Studies of porosity and diffusion coefficient in porous matrices by computer simulations

We present a series of molecular dynamics simulations to study the porosity on different matrix configurations. The matrices were prepared using two different processes. In the fist method we used direct simulations of a fluid at a fixed density and the matrix was taken from the last configuration of its particles. In the second method we simulated a binary mixture where one of the components served as a template material and the final porous matrix configuration was obtained by removing template particles from the mixture. Matrices were prepared at different densities and at different matrix particle interactions. The results showed that the matrix structure and the matrix porosity were affected by the way the porous matrices were prepared. Finally, we also investigated the diffusion of a fluid inside the matrices. The diffusion coefficient was measured by mean square displacements of the particles in the fluid. It was observed that this quantity was also affected by the kind of porous matrix employed. The calculations were performed for several fluids at different densities in the different porous matrices. From these studies we observed that the highest porosity and diffusion coefficient were found in matrices prepared with attractive particle interactions and without any template.     

Pore matrices prepared at supercritical temperature by computer simulations: matrix characterization and studies of diffusion coefficients of adsorbed fluids

We present series of molecular dynamics simulations to study the structure of different porous matrix configurations. The present simulations are an extension of recently reported data at a temperature below the critical. Here, we show how temperature modifies the structure and porosity of pore matrices during their preparation in comparison with the previous work. Moreover, in these studies at a higher temperature, we studied in more detail the structure of the porous matrix. Matrices were prepared by two different processes. The first method consisted of a single Lennard-Jones fluid simulated at a fixed density and at a supercritical temperature. Then, the matrix configuration was taken from the last configuration of the fluid. The second method was prepared from a binary mixture, where one of the components served as a template material. The final porous matrix configuration was obtained by removing template particles from the mixture. Matrices were prepared at two different densities and at different matrix particle interactions. The volume distribution, the cluster formation and the connectivity between the particles in the pore matrix were investigated. The importance of using template particles was clear since they produced larger voids and higher porosities. On the other hand, the temperature of preparation seems to modify the size of the voids and the porosity in the matrices. For instance, at this high temperature, the matrix porosity is higher when template particles are present in the system. These results point in the opposite direction compared to those found in a previous paper at a lower temperature. The diffusion of fluids immersed in the different matrices was also analysed by calculating the mean square displacements of their particles. It was observed that this quantity was higher when matrices were prepared with template particles. These results also point to different directions in contrast with pore matrices prepared at a lower temperature. Finally, the results show that temperature plays an important role in the pore matrix formation.     

Dewetting in associating lattice gas model confined by hydrophobic walls

The phase behavior of a two dimensional fluid confined within hydrophobic walls is obtained by Monte Carlo simulations. The fluid is described by the associating lattice gas model which reproduces the density and diffusion anomalous behavior of water.The confined fluid exhibits a liquid-liquid critical temperature which decreases with the decrease of the distance between the confining walls. In contact with the wall a dewetting is observed. The thickness of this interfacial layer is independent of the distance between the two walls. Even for very small distances between the two walls no total depletion is observed and consequently no drying transition is present.     

Depolarised Rayleigh light scattering in argon layer confined between graphite plains: MD simulation

The interaction-induced light scattering many-body correlation functions and their spectra in a thin argon layer located between two parallel graphite walls have been investigated by molecular dynamics simulation method. The calculations have been performed for three different distances between graphite plates. Our simulations show the increased intensity of the interaction-induced light scattering spectra at low frequencies for argon atoms in confined space, in comparison to the bulk (unconfined) sample. Moreover, we show a substantial dependence of the interaction-induced light scattering correlation functions of argon on the distances between graphite walls, that is, on the density of argon layer. The mean square displacement and related diffusion coefficient of argon atom in the confined space has been also investigated. Moreover, the structural feature of the thin layer has been studied by calculating the argon density profile, perpendicular to the graphite walls. An interesting observation is the development of a fluid phase in the innermost region of the confined argon layer.     

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.     

约束条件下的硬球流体

利用密度泛函理论和分子动力学方法 ,对处于两平行硬墙之间的硬球流体的密度分布进行了计算 .通过比较两种方法的结果 ,发现在墙之间距离较大时 ,Rosenfeld密度泛函理论的结果与分子动力学模拟的结果符合很好 ;当两堵墙间的距离很小时 ,这两个结果之间存在明显的不一致 .另外 ,还研究了约束条件下密度分布的结构     

Slit-pore sorption isotherms by the grand-canonical Monte Carlo method

The grand-canonical ensemble Monte Carlo method has been used to study sorption isotherms (plots of density versus chemical potential) for a rare-gas fluid in a slit-pore whose plane-parallel walls comprise rigidly fixed rare-gas atoms. Hysteresis in the vicinity of the capillary-condensation transition is evinced by fluctuations between liquid and vapour phases as the length of the Markov chain increases (at fixed chemical potential). The degree of hysteresis, which reflects the non-ergodic behaviour of the Markov chain, can be diminished by increasing the surface area of the pore wall and by extending the Markov chain. It is concluded that hysteresis is a non-equilibrium phenomenon that cannot be properly treated by equilibrium statistical-mechanical methods.     

Computer simulations of structure, dynamics, and phase behavior of colloidal fluids in confined geometry and under shear

Using computer simulations, colloidal systems in different external fields are investigated. Colloid-polymer mixtures, described in terms of the Asakura-Oosawa (AO) model, are considered under strong confinement. Both in cylindrical and spherical confinement, the demixing transition of the three-dimensional AO model is rounded and, using Monte Carlo simulations, we analyze in detail the consequences of this rounding (occurrence of multi-domain states in cylindrical geometry, non-equivalence of conjugate ensembles due to different finite-size corrections in spherical geometry etc.). For the case of the AO model confined between two parallel walls, spinodal decomposition is studied using a combination of molecular dynamics simulation and the multiparticle collision dynamics method. This allows us to investigate the influence of hydrodynamic interactions on the domain growth during spinodal decomposition. For a binary glass-forming Yukawa mixture, non-linear active micro-rheology is considered, i.e. a single particle is pulled through a deeply supercooled liquid. The diffusion dynamics of the pulled particle is analyzed in terms of the van Hove correlation function. Finally, the Yukawa mixture in the glass state, confined between walls, is studied under the imposition of a uniform shear stress. Below and around the yield stress, persistent creep in the form of shear-banded structures is observed.     

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.     

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.     

Water in the presence of inert Lennard-Jones obstacles

Water confined by the presence of a ‘sea’ of inert obstacles was examined. In the article, freely mobile two-dimensional Mercedes-Benz (MB) water put to a disordered, but fixed, matrix of Lennard-Jones disks was studied by the Monte Carlo computer simulations. For the MB water molecules in the matrix of Lennard-Jones disks, we explored the structures, hydrogen-bond-network formation and thermodynamics as a function of temperature and size and density of matrix particles. We found that the structure of model water is perturbed by the presence of the obstacles. Density of confined water, which was in equilibrium with the bulk water, was smaller than the density of the bulk water and the temperature dependence of the density of absorbed water did not show the density anomaly in the studied temperature range. The behaviour observed as a consequence of confinement is similar to that of increasing temperature, which can for a matrix lead to a process similar to capillary evaporation. At the same occupancy of space, smaller matrix molecules cause higher destruction effect on the absorbed water molecules than the bigger ones. We have also tested the hypothesis that at low matrix densities the obstacles induce an increased ordering and ‘hydrogen bonding’ of the MB model molecules, relative to pure fluid, while at high densities the obstacles reduce MB water structuring, as they prevent the fluid to form good ‘hydrogen-bonding’ networks. However, for the size of matrix molecules similar to that of water, we did not observe this effect.     

X-Ray waveguiding studies of ordering phenomena in confined fluids

We have determined the structure of a colloidal fluid confined in a gap between two walls by making use of the waveguiding properties of the gap at x-ray wavelengths. The method is based on an analysis of the coupling of waveguide modes induced by the density variations in the confined fluid. Studies on suspensions confined within gaps of a few hundred nanometers showed strongly selective mode coupling effects, indicative of an ordering of the colloidal particles in layers parallel to the confining walls.     

Charge dynamics in a model for grains electrization

In this work we investigate a model for the dynamics of granular electrization using event-driven simulations and approximate calculations. The model is defined as a mixture of isolating grains of different species confined in a cubic box. During the collisions, the grains and the walls can acquire electric charge via tribocharging. We focus on the dynamics of charge exchange, and calculate the time evolution of the total charge in each species, that presents a double exponential behavior in the case of zero gravitational field. For non-zero field, a stretching of the curve is present, caused by the resulting density and velocity profiles.     

Layering and position-dependent diffusive dynamics of confined fluids

We study the diffusive dynamics of a hard-sphere fluid confined between parallel smooth hard walls. The position-dependent diffusion coefficient normal to the walls is larger in regions of high local packing density. High density regions also have the largest available volume, consistent with the fast local diffusivity. Indeed, local and global diffusivities as a function of the Widom insertion probability approximately collapse onto a master curve. Parallel and average normal diffusivities are strongly coupled at high densities and deviate from bulk fluid behavior.     

Molecular simulation study of triangle-well fluids confined in slit pores

Grand canonical Monte Carlo simulations are used to study the behaviour of triangle-well (TW) fluids with variable well widths confined inside slit pores. The effect of individual factors influencing the properties of confined fluids such as fluid–fluid interactions, pore size and pore wall–fluid interactions are obtained using simulations as it is difficult to experimentally determine the same. An interesting observation of this study is that inside the narrow pore of slit height h* = 5 at the high-pressure condition of P* = 0.8, for the TW fluid with long-range attraction or for the fluid at a low temperature for even a short-range attraction, the density profiles show layering such that there is a sticking tendency of the particles at centre, while there is a depletion of particles near the wall (as the layers at the centre have higher density peak heights than near the walls).     

Simulation of solid–fluid mixture flow using moving particle methods

A new basic framework for solid–fluid mixture flow simulation was developed using moving particle methods. The interactions between solid and fluid were modeled by the finite volume particle (FVP) method. The distinct element method (DEM) together with a multi-time-step algorithm was introduced into the FVP method to calculate the effects of contact between solid bodies and between solid bodies and walls. The introduced DEM model was verified by experimental analyses for the collapse of multiple solid cylinder layers. The proposed algorithm using the optimized DEM model was then applied to a water dam breaking, involving multiple solid cylinder layers. A comparison between experiments and simulations demonstrated the DEM model introduced into the FVP method is effective in representing solid–fluid mixture flows reasonably well.     

Computer simulation and replica Ornstein—Zernike integral equation studies of a hard-sphere dipolar fluid adsorbed into disordered porous media

By means of extensive grand canonical Monte Carlo simulations and replica Ornstein-Zernike integral equation calculations, we explore the thermodynamics and dielectric behaviour of a dipolar fluid confined in disordered matrices. Different matrix topologies are modelled using, on the one hand, quenched hard-sphere configurations and, on the other, randomly positioned spheres. This illustrates the influence of the pore size and shape on the properties of the adsorbed fluid. For the same purpose, various sizes of the matrix particles have been considered. The integral equation calculations in the hypernetted chain approximation agree quantitatively with the simulation results. As in other studies on quenched disorder, one observes that the effect of confinement when considering hard-sphere matrices is rather limited, exhibiting however a tendency to facilitate the transition to ferroelectric states, as well as favouring the gas-liquid transition. Additionally, one observes that the sensitivity of the fluid properties to changes in the size of the matrix particles is considerably larger when these are randomly positioned.     

Slow dynamics of embedded fluid in mesoscopic confining systems as probed by NMR relaxometry

Mesoscopic media such as porous materials or colloidal dispersions strongly influence the dynamics of the embedded fluid. In the strong-adsorption regime, it was recently proposed that the effective surface diffusion on flat surface is anomalous and exhibits long-time pathology, enlarging the time domain of the embedded-fluid dynamics towards the low-frequency regime. An interesting way to probe such a slow interfacial process is to use the field-cycling NMR relaxometry. This technique is used here to probe the fluid dynamics in two types of interfacial systems: i) a colloidal glass made of thin and flat particles; ii) a fully saturated porous media, the Vycor glass. Experimental results are critically compared to either a simple theoretical model of NMR dispersion involving elementary steps of the fluid dynamics near an interface (loops, trains, tails) or Brownian-dynamics simulations performed inside 3D reconstructions of these confined systems.Received: 1 January 2003, Published online: 14 October 2003PACS: 76.60.Es Relaxation effects - 61.43.Gt Powders, porous materials - 82.70.Dd Colloids     

多孔介质中流体流动及扩散的耦合格子Boltzmann模型

多孔介质中高Péclet数和大黏性比下混溶流体的流动和扩散广泛存在于二氧化碳驱油、化工生产等工业过程中.用数值方法对该问题进行研究时,关键在于如何正确描述高Péclet数和大黏性比下多孔介质内流体的行为.为此,提出了一种基于多松弛模型和格子动理模型的耦合格子Boltzmann模型.通过Chapman-Enskog分析,证明该模型能有效求解不可压Navier-Stokes方程和对流扩散方程.数值结果表明,该模型不仅具有二阶精度和良好的稳健性,而且对于高Péclet数和大黏性比的问题具有良好的数值稳定性,为模拟此类问题提供了有效工具.     

Integral equation theory of a one dimensional hard-ellipse fluid between hard walls

Density functional theory is used to study the structure of a one dimensional fluid model of hard-ellipse molecules with their axes freely rotating in a plane, confined between hard walls. A simple Hypernetted chain (HNC) approximation is used for the density functional of the fluid and the integral equation for the density is obtained from the grand potential. The only required input is the direct correlation function of the one dimensional hard-ellipse fluid. For this model, the pressure, sum rule and the density at the walls are obtained. The Percus Yevick (PY), for lower density, and HNC, for higher density, integral equations are also solved to obtain the direct correlation function of hard-ellipse model introduced here. We obtain the average density at the wall as well as the radial density profile. We compare these with Monte Carlo simulations of the same model and find reasonable agreement.