Phase Transitions In Pores Research Articles

Pore-scale model of freezing inception in a porous medium

The article describes a new model of water–ice phase transition in pores of a saturated porous medium. The model takes into account the difference in specific volume between ice and water which causes structural changes in the porous medium. Describing details of heat, phase, and structure dynamics, the model contributes to a deeper understanding of phenomena in upper soil layers subjected to either seasonal conditions or climate changes. Governing multi-physics system of equations includes the conservation of mass, momentum and energy at the pore level and includes the anisotropic Allen–Cahn equation for tracking the position of ice during nucleation and growth inside the pores. The model provides space–time behavior of key quantities and describes the interaction of growing ice with pore geometry and surrounding grains. The governing system of equations is solved by the finite-element method to provide several qualitative computational studies of the ice growth inside a porous structure.

A thermodynamic study of the mid-density scheme to determine the equilibrium phase transition in cylindrical pores

We present a thermodynamic analysis of the mid-density scheme that was introduced recently as a simple alternative to more complex methods (such as the gauge cell and the thermodynamic integration procedures) for determining the equilibrium phase transition in pores. A construction of configurations having densities falling between those of the low- and high-density states inside the hysteresis loop is carried out with a canonical Monte Carlo simulation (NVT, where the number of particles N, the system volume V and the temperature T in the system are constant), and it is found that this ‘nucleation’ state has two coexisting phases of alternating liquid bridges and gas cavities. This state is maintained at a threshold chemical potential, μeq (obtained with the Widom insertion method), which is the coexistence chemical potential. When this ‘nucleation’ state is in contact with an infinite reservoir (grand canonical ensemble) of chemical potential less than μeq, the system will evolve to a low-density state. On the other hand, when it is exposed to a chemical potential greater than μeq, the system will evolve to a high-density state. We test this method with a number of examples of cylindrical pores, and investigate the effects of the pore size, the simulation box length for the case of infinite pores, the pore length for the case of finite pores and the temperature. When the pore is finite in length, the coexistence is no longer a first-order transition, like the one observed in an infinitely long pore, but instead shows a second-order transition with a continuous, but sharp change from a low-density state to a high-density state.

Photon Correlation Spectroscopy of Confined Liquid Crystals in Isotropic, Nematic, and Smectic Phases

Photon correlation experiments were performed to study the influence of confinement on the dynamic behavior of liquid crystals (LCs) in isotropic, nematic and smectic-A phases. In isotropic phase the only fast, single exponential decay due to the order parameter fluctuations is observed. While decreasing the temperature, but still above I-N phase transition in pores, the contribution from this decay decreases and second decay due to director fluctuations appears. The existence of this decay is the evidence for the formation of ordered (paranematic) phase preceding I-N transition in confined LCs. In nematic phase the decay due to order parameter fluctuations vanishes, the relaxation due to director fluctuations dominates, and slow process absent in the bulk LC appears. In smectic phase the director fluctuations are almost frozen.

Phase Transitions in Pores: Experimental and Simulation Studies of Melting and Freezing

We report both experimental measurements and molecular simulations of the melting and freezing behavior of simple fluids in porous media. The experimental studies are for carbon tetrachloride and nitrobenzene in controlled pore glass (CPG) and Vycor. Differential scanning calorimetry (DSC) was used to determine the melting point in the porous materials for each of the glass samples. In the case of nitrobenzene (which has a nonzero dipole moment), dielectric spectroscopy was also used to determine melting points. Measurements by the two methods were in excellent agreement. The melting point was found to be depressed relative to the bulk value for both fluids. With the exception of smallest pores, the melting point depression was proportional to the reciprocal of the pore diameter, in agreement with the Gibbs-Thomson equation. Structural information about the different confined phases was obtained by measuring the dielectric relaxation times using dielectric spectroscopy. Monte Carlo simulations were used to determine the shift in the melting point, T m , for a simple fluid in pores having both repulsive and strongly attractive walls. The strength of attraction to the wall was shown to have a large effect on the shift in T m , with T m being reduced for weakly attracting walls. For strongly attracting walls, such as graphitic carbon, the melting point increases for slit-shaped pores. For such materials, the adsorbed contact layer is shown to melt at a higher temperature than the inner adsorbed layers. A method for calculating the free energies of solids in pores is presented, and it is shown that the solid-liquid transition is first order in these systems.

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.

Structure and Adsorption of a Hard Sphere Fluid in a Cylindrical and Spherical Pore Filled by a Disordered Matrix: A Monte Carlo Study

We have studied a model of a hard sphere fluid adsorbed in a cylindrical and spherical pore filled with a quenched disordered matrix of hard sphere particles using Grand canonical Monte Carlo simulations. The interactions of both matrix and fluid species with pore walls are assumed to be of a hard sphere type. We discuss the adsorption isotherms and density profiles of fluid particles in pores with different microporosities for several values of pore radius. We have observed that, similar to 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 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.

Molecular Simulations of Phase Transitions in Pores

Abstract Methods for simulating phase transitions in narrow pores are reviewed, and the advantages and disadvantages of different techniques are discussed. Examples are given of applications to vapor-liquid, liquid-liquid, melting and freezing, solid-solid and layering transitions. While there has been a considerable body of simulation work on vapor-liquid, wetting and layering transitions for simple fluids and pore geometries, much remains to be done on more complex geometries and network effects, on heterogeneous surfaces, and on liquid-liquid, melting and solid-solid transitions in pores.

Liquid-liquid phase transitions in pores

The behaviour of symmetric binary Lennard-Jones mixtures in narrow slit-like pores has been studied. Gibbs ensemble Monte Carlo simulation techniques have been used to examine the influence of confinement on phase separation for pores of different size and for different kinds of wall-fluid interaction. The phase coexistence curves and density profiles have been calculated. The critical point is shifted towards higher densities when the external potential becomes stronger or the pore width becomes smaller. Thus, the effect of confinement is to reduce the region of immiscibility.