Capillary condensation in a fractal porous medium

Small-angle x-ray and neutron scattering are used to characterize the surface roughness and porosity of a natural rock which are described over three decades in length scales and over nine decades in scattered intensities by a surface fractal dimension D = 2.68+/-0.03. When this porous medium is exposed to a vapor of a contrast-matched water, neutron scattering reveals that surface roughness disappears at small scales, where a Porod behavior typical of smooth interfaces is observed instead. Water-sorption measurements confirm that such interface smoothing is due predominantly to the water condensing in the most strongly curved asperities rather than covering the surface with a wetting film of uniform thickness.     

Capillary condensation in a disordered mesoporous medium: a grand canonical Monte Carlo study

The adsorption of rare gases in a disordered mesoporous silica glass has been studied by means of grand canonical Monte Carlo (GCMC) simulation. A series of porous samples has been obtained by using an off-lattice 3D reconstruction method recently introduced to reproduce topological and morphological properties of correlated disordered porous materials such as Vycor. The off-lattice functional of 115m²g^?1 Vycor is applied to a simulation box containing silicon and oxygen atoms of cubic cristoballite with a homothetic reduction in order to obtain porous samples with mean pore size around 35 Å and specific surface around 220 m² g^?1. A realistic surface chemistry is then obtained by saturating all the dangling oxygen bonds with hydrogen. Some topological properties of the different 3D reconstructions of the Vycor-like material are analysed using chord length distributions and small angle scattering data. The GCMC Ar, Kr and Xe adsorption/desorption isotherms are calculated at different temperatures. At sufficiently low temperatures, they exhibit a capillary condensation transition with an adsorption branch having a finite slope accompanied by a hysteresis loop upon desorption. It has been shown on a set of simulated argon isotherms, that evolution with temperature of the GCMC results is similar to experiment. At the temperature at which the hysteresis loop disappears, it was found that the compressibility of the dense liquid-like phase at the maximum of the so-called hysteretic coexistence curves increases significantly. In the low pressure and temperature domain, different adsorption scenarios can be interpreted on the basis of a Zisman-type of criterion for wetting. The BET surface area is shown to be strongly related to this criterion. At higher pressure, it was found that the pore size distribution obtained by using the standard BJH analysis applied to both simulated adsorption and desorption data qualitatively reproduces the main features of the chord length distribution.     

Capillary condensation in porous alumina observed by positronium lifetime spectroscopy

The PALS method based on time distribution measurements has been used to study capillary condensation of different gases adsorbed in microporous alumina powder. The isotherms exhibit features which are associated with a shifted gas-liquid transition. The sorption and desorption processes are irreversible presenting a hysteresis effect.Suggestions on some new aspects of the capillary condensation dynamics are made.     

Capillary condensation in liquid-crystal colloids

We study capillary condensation between two spherical particles dispersed in the isotropic phase of a nematic liquid crystal. Within the Landau-de Gennes theory, we calculate interaction energies due to the formation of capillary bridges that reproduce experimental observations. Close to the critical point of the transition line separating the no-bridge from the bridge configuration, fluctuations in the particle cluster might be described by an effective two-state system. We show that the transition line vanishes for small particles and that the shape of the interaction potential depends on particle size.     

Asymptotic behavior of electron wave functions in disordered granular materials

For weakly localized electrons in disordered granular materials, the wave function decay length that determines the spatial decrease of the wave functions in the superlocalization mode is calculated. The dependence of the decay length on the energy and structural parameters of the system is established.     

Polariton condensation in a one-dimensional disordered potential

We study the coherence and density modulation of a nonequilibrium exciton-polariton condensate in a one-dimensional valley with disorder. By means of interferometric measurements we evidence a modulation of the first-order coherence function and we relate it to a disorder-induced modulation of the condensate density, that increases as the pump power is increased. The nonmonotonic spatial coherence function is found to be the result of the strong nonequilibrium character of the one-dimensional system, in the presence of disorder.     

Diffusion hysteresis in mesoporous materials

The pulsed field gradient NMR method has been used to study molecular diffusion of adsorbate molecules in mesoporous materials with different pore morphologies as a function of the external gas pressure. The obtained experimental results reveal that in line with the adsorption hysteresis, the measured diffusivities as well exhibit a history-dependent behavior, i.e. diffusivities on the adsorption branch do not follow those measured on the desorption branch. Particular details of the diffusion hysteresis loop depend on the structure of the used porous material. New insight into the mechanisms of adsorption hysteresis, namely indication of two-step relaxation process in the hysteresis region, is gained by comparing the dynamical properties of the adsorbate obtained under equilibrium and non-equilibrium conditions.     

Capillary hysteresis in a confined swirling two-fluid flow

This experimental study describes a hysteresis—a vivid manifestation of strongly nonlinear flow physics. A sealed vertical cylindrical container of radius 45 mm and height 90 mm is filled with water and sunflower oil. The rotating lid drives swirl and themeridional circulation of both fluids. As the rotation strength Re increases, the oil–water interface rises near the axis, touches the lid at Re = Re₁, and moves toward the container sidewall. Then as Re decreases, the interface returns to the axis and separates fromthe lid at Re = Re₂ < Re₁. At each Re from the range, Re₂ < Re < Re₁, two different stable steady flow states are observed, which is typical of hysteresis. The hysteresis only occurs if a volume fraction of oil is small. The hysteresis disappears as the oil fraction exceeds a threshold, which is around 0.4.     

Capillary rise in porous media

Capillary rise experiments were performed in columns filled with glass beads and Berea sandstones, using visual methods to register the advance of the water front. For the glass-bead-filled columns, early time data is well fitted by the Washburn equation. However in the experiments, the advancing front exceeded the predicted equilibrium height. For large times, an algebraic behavior of the velocity of the front is observed (T. Delker, D. Pengra and P. Wong, Phys. Rev. Lett. 76 (1996) 2902). A model to study the capillary pressure evolution in a regular assembly of spheres is proposed and developed. It is based on a quasi-static advance of the meniscus with a piston-like motion, and allows to estimate the hydraulic equilibrium height, with values very close to those obtained by fitting early time data to a Washburn equation. The change of regime is explained as a transition in the mechanism of advance of the meniscus. On the other hand, only the Washburn regime was observed for the sandstones. The front velocity was fitted to an algebraical form with an exponent close to 0.5, a value expected from the asymptotic limit of the Washburn equation.     

Optical phonons in disordered materials

A quantized Hamiltonian, including both the short- and long-range forces, for optical phonons in disordered materials is presented in this paper. The spectra of the optical phonons and the LO---TO splitting are calculated under the coherent potential approximation. The competed effects of the long-range correlation and the local structural fluctuation on the optical phonons are also discussed.     

Self-trapped carrier states and dielectric hysteresis in disordered dipole systems

Theory of self-trapped free-carrier states (fluctuons) at polarization fluctuations in KTaO₃:Li,Nb-type disordered ferroelectrics has been developed. The main characteristics of the fluctuon, viz. its energy, localization length, effective mass, and mobility have been calculated as functions of impurity-dipole concentration and temperature. The theory predicts the appearance of stable fluctuon states both in the ferroelectric-dipole-glass mixed phase and in the dipole glass state of disordered ferroelectrics. The possible participation of fluctuons in conduction and other transport phenomena in disordered ferroelectrics is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 722–727 (April 1998)     

Capillary condensation in linear mesopores of different shape

The hysteresis and kinetics of capillary condensation of N2 and Ar in linear mesopores, produced by etching of Si wafers, have been studied for different pore shapes, including the ink bottle geometry. Pore blocking has been observed in the solid state of the pore fillings, but not in the liquid state. We conclude that individual local geometries such as the pore mouth, a blind end, or a single constriction have no effect on the shape of sorption isotherms, that the pore space should be regarded as a statistical ensemble of pore segments with a lot of quenched disorder.     

Coherent transport in disordered metals out of equilibrium

We derive a formula for the quantum corrections to the electrical current for a metal out of equilibrium. In the limit of linear current-voltage characteristics our formula reproduces the well known Altshuler-Aronov correction to the conductivity of a disordered metal. The current formula is obtained by a direct diagrammatic approach, and is shown to agree with what is obtained within the Keldysh formulation of the non-linear sigma model. As an application we calculate the current of a mesoscopic wire. We find a current-voltage characteristics that scales with eV/kT, and calculate the different scaling curves for a wire in the hot-electron regime and in the regime of full non-equilibrium. Received 13 June 2001     

Off equilibrium dynamics in disordered quantum spin chain

We study the non-equilibrium time evolution of the average transverse magnetisation and end-to-end correlation functions of the random Ising quantum chain. Starting with fully magnetised states, either in the x or z direction, we compute numerically the average quantities. They show similar behaviour to the homogeneous chain, that is an algebraic decay in time toward a stationary state. During the time evolution, the spatial correlations, measured from one end to the other of the chain, are building up and finally at long time they reach a size-dependent constant depending on the distance from criticality. Analytical arguments are given which support the numerical results. Received 11 July 2002 / Received in final form 9 September 2002 Published online 29 October 2002     

Minor hysteresis loop effects in magnetic materials

In this report we discuss the nature of minor hysteresis in ferromagnets. In particular we have examined the formation of minor loops in which the magnetisation in the second quadrant is at a higher positive value, as the magnitude of H is increasing in a negative sense, than it is during a reverse sweep back to zero field. This behaviour produces the well known elliptical minor loops, or the ‘eye effect’. We have examined in some detail changes in magnetisation on these minor loops and we infer that these are changes in the irreversible component of magnetisation. Such changes are anomalous as the sample is maintained in a negative field at all times. We have also observed time dependent changes in magnetisation in which M increases in a positive sense in a negative field. Analysis of this effect shows that it alone is insufficient to account for the eye effect. We discuss the origins of the eye effect in terms of interaction effects in materials and show that this phenomenon can occur in dipolar and exchange coupled materials and may provide a technique for probing local interaction effects.     

A neural model of hysteresis in amorphous materials and piezoelectric materials

A new approach to constructing hysteretic operator (HO) is proposed in this paper. Based on the HO, the input space of neural networks is expanded from one-dimension to two-dimension and the multi-value mapping of hysteresis is transformed into a continuous mapping comprised of one-to-one mapping and multiple-to-one mapping. Based on the expanded input space, a neural network is employed to approximate hysteresis. The results of experimental examples suggest the proposed approach is effective.