrophobic surfaces we confirm the formerly reported logarithmic increase. A model for the thermally activated nucleation of water bridges between tip and sample asperities fully reproduces the experimental data.     

Effect of nucleon correlations on pion condensation in neutron stars

It is pointed out that nucleon-nucleon correlations, play an important role in the pion condensation problem. Their effect is estimated by a simple analytical method. It is seen that they inhibit condensation by reducing the attraction a pion might otherwise feel in nuclear matter. Further the effect of correlations increases with nucleon density ?, offsetting the corresponding increase in the attraction felt by pions in free neutron matter.     

Direct observation of capillary condensation of a solid

We describe the direct condensation of a solid from vapor in an annular mica wedge. Neo-pentanol initially condenses as a liquid from 8 to 57 degrees C (the melting point T(m)), followed by nucleation of a solid from vapor for T<45 degrees C. Menthol (T(m) = 42 degrees C) gives only liquid condensates down to 12 degrees C. The adsorbed films of neo-pentanol, which unlike those of menthol show layering transitions, and the disordered crystalline phase of bulk neo-pentanol appear to facilitate condensation of the solid phase. There is evidence for a change in the nature of the solid neo-pentanol condensate with T.     

Evolution of large-scale correlations in a strongly nonequilibrium Bose condensation process

The evolution of off-diagonal correlation functions (for the example of a single-particle density matrix) in the process of Bose condensation of an initially nonequilibrium interacting gas is discussed. Special attention is given to the character of the decay of the density matrix at distances much greater than the size of the quasicondensate region. Specifically, it is shown that the exponential decay of the density matrix necessarily presupposes the presence of a chaotic vortex structure — a tangle of vortex lines — in the system. When topological order is established but there is no off-diagonal long-range order, the density matrix decays with distance according to a power law. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 495–501 (10 April 1998)     

The ins and outs of capillary condensation in cylindrical pores

A series of simulations has been performed of adsorption and desorption of a simple model of xenon in cylindrical pores of a silica-like material. Closed-ended, open-ended, and single-ended pores of either 3 nm or 4 nm diameter were considered, and the pore length was varied between 8nm and 108 nm. This study exposes some of the possible mechanisms of pore filling and emptying, and demonstrates that hysteresis can be almost entirely suppressed in certain pore geometries. The effects of pore length are considered, and the thermodynamics of 1-dimensional systems and the nature of ‘capillary critical points’ are discussed.     

Influence of capillary condensation on the near-critical solvation force

We argue that in a fluid, or magnet, confined by adsorbing walls which favor liquid, or the (+) phase, the solvation (Casimir) force in the vicinity of the critical point is strongly influenced by capillary condensation which occurs below the bulk critical temperature T(c). At T slightly below and above T(c), a small bulk field h<0, which favors gas, or the (-) phase, leads to residual condensation and a solvation force which is much more attractive (at the same large wall separation) than that found exactly at the critical point. Our predictions are supported by results obtained from density-matrix renormalization-group calculations in a two-dimensional Ising strip subject to identical surface fields.     

Novel insights into nanopore deformation caused by capillary condensation

By means of in situ small-angle x-ray diffraction experiments and semi-grand-canonical ensemble Monte Carlo simulations we demonstrate that sorption and condensation of a fluid confined within nanopores is capable of deforming the pore walls. At low pressures the pore is widened due to a repulsive interaction caused by collisions of the fluid molecules with the walls. At capillary condensation the pores contract abruptly on account of attractive fluid-wall interactions whereas for larger pressures they expand again. These features cannot solely be accounted for by effects related to pore-wall curvature but have to be attributed to fluid-wall dispersion forces instead.     

Nuclear track evolution by capillary condensation during etching in SSNT detectors

The microscopic process taking place during chemical etching is described in terms of a dynamic framework governed by capillary condensation. The aim is to obtain physical information on how the cone shaped tracks with curved walls evolve during chemical etching under a close examination of first principles. The results obtained with the proposed theory are compared with published values to establish their range of validity.     

Two-photon correlations of luminescence at the Bose-Einstein condensation of dipolar excitons

Correlations of the luminescence intensity (the second-order correlation function g ⁽²⁾(τ)), where τ is the delay time between the photons detected in pairs) under the conditions of the Bose-Einstein condensation (BEC) of dipolar excitons has been studied in a temperature range of 0.45–4.2 K. Photoexcited dipolar excitons have been accumulated in a lateral trap in a GaAs/AlGaAs Schottky diode with a 25-nm wide single quantum well with an electric bias applied across the heterolayers. Two-photon correlations have been measured with the use of a two-beam intensity interferometer with a time resolution of }~0.4 ns according to the well-known classical Hanbury-Brown-Twiss scheme. The photon bunching has been observed at the onset of Bose-Einstein condensation manifested by the appearance of a narrow exciton condensate line in the luminescence spectrum at an increase in the optical pumping (the line width near the threshold is ?200 μeV). At the same time, the two-photon correlation function itself obeys the super-Poisson distribution, g ⁽²⁾(τ) > 1, at time scale τ_c ? 1 ns of the system coherence. The photon bunching is absent at a pumping level substantially below the condensation threshold. The effect of bunching also decreases at pumping significantly above the threshold, when the narrow exciton condensate line starts to dominate in the luminescence spectra, and finally disappears with the further increase in the optical excitation. In this region, the distribution of pair photon correlations is a Poisson distribution manifesting the united quantum coherent state of the exciton condensate. Under the same conditions, the first-order spatial correlation function g ⁽¹⁾(r) determined from the interference pattern of the luminescence signals from the spatially separated parts of the condensate at constant pumping remains noticeable at distances of no less than 4 μm. The discovered effect of photon bunching is very sensitive to temperature and decreases by several times with a temperature increase in the range of 0.45–4.2 K. Assuming that the luminescence of the dipolar excitons directly reflects the coherence properties of the gas of interacting excitons, the discovered photon bunching at the onset of condensation, where the fluctuations of the exciton density and, consequently, of the luminescence intensity are most significant, indicates a phase transition in the interacting Bose gas of excitons, which is an independent way of detecting the Bose-Einstein condensation of excitons.     

Universal correlations, pseudo‐gaps and condensation mechanisms in high‐Tc superconductors

We combine universal correlations between T_c and n_s/m^* (superconducting carrier density/effective mass) found by μSR with the pseudo gap behavior found in the underdoped region by various techniques to develop a picture for explaining doping dependence of high‐T_c superconductivity in terms of a crossover from Bose–Einstein to BCS condensation. μSR results in overdoped and Zn‐substituted systems suggest that behavior on the high‐density side is more complicated than expected in a simple BCS model. This revised version was published online in August 2006 with corrections to the Cover Date.     

Freezing and pressure-driven flow of solid helium in Vycor

The recent torsional oscillator results of Kim and Chan suggest a supersolid phase transition in solid 4He confined in Vycor. We have used a capacitive technique to directly monitor density changes for helium confined in Vycor at low temperature and have used a piezoelectrically driven diaphragm to study the pressure-induced flow of solid helium into the Vycor pores. Our measurements showed no indication of a mass redistribution in the Vycor that could mimic supersolid decoupling and put an upper limit of about 0.003 microm/s on any pressure-induced supersolid flow in the pores of Vycor.     

Study of vapor condensation on curvilinear fins under influence of capillary forces and gravity

The problem on film vapor condensation on curvilinear fins is solved numerically with regard to surface tension and gravity. Steam condensation on a fin of optimized and semicircular form was calculated for various inclinations of the fin relative to the direction of gravity vector and the levels of groove flooding. The calculations showed that the average heat transfer coefficient for a semicircular fin slightly decreases with changing the fin position relative to the direction of gravity vector, at that, the “zero” flow point from which the condensate flows in different directions shifts. For an optimized fin, there is no such shift, and the heat transfer coefficient does not practically change. The top of an optimized fin with a large curvature serves as a kind of "barrier" for the flow. The heat transfer coefficients on optimized fins are significantly higher than those on semicircular fins. The amount of the level of groove flooding significantly affects the condensation efficiency as a whole; however, it does not affect the process of condensation on the convex part of fins.     

Grand canonical Monte Carlo and molecular dynamics simulations of capillary condensation and evaporation of water in hydrophilic mesopores

A combination of grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations (GCMD simulations) was performed to investigate the effect of the difference in chemical potential between the pore surface and gas phase on the adsorption and desorption kinetics of water in a hydrophilic mesopore. The chemical potential of water vapour was controlled by the GCMC method and the adsorption/desorption of water to/from a hydrophilic mesopore was simulated by the MD method. The calculation results showed that when the stepwise change in chemical potential of the gas phase was small, the initial rate of adsorption or desorption of water could be well predicted by the kinetic theory of gases. However, the rate greatly varied depending on the transport mechanisms of water in mesopores such as film flow and column flow. On the other hand, when the stepwise change in chemical potential was large, the initial rate was overestimated by the kinetic theory of gases; however, it did not change greatly with time.