Reentrant filling transitions in Lennard-Jones fluids confined in nanoscopic slit-like pores

The reentrant first-order pore filling transition in a Lennard-Jones (LJ) fluid confined between the parallel (100) planes of the face centered cubic (fcc) crystal is studied by means of Monte Carlo simulations in the canonical as well as grand canonical ensembles. Slit-like pores of the width allowing for the formation of only three adsorbate layers are used. It is demonstrated that the reentrant first-order pore filling transition, associated with the condensation of the middle layer, appears only when the height of the potential barrier between adjacent sites, V_D, fulfills a condition V_D∈[V_D,min,V_D,max]. The lower limit of V_D (V_D,min) is primarily determined by the stability of commensurate monolayers formed at both pore walls during the first step of adsorbate condensation, while the upper limit of V_D (V_D,max) depends on the stability of the commensurate three-layer structure. It is also shown that both the misfit between the size of adsorbed atom and the surface lattice as well as the pore width have a great influence on the phase behavior of confined fluids. Moreover, the effects of the phase shift between the confining lattices on the reentrant first-order filling transition are discussed.     

Application of the density functional method to study phase transitions in an associating Lennard-Jones fluid adsorbed in energetically heterogeneous slit-Like pores

A density functional approach is used to study the adsorption of the four-bonding-site model associating Lennard-Jones fluid in slit-Like pores with energetically heterogeneous walls. The fluid-wall potential is qualitatively similar to that invoked by Röcken, P., Somoza, A., Tarazona, P., and Findenegg, G. H., 1999, J. chem. Phys., 108, 8089, i.e. it consists of a homogeneous part that varies in the direction perpendicular to the wall and a periodic part, varying also in one direction parallel to the wall. Both parts are modelled by Lennard-Jones 9,3-type functions. The structure of the adsorbed film is characterized by the local densities of all particles and the densities of the monomers. The phase diagrams are evaluated for several systems characterized by different corrugation of the adsorbing potential. The adsorbing field is strong enough to allow for the layering transition. As well as the formation of the so-called bridge phase that fills the pore space over the most energetic parts of the wall and of capillary condensation, the layering transition is observed within the first layer adjacent to the pore walls. If the adsorbing potential due to each pore wall is shifted in phase by π/2, the bridge phase is not formed.     

Demixing transitions in a binary Gaussian-core fluid confined in narrow slit-like pores

Using a mean-field density functional approach we investigate phase separation transition in a binary mixture of Gaussian-core molecules confined in narrow slit-like pores. We consider pores with repulsive and attractive walls. In the case of fluid confinement in pores with repulsive and non-selective attracting walls, no phase separation in the confined fluid, prior to the bulk separation transition, was observed. However, in the case of pores with the walls selectively attracting fluid particles, we reveal that the separation transition may take place as a two-step process. During the first step the composition change occurs within a few layers adjacent to the pore walls, whereas in the second step, it takes place in the pore interior.     

Structure and phase transitions in a network-forming associating Lennard-Jones fluid in a slit-like pore: a density functional approach

A study is reported of adsorption of an associating Lennard-Jones fluid with four associative sites per molecule in a slit-like pore. The density distribution of particles in the pore and thermodynamics properties are evaluated by using a density functional method. It is found that at low temperatures the fluid exhibits a set of layering transitions, followed by capillary condensation. Transitions are localized by analysing the grand canonical potential. The density profiles of particles and the distribution of unbound and differently bonded particles demonstrate changes in the structure of the fluid in the pore along the phase coexistence. The critical temperature is lower for a confined fluid, compared with the bulk counterpart. However, an increase in the energy of association increases the critical temperature. The envelope of the capillary condensation is narrower than the bulk liquid-vapour phase diagram. The dependence of the solvation force on the energy of association and on the bulk density is discussed.     

Orientational fluctuations and phase transitions in 8CB confined by cylindrical pores of the PET film

Abstract

Results of optical investigations of the isotropic-nematic and nematic-smectic A phase transitions in porous polyethyleneterephthalate (PET) films filled with octyl-cyanobihenyl (8CB) liquid crystal (LC) are reported. Samples of porous films of thickness 23 µm with normally oriented cylindrical pores of a radius R ranging from 10 nm to 1000 nm were prepared using the track-etched membrane technology. The dynamic light scattering method was used to probe the nematic orientational fluctuations of confined LC samples. The corresponding relaxation time τ was measured as a function of R and temperature T at slow enough cooling rates (0.3–0.6 K/h) to locate the phase transition temperatures. Changes in τ(T) dependencies relatively sensitivity fingerprint the LC phase transformations. Experimental results are analysed using the Landau-de Gennes-Ginzburg phenomenological approach.     

Molecular dynamics study of the formation of small crystallites of Lennard-Jones particles in slit-like pores with (100) fcc walls

Canonical ensemble molecular dynamics is used to investigate the formation of small crystallites in slit-like pores with crystalline walls of (100) fcc symmetry. We study the influence of the pore width and the energetic parameters of the fluid particle-pore wall potential on the structure of the frozen droplets. We analyse the density profiles, in-plane radial distribution functions, in-plane diffusion coefficients, the distributions of Voronoi polygons, as well as the snapshots of consecutive configurations.     

The dynamics of random interfaces in phase transitions

A review is given of recent developments involving the dynamics of random interfaces formed in the evolution of metastable and unstable systems. Topics which are discussed include interface growth and nonequilibrium dynamical scaling. The possibility of there being dynamical universality classes in first-order phase transitions is also discussed. There are a large number of systems of experimental interest which include binary alloys, binary fluids, and polymer mixtures. Other systems studied by computer simulation include the kinetic Ising, Potts, andZ _N models.Work supported by NSF grant No. DMR-8013700.     

Effect of a random field conjugate to a non-critical variable on phase transitions

We consider the phase transition in an anisotropic system with a random field conjugate to a non-critical variable. It is shown that in this case a sufficiently weak random field acts like random potentials.     

Molecular dynamics simulations of surface-controlled phase separation of binary fluid mixtures confined in slit pores - Online only

When binary mixtures are confined into nanoscopic slit pores, an intricate interplay between surface enrichment (wetting) of one component and lateral phase separation occurs. After a brief review of the static equilibrium phase diagram of such systems, a discussion of the kinetics of phase separation is given. Considering quenches from an initially homogeneous distribution of the two species in the slit, it is shown by molecular dynamics simulation that typically in the initial stages a stratified structure develops, with enrichment layers of the preferred component at the walls of the slit pore. Then this laterally homogeneous structure breaks up into domains, which coarsen with time according to a power law with a 2/3 exponent. This growth law must be attributed to a hydrodynamic mechanism, since corresponding simulations of a diffusive Ginzburg-Landau model yield an exponent of 1/3 only. The relation to spinodal decomposition in d=2 space dimensions is briefly discussed.     

The influence of external noise on non-equilibrium phase transitions

The method of Ito stochastic differential equations is used to analyze the influence of external noise in non-equilibrium phase transitions. It is found that external noise deeply affects the behaviour of the system and gives rise to new phenomena not predicted by the deterministic analysis.     

Adsorption and interfacial phenomena of a Lennard-Jones fluid adsorbed in slit pores: DFT and GCMC simulations

ABSTRACT

Confinement of fluids in porous media leads to the presence of solid–fluid (SF) interfaces that play a key role in many different fields. The experimental characterisation of SF interfacial properties, in particular the surface tension, is challenging or not accessible. In this work, we apply mean-field density functional theory (DFT) to determine the surface tension and also density profile of a Lennard-Jones fluid in slit-shaped pores for realistic amounts of adsorbed molecules. We consider the pore walls to interact with fluid molecules through the well-known 10-4-3 Steele potential. The results are compared with those obtained from Monte Carlo simulations in the Grand Canonical Ensemble (GCMC) using the test-area method. We analyse the effect on the adsorption and interfacial phenomena of volume and energy factors, in particular, the pore diameter and the ratio between SF and fluid–fluid dispersive energy parameters, respectively. Results from DFT and GCMC simulations were found to be comparable, which points to their reliability.     

Rashba自旋-轨道相互作用对一维谐振子势场中电子性质的影响

分析了Rashba自旋-轨道相互作用对一维谐振子势场中电子性质的影响,发现该自旋-轨道相互作用能够导致能级之间的自旋翻转,并且自旋翻转的性质明显依赖于自旋-轨道耦合系数和参考系坐标之间的关系.     

The sequence of structural phase transitions in systems with a four-well potential

The theory of order-disorder phase transitions is discussed for systems with a four-well potential. It is shown that the model can be generalized to the case of the well-known Ising model in a transverse field. It is also shown that two or three crystalline phases can emerge as the temperature is reduced. Fiz. Tverd. Tela (St. Petersburg) 39, 2217–2219 (December 1997)     

The influence of multiphonon transitions on interband optical absorption in semiconductors with a Coulomb-type random field

The temperature dependence of the optical absorption coefficient in the tail region is studied in semiconductors with a random Coulomb-type field. The results of numerical calculation are compared with data for chalcogenide glasses.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 63–67, September, 1981.The author is grateful to V. L. Bonch-Bruevich for his attention to the work.     

Equilibrium partitioning of a simple fluid in a matrix-filled cylindrical pores with adsorbing walls: A grand canonical Monte Carlo study

We have studied a model of a hard sphere fluid adsorbed in a cylindrical pore filled with quenched disordered matrix of hard sphere particles using Grand canonical Monte Carlo simulations. The interactions between matrix species and pore walls are assumed of a hard sphere type. However, the pore walls exert a short-range attraction upon adsorbed fluid particles. We discuss the adsorption isotherms and the density profiles of fluid particles in pores with different microporosity for several values of the pore radius. We have observed that like in homogeneous microporous media the adsorption increases with increasing porosity. However, trends of behavior of the isotherms also reflect layering of adsorbed fluid. The data obtained in this study may serve as a benchmark for the development of the theory of confined quenched-annealed systems and for computer simulation investigation of models permitting phase transitions in pores. This project has been supported in parts by DGAPA of the UNAM under research grant IN111597, by the National Council for Science and Technology (CONACyT), grant No. 25301-E.     

The influence of magnetoelastic interaction on structural phase transitions in cubic ferromagnetics

In the framework of the Landau theory of phase transitions, the influence of the magnetoelastic interaction on structural transitions in cubic ferromagnetics with a positive first magnetic anisotropy constant is analyzed. It is shown that structural transitions are not accompanied by a reorientation of magnetization in this case. The phase diagrams of such ferromagnetics either contain a termination point of the structural transition or a critical point in which the first-order transition is replaced by a second-order one. Magnetoelastic interaction also leads to the appearance of an interval of the ferromagnetic parameters in which a coupled first-order structural-magnetic transition exists. The phase T?x diagram for Heusler Ni_2+x Mn_1?x Ga alloys is calculated, which is in good agreement with the experimental phase diagram of these alloys.     

Electric field effects on nucleation during phase transitions of a dielectric fluid

Using the thermodynamic theory of phase transitions, it is shown that an electric field increases the critical size and work for bubble nucleation in a superheated liquid and impedes the process; it decreases the critical size and work for liquid droplet formation in a supercooled vapour and enhances the process.     

The friction coefficient of a Lennard-Jones fluid from the random force autocorrelation function determined as a memory function by molecular dynamics calculations

A recent molecular dynamics (MD) study showed that the friction coefficient of a simple fluid is obtainable by the integral over the autocorrelation function (ACF) of the total force of a Brownian-type particle. The results indicated that mass ratios 50M/m200 of the massive and the light particle suffice to yield accurate friction coefficients. Complementarily, we calculate the random force ACF of the light particle, which is the memory function force of the ACF of the velocity apart from a constant factor, for all the states of the Lennard-Jones system investigated previously. A detailed comparison is presented of the memory function, the total force ACF of the fluid particle, and the total force ACF of the massive particle. The MD results confirm quantitatively our theoretical predictions: (i) on a time scale corresponding to the dynamics of the massive particle the total force ACF of that particle approximates well the memory function, while there are slight differences between them on a short time scale, (ii) the total force ACF of the liquid particle deviates significantly from the memory function already after extremely short time and is thus completely useless for the determination of the friction coefficient, (iii) using the total force ACF of a heavy particle for the determination of the friction constant with mass ratios ofM/m=50 up to 200, the pseudo plateau value of the time integral is often not very noticeable, as the memory function is only approximated and the total force ACF of the massive particle has a negative part at medium times. In those cases the integration has to be extended to include the negative part.