Symmetry breaking of the fluid density profiles in closed nanoslits

The density profiles in a fluid interacting with the two identical solid walls of a closed long slit were calculated for wide ranges of the number of fluid molecules in the slit and temperature by employing a nonlocal density functional theory. Using argon as the sample fluid and considering the walls composed of solid carbon dioxide, it is shown that the density profile corresponding to the stable state of the fluid considerably changes its shape with increasing average density rho(av) of the fluid inside the slit. Temperature dependent critical values rho(sb1) and rho(sb2) of rho(av) were identified, such that for rho(sb1)相似文献   

Fluid density profile transitions and symmetry breaking in a closed nanoslit

The density profiles in a fluid interacting with the two identical solid walls of a closed long slit were calculated for wide ranges of the number of fluid molecules in the slit and temperature by employing density functional theory in the local density approximation. Two potentials, the van der Waals and the Lennard-Jones, were considered for the fluid-fluid and the fluid-walls interactions. It was shown that the density profile corresponding to the stable state of the fluid considerably changes its shape with increasing average density (rhoav) of the fluid inside the slit, the details of changes being dependent on the selected potential. For the van der Waals potential, a single temperature-dependent critical value rhosb of rhoav was identified, such that for rhoav < rhosb the stable state of the system is described by a symmetric density profile, whereas for rhoav >/= rhosb it is described by an asymmetric one. This transition constitutes a spontaneous symmetry breaking of the fluid density distribution in a closed slit with identical walls. For rhoav >/= rhosb, a metastable state, described by a symmetric density profile, was present in addition to the stable asymmetric one. The shape of the symmetric profile changed suddenly at a value rhoc-h > rhosb of the average density, the density rhoc-h being almost independent of temperature. Because of the shapes of the profiles before and after the transformation, this transition was named cup-hill transformation. At the transition point, the density of the fluid near the walls decreased suddenly from a liquid-like value becoming comparable with the density of a gaseous phase, and the density in the middle of the slit increased suddenly from a gaseous-like value becoming on the order of the density of a liquid phase. For the Lennard-Jones potential, there are two temperature-dependent critical densities, rhosb1 and rhosb2, such that the stable density profile is asymmetric (symmetry breaking occurs) for rhosb1 相似文献   

Two-dimensional symmetry breaking of fluid density distribution in closed nanoslits

Stable and metastable fluid density distributions (FDDs) in a closed nanoslit between two identical parallel solid walls have been identified on the basis of a nonlocal canonical ensemble density functional theory. Similar to Monte Carlo simulations, periodicity of the FDD in one of the lateral (parallel to the walls surfaces) directions, denoted as the x direction, was assumed. In the other lateral direction, y direction, the FDD was considered uniform. It was found that depending on the average fluid density in the slit, both uniform as well as nonuniform FDDs in the x direction can occur. The uniform FDDs are either symmetric or asymmetric about the middle plane between walls; the latter FDD being the consequence of a symmetry breaking across the slit. The nonuniform FDDs in the x direction occur either in the form of a bump on a thin liquid film covering the walls or as a liquid bridge between those walls and provide symmetry breaking in the x direction. For small and large average densities, the stable state is uniform in the x direction and is symmetric about the middle plane between walls. In the intermediate range of the average density and depending on the length L(x) of the FDD period, the stable state can be represented either by a FDD, which is uniform in the x direction and asymmetric about the middle of the slit (small values of L(x)), or by a bump- and bridgelike FDD for intermediate and large values of L(x), respectively. These results are in agreement with the Monte Carlo simulations performed earlier by other authors. Because the free energy of the stable state decreases monotonically with increasing L(x), one can conclude that the real period is very large (infinite) and that for the values of the parameters employed, a single bridge of finite length over the entire slit is generated.     

Phase Equilibria of Hydrogen Bonding Fluid in a Slit Pore with Broken Symmetry

Phase equilibria of hydrogen bonding (HB) fluid confined in a slit pore with broken symmetry were investigated by the density functional theory incorporated with modified fundamental measure theory, where the symmetry breaking originated from the distinct interactions between fluid molecules and two walls of the slit pore. In terms of adsorption-desorption isotherms and the corresponding grand potentials, phase diagrams of HB fluid under various conditions are presented. Furthermore, through phase coexistences of laying transition and capillary condensation, the effects of HB interaction, pore width, fluid-pore interaction and the broken symmetry on the phase equilibrium properties are addressed. It is shown that these factors can give rise to apparent influences on the phase equilibria of confined HB fluid because of the competition between intermolecular interaction and fluid-pore interaction. Interestingly, a significant influence of broken symmetry of the slit pore is found, and thus the symmetry breaking can provide a new way to regulate the phase behavior of various confined fluids.     

Fluid in a closed narrow slit

The behavior of a fluid inside a closed narrow slit between solid walls is examined on the basis of the density functional theory. It is shown that the constraint of constant number of molecules leads to interesting effects which are absent when the slit is open and in contact with a reservoir. If the slit walls are identical, the density profiles at low temperatures or at high average densities rhoav of the fluid molecules in the slit have a sharp maximum in the middle of the slit, the value of the density at maximum being comparable to that of a liquid. The density of fluid at the walls is in this case comparable to the density of a vapor phase. At high temperatures or at low rhoav the fluid density in the middle of the slit is of the same order of magnitude as at the walls. For nonidentical walls the density maximum is shifted towards the wall with a stronger wall-fluid interaction. The transition between the two types (with and without the sharp maximum) of density profiles with the change of temperature in the slit occurs in a narrow range of temperatures, this range being larger for narrower slits. The pressures which the fluid exerts on the walls as well as the forces per unit area arising due to stresses in the sidewalls of the system can decrease with increasing rhoav. Such a behavior is not possible for homogeneous systems and can be explained by analyzing the fluid density at the walls when rhoav increases. The normal and transversal components of the pressure tensor were calculated as functions of the distance from the wall using the equation of hydrostatic equilibrium and direct calculation of the forces between molecules, respectively. The normal component decreases with increasing distance near the wall in contrast to the normal component near the liquid-vapor interface reported previously in the literature. The behavior of the transverse component does not depend on the fluid-solid interaction and is comparable to that for a liquid-vapor interface.     

Symmetry breaking in binary mixtures in closed nanoslits

The symmetry breaking (SB) of the fluid density distribution (FDD) in closed nanoslits between two identical parallel solid walls described by Berim and Ruckenstein [J. Chem. Phys. 128, 024704 (2008)] for a single component fluid is examined for binary mixtures on the basis of a nonlocal canonical ensemble density functional theory. As in Monte Carlo simulations, the periodicity of the FDD in one of the lateral (parallel to the wall surfaces) directions, denoted as the x direction, was assumed. In the other lateral direction, y direction, the FDD was considered to be uniform. The molecules of the two components have different diameters and their Lennard-Jones interaction potentials have different energy parameters. It was found that depending on the average fluid density in the slit and mixture composition, SB can occur for both or none of the components but never for only one of them. In the direction perpendicular to the walls (h direction), the FDDs of both components can be asymmetrical about the middle plane between walls. In the x direction, the SB occurs as bumps and bridges enriched in one of the components, whereas the composition of the mixture between them is enriched in the other component. The dependence of the SB states on the length Lx of the FDD period at fixed average densities of the two components was examined for Lx in the range from 10 to 120 molecular diameters of the smaller size component. It was shown that for large Lx, the stable state of the system corresponds to a bridge. Because the free energy of that state decreases monotonically with increasing Lx, one can conclude that the real period is very large (infinite) and that a single bridge exists in the slit.     

Understanding self-assembly of rod-coil copolymer in nanoslits

Rod-coil diblock copolymers are a special kind of molecule containing a rigid rod and a flexible part. We present a systematic study on self-assembly of the rod-coil copolymers in nanoslits using a hybrid density functional theory. The self-assembly of the rod-coil molecule is driven by the bulk concentration, and there exists a critical bulk concentration beyond which the rod-coil molecule self-assembled into ordered lamellar structures in the slit, otherwise it is in a disordered state. By monitoring the effect of the interaction (epsilon(TT)(*)) of molecular tail on the self-assembly, we found that in the nanoslit of H=13sigma, it is at epsilon(TT)(*)=8 rather than epsilon(TT)(*)=10 or epsilon(TT)(*)=12 that the minimal critical bulk concentration occurs. It may be because the strong tail-tail interaction leads to aggregation of the copolymer molecules in bulk phase, and the resulting supramolecular structures are fairly difficult to enter the slit due to the depletion effect. At a fixed slit, the structural evolution of the self-assembled film with the bulk concentration is observed, including trilayer and five-layer lamellar structures, smectic-A, smectic-C, and a mixture of smectic-A and smectic-C liquid crystal phases and so on. We found that the critical bulk concentration, corresponding to the disordered-ordered phase transition, greatly depends on the separation between two walls, and it changes periodically with the increase of the slit width. In addition, it is also found that the molecular flexibility is one of key factors determining the self-assembled structure in the slit, and the critical bulk density increases with the molecular flexibility.     

A hybrid approach for microscopic properties and self-assembly of dendrimers between two hard walls

Dendrimers are of interest in a number of applications and theoretical studies due to their interesting and complex architectures. We use a hybrid approach to investigate the microstructure of hard dendrimers and self-assembly of diblock dendrimers confined between two hard walls. In the hybrid approach, a single-chain Monte Carlo simulation is used to evaluate the ideal-gas contribution of the Helmholtz energy and a density functional theory is employed to calculate the excess Helmholtz energy. In our calculations, a coarse-grained model is used to represent the dendrimers of generations 1-4. The effects of generation and bulk packing fraction on the microscopic properties of the hard dendrimers are explored. With the increase of generations, the complexity of the dendritic architecture increases. Accordingly, the depletion effect becomes stronger with the generation at etabulk = 0.1. Furthermore, it is found that the more complex the molecular architecture and the higher the molecular stiffness, the smaller is the partitioning coefficient of confined dendrimers. In addition, we also investigate the effects of the width of the slit and the interaction (epsilon*AA) between hydrophilic segments on the self-assembly of diblock dendrimers in the slit. With the increase of epsilon*AA, we observe that the curves of average packing fraction of the dendrimers in the slit exist an abrupt jump, which corresponds to the first-order phase transition from a disordered state to a lamellar ordered structure. In the slit of H = 11sigma, it is at epsilon*AA = 8 rather than epsilon*AA = 10 or epsilon*AA = 12 that the minimum critical bulk packing fraction appears. This observation is distinctively different from the case of self-assembly of rod-like molecules in the slit, where the critical bulk concentration increases with the decrease of the head-head interaction linearly.     

Polymer-induced depletion interaction between weakly attractive plates

Lattice Monte Carlo simulations have been employed to calculate depletion interaction of excluded volume chains in a weakly attractive slit, particularly in the region around the critical point of adsorption. The simulations were performed under full equilibrium conditions where a dilute solution in a slit was in contact with the reservoir. The free energy of confinement deltaA, the force f, and the relative pressurepI/pE on the slit walls were calculated as a function of slit width D and the attraction strength epsilon. The depletion region in the pressure profile pI/pE vs D is reduced by an increase in the attraction potential epsilon in a manner resembling the influence of polymer concentration. At the critical point of adsorption epsilonc the depletion interaction vanishes both in the pressure pI/pE and in the intraslit concentration profile phiI(x). The parameters used to assess the stability of colloidal dispersions such as the depletion potential W(D) (an integral of the net pressure deltap) reach a unique value at the critical condition. A monotonic repulsive profilepI vs D was found for chains trapped in the slit at restricted equilibrium. The mean dimensions (R2) of chains compressed in attractive slits feature a distinct minimum at intermediate slit widths.     

Fluid transport in nanochannels induced by temperature gradients

We investigate the mechanisms of fluid transport driven by temperature gradients in nanochannels through molecular dynamics simulations. It is found that the fluid-wall interaction is critical in determining the flow direction. In channels of very low surface energy, where the fluid-wall binding energy ε(fw) is small, the fluid moves from high to low temperature and the flow is induced by a potential ratchet near the wall. In high surface energy channels, however, the fluid is pumped from low to high temperature and the pressure drop caused by the temperature gradient is the major driving force. In addition, as the fluid-wall interaction is strengthened, the flow flux assumes a maximum, where ε(fw) is close to the lower temperature T(L) of the channel and ε(fw)/kT(L) ≈ 1 is roughly satisfied.     

Pressure driven flow of polymer solutions in nanoscale slit pores

Polymer solutions subject to pressure driven flow and in nanoscale slit pores are systematically investigated using the dissipative particle dynamics approach. The authors investigated the effect of molecular weight, polymer concentration, and flow rate on the profiles across the channel of the fluid and polymer velocities, polymer density, and the three components of the polymers radius of gyration. They found that the mean streaming fluid velocity decreases as the polymer molecular weight and/or polymer concentration is increased, and that the deviation of the velocity profile from the parabolic profile is accentuated with increase in polymer molecular weight or concentration. They also found that the distribution of polymers conformation is highly anisotropic and nonuniform across the channel. The polymer density profile is also found to be nonuniform, exhibiting a local minimum in the center plane followed by two symmetric peaks. They found a migration of the polymer chains either from or toward the walls. For relatively long chains, as compared to the thickness of the slit, a migration toward the walls is observed. However, for relatively short chains, a migration away from the walls is observed.     

Phase behavior of binary symmetric mixtures in pillared slit-like pores: a density functional approach

Density functional approach is applied to study the phase behavior of symmetric binary Lennard-Jones(12,6) mixtures in pillared slit-like pores. Our focus is in the evaluation of the first-order phase transitions in adsorbed phases and lines delimiting mixed and demixed adsorbed phases. The scenario of phase changes is sensitive to the pore width, to the energy of fluid-solid interaction, the amount, and the length of the pillars. Quantitative trends and qualitative changes of the phase diagrams topology are examined depending on the values of these parameters. The presence of pillars provides additional excluded volume effects, besides the confinement due to the pore walls. The effects of attraction between fluid species and pillars counteract this additional confinement. We have observed that both the increasing surface pillar density and the augmenting strength of fluid-solid interactions can qualitatively change the phase diagrams topology for the model with sufficiently strong trends for demixing. If the length of pillars is sufficiently large comparing to the pore width at low temperatures, we observe additional phase transitions of the first and second order due to the symmetry breaking of the distribution of chain segments and fluid species with respect to the slit-like pore center. Re-entrant symmetry changes and additional critical points then are observed.     

Density profiles of Ar adsorbed in slits of CO2: spontaneous symmetry breaking revisited

A recently reported symmetry breaking of density profiles of fluid argon confined by two parallel solid walls of carbon dioxide is studied. The calculations are performed in the framework of a nonlocal density functional theory. It is shown that the existence of such asymmetrical solutions is restricted to a special choice for the adsorption potential, where the attraction of the solid-fluid interaction is reduced by the introduction of a hard-wall repulsion. The behavior as a function of the slit's width is also discussed. All the results are placed in the context of the current knowledge on this matter.     

Asymptotics of pressure tensor in wedge-shaped film between solids characterized by dispersion forces

Asymptotic equations are derived for all four (three diagonal and one off-diagonal) components of the Irving-Kirkwood pressure tensor in a wedge-shaped film of a fluid located between solid phases characterized by dispersion forces. The equations describe the dependence of the aforementioned components on the position inside the film at large distances from the solid surfaces. It is shown that, when the walls of a slit are of different natures, the pressure tensor depends not only on the opening angle of the wedge-shaped slit, bus also on the position of the interface between the solids in contact outside of the slit. As a consequence, the symmetry of the spatial dependence of the tensor components with respect to the median plane is violated.     

Diffusioosmosis and electroosmosis in a capillary slit with surface charge layers

This study analytically examines the steady diffusioosmotic and electroosmotic flows of an electrolyte solution in a fine capillary slit with each of its inside walls covered by a layer of adsorbed polyelectrolytes. In this solvent-permeable and ion-penetrable surface charge layer, idealized polyelectrolyte segments are assumed to distribute at a uniform density. The electric double layer and the surface charge layer may have arbitrary thicknesses relative to the gap width between the slit walls. The electrostatic potential distribution on a cross section of the slit is obtained by solving the linearized Poisson–Boltzmann equation, which applies to the case of low potentials or low fixed-charge densities. Explicit formulas for the fluid velocity profile due to the imposed electrolyte concentration gradient or electric field through the slit are derived as the solution of a modified Navier–Stokes/Brinkman equation. The results demonstrate that the structure of the surface charge layer can lead to an augmented or a diminished electrokinetic flow (even a reversal in direction of the flow) relative to that in a capillary with bare walls, depending on the characteristics of the capillary, of the surface charge layer, and of the electrolyte solution. For the diffusioosmotic flow with an induced electric field, competition between electroosmosis and chemiosmosis can result in more than one reversal in direction of the flow over a range of the Donnan potential of the adsorbed polyelectrolyte in the capillary.     

Chiral symmetry breaking in a microscopic model with asymmetric autocatalysis and inhibition

Asymmetric autocatalysis and inhibition have been proposed as key processes in the spontaneous emergence of chiral symmetry breaking in a prebiotic world. An elementary lattice model is formulated to simulate the kinetics of chiral symmetry breaking via autocatalysis and inhibition in a mixture of prochiral reactants, chiral products, and inert solvent. Starting from a chirally unbiased initial state, spontaneous symmetry breaking occurs in spite of equal a priori probability for creating either product enantiomer, and the coupled reaction-diffusion processes subsequently amplify the random early-stage symmetry breaking. The processes of reaction and diffusion are kinetically intertwined in a way leading to competition in the appearance of enantiomeric excess. An effective transition temperature can be identified below which spontaneous symmetry breaking appears. In the absence of inhibition, reactions are predominantly autocatalytic under both reaction control (fast diffusion, slow reaction) or diffusion control (fast reaction, slow diffusion) conditions. In the presence of inhibition, simulations with different system sizes converge to the same transition temperature under reaction control conditions, and in this limit the reactions are predominantly nonautocatalytic.     

Casimir amplitudes and capillary condensation of near-critical fluids between parallel plates: renormalized local functional theory

We investigate the critical behavior of a near-critical fluid confined between two parallel plates in contact with a reservoir by calculating the order parameter profile and the Casimir amplitudes (for the force density and for the grand potential). Our results are applicable to one-component fluids and binary mixtures. We assume that the walls absorb one of the fluid components selectively for binary mixtures. We propose a renormalized local functional theory accounting for the fluctuation effects. Analysis is performed in the plane of the temperature T and the order parameter in the reservoir ψ(∞). Our theory is universal if the physical quantities are scaled appropriately. If the component favored by the walls is slightly poor in the reservoir, there appears a line of first-order phase transition of capillary condensation outside the bulk coexistence curve. The excess adsorption changes discontinuously between condensed and noncondensed states at the transition. With increasing T, the transition line ends at a capillary critical point T=T(c) (ca) slightly lower than the bulk critical temperature T(c) for the upper critical solution temperature. The Casimir amplitudes are larger than their critical point values by 10-100 times at off-critical compositions near the capillary condensation line.     

Effects of the juxtaposition of carbonaceous slit pores on the overall transport behavior of adsorbed fluids

It is a common approximation in the modeling of adsorption in microporous carbons to treat the pores as slit pores, whose walls are considered to consist of an infinite number of graphitic layers. In practice, such an approximation is appropriate as long as the number of graphitic layers in the wall is greater than three. However, it is understood that pore walls in microporous carbons commonly consist of three or fewer layers. As well as affecting the solid--fluid interaction within a pore, such narrow walls permit the interaction of fluid molecules through the wall, with consequences for the adsorption characteristics. We consider the effect that a distributed pore-wall thickness model can have on transport properties. At low density we find that the only significant deviation in the transport properties from the infinite pore-wall thickness model occurs in pores with single-layer walls. For a model of activated carbons with a distribution of pore widths and pore-wall thicknesses, the transport properties are generally insensitive to the effects of finite walls, in terms of both the solid-fluid interaction within a pore and fluid-fluid interaction through the pore walls.     

Effects of wetting and anchoring on capillary phenomena in a confined liquid crystal

A fluid of hard spherocylinders of length-to-breadth ratio L/D=5 confined between two identical planar, parallel walls--forming a pore of slit geometry--has been studied using a version of the Onsager density-functional theory. The walls impose an exclusion boundary condition over the particle's centers of mass, while at the same time favoring a particular anchoring at the walls, either parallel or perpendicular to the substrate. We observe the occurrence of a capillary transition, i.e., a phase transition associated with the formation of a nematic film inside the pore at a chemical potential different from micro(b)-the chemical potential at the bulk isotropic-nematic transition. This transition terminates at an Ising-type surface critical point. In line with previous studies based on the macroscopic Kelvin equation and the mesoscopic Landau-de Gennes approach, our microscopic model indicates that the capillary transition is greatly affected by the wetting and anchoring properties of the semi-infinite system, i.e., when the fluid is in contact with a single wall or, equivalently, the walls are at a very large distance. Specifically, in a situation where the walls are preferentially wetted by the nematic phase in the semi-infinite system, one has the standard scenario with the capillary transition taking place at chemical potentials less than micro(b) (capillary nematization transition or capillary ordering transition). By contrast, if the walls tend to orientationally disorder the fluid, the capillary transition may occur at chemical potentials larger than micro(b), in what may be called a capillary isotropization transition or capillary disordering transition. Moreover, the anchoring transition that occurs in the semi-infinite system may affect very decisively the confinement properties of the liquid crystal and the capillary transitions may become considerably more complicated.