Capillary freezing or complete wetting of hard spheres in a planar hard slit?

Extensive simulations of a hard sphere fluid confined between two planar hard walls show the onset of crystalline layers at the walls at about 98.3% of bulk crystallization density rho(f) independent of the wall separations L(z), and is, hence, a single wall phenomenon. As the bulk density far from the wall rho(b) increases, the thickness of the crystalline film appears to increase logarithmically, with (rho(f)-rho(b)) indicating complete wetting by the hard sphere crystal of the wall-fluid interface. Increasing rho(b) further, we observe a jump in the adsorption which depends on L(z) and corresponds to capillary freezing. The formation of crystalline layers below bulk crystallization, the logarithmic growth of the crystalline film, its independence of L(z), and its clear distinction from capillary freezing lend strong evidence for complete wetting by the hard sphere crystal at the wall-fluid interface.     

Interface mixing behaviour of Lennard–Jones FCC (100) thin film

A modified Monte Carlo method combined with quenched molecular dynamics simulation is used to determine mixing energetics and concentration profiles at interface for systems containing mono-and bilayers of adatoms adsorbed on FCC (100) crystal surface. The systems under consideration are constructed via Lennard–Jones potential at temperatures near 0 K. For systems with monolayer of adatoms, intermixing at the interface becomes preferable with increasing magnitude of the potential well-depth ratio of adatom to substrate atom. The increasing tendency of intermixing is linearly enhanced when the adatom becomes smaller than the substrate atom, otherwise, the intermixing trend is non-linear and weaker. For systems with bilayers of adatoms, complex development of concentration profile is observed along with increasing magnitude of the potential well-depth ratio and atomic size difference between adatom and substrate atom. This behaviour is related to the interplay between contributions of asymmetric bond interaction and relaxation to minimise the total energy of the system.     

Dynamic broadening of the crystal-fluid interface of colloidal hard spheres

We investigate the structure and dynamics of the crystal-fluid interface of colloidal hard spheres in real space by confocal microscopy. Tuning the buoyancy of the particles allows us to study the interface close to and away from equilibrium. We find that the interface broadens from 8-9 particle diameters close to equilibrium to 15 particle diameters away from equilibrium. Furthermore, the interfacial velocity, i.e., the velocity by which the interface moves upwards, increases significantly. The increasing gravitational drive leads to supersaturation of the fluid above the crystal surface. This dramatically affects crystal nucleation and growth, resulting in the observed dynamic broadening of the crystal-fluid interface.     

Long range relationship between Morse and Lennard–Jones potential energy functions

Arising from the use of the Morse function–which is well-known for its applicability for describing bonded interaction energy–in van der Waals systems, an attempt is made herein to express parameters of the Lennard–Jones potential function in terms of the Morse function to enable normalized comparison. In a departure from previous work where the parameter relationships enforce equal curvature at the minimum well-depth, the present approach replaces this rule with equal area above the curves for 1?≤?(r/R)?≤?∞. Results show good approximations of the Morse function to the Lennard–Jones curve and vice versa. Comparison with the previous relation for short range interaction shows that the present relations offer superior agreement with the Lennard–Jones function over a longer range. The conversion relations provide a cost-effective, less time-consuming and reasonably reliable method for obtaining Morse parameters from those of the Lennard–Jones function and vice versa.     

Analytical equations of state for multi-Yukawa fluids based on the Ross variational perturbation theory and the Percus-Yevick radial distribution function of hard spheres

Analytical expressions for equation of state and thermodynamic properties have been derived for the multi-Yukawa fluids, based on the Ross variational perturbation theory and the analytical Percus–Yevick (PY) expression for the radial distribution function of hard spheres. It is shown that the variational procedure is absolutely convergent and the calculations are convenient and fast. By using the parameters fitted to the Lennard–Jones 12–6 potential from the literature, numerical calculations have been made within wide temperature and density ranges. Comparison with computer simulations shows that the precision of the analytical Ross theory based on the Yukawa-type potential is equivalent to the non-analytical Ross theory based on the Lennard–Jones 12–6 potential. It is concluded that the analytical theory based on the Yukawa-type potential can be applied to research practical fluids within wide temperature and density ranges.     

Homogeneous bulk, surface, and edge nucleation in crystalline nanodroplets

The birth of a crystal is initiated by a nucleus from which the crystal grows--a dust grain in a snowflake is a familiar example. These nuclei can be heterogeneous defects, like the dust grain, or homogeneous nuclei which are intrinsic to the material. Here we study homogeneous nucleation in nanoscale polymer droplets on a substrate which itself can be crystalline or amorphous. We observe a large difference in the nucleating ability of the substrate. Furthermore, the scaling dependence of nucleation on the size of the droplets proves that the birth of the crystalline state can be directed to originate predominantly within the bulk, at the substrate surface, or at the droplets' edge, depending on how we tune the substrate.     

Phase diagram of hard colloidal platelets: a theoretical account

We construct the complete liquid crystal phase diagram of hard plate-like cylinders for variable aspect ratio using Onsager's second virial theory and employing the Parsons–Lee decoupling approximation to account for higher-body interactions in the isotropic and nematic fluid phases. The stability of the solid (columnar) state at high packing fraction is included by invoking a simple equation of state based on a Lennard–Jones–Devonshire cell model which has proven to be quantitatively reliable over a large range of packing fractions. By employing an asymptotic analysis based on the Gaussian approximation we are able to show that the nematic–columnar transition is universal and independent of particle shape. The predicted phase diagram is in qualitative agreement with simulation results.     

Effects of magnetoelastic coupling: critical behavior and structure deformation

We study a 3D crystal where each atom interacts with neighbors via elastic and magnetic interactions by Monte Carlo simulation. The distance dependence of both interactions is supposed to be the Lennard–Jones potential and the spins are of the Ising type. When the magnetic interaction strength is much smaller than the elastic one, the magnetic transition remains in the 3D Ising criticality. With larger magnetic interaction, the critical exponents get very close to those of the 3D XY universality and not far from Fisher renormalized Ising exponents. For strong magnetic interaction, we show that as the temperature increases the crystal is broken into ferromagnetic domains separated by domain walls consisting of contracted antiferromagnetic spin pairs.     

Comprehensive study of the vapour–liquid coexistence of the truncated and shifted Lennard–Jones fluid including planar and spherical interface properties

Vapour–liquid equilibria of the Lennard–Jones potential, truncated and shifted at 2.5σ, are studied using molecular dynamics simulations, an attractive option for studying inhomogeneous systems. Comprehensive simulation data are reported for three cases: no interface, a planar interface, and a spherical interface between the coexisting phases, covering a wide range of temperatures. Spherical droplets are also studied for a range of radii between 5 and 16σ. The size dependence of the surface tension, based on the Irving–Kirkwood pressure tensor, and other properties is quantified for spherical interfaces. All simulation results are correlated with a consistent set of empirical equations. A comparison with the results of other authors as well as with experimental data for noble gases and methane is also presented.     

Determination of the colloidal crystal nucleation rate density

Colloidal suspensions of charged latex microspheres in water exhibit liquid-like or crystalline ordering depending on particle interaction and concentration. By virtue of large particle spacing and slow dynamics, colloidal systems offer a unique opportunity to study interfacial structure and dynamics. This paper presents the first reported experimental study of the nucleation rate density, c, of an nonequilibrium (supercooled) colloidal liquid to colloidal crystal first order phase transition. Local and global observations of colloidal crystals growing from a metastable colloidal liquid were used to determine c. Microscopic local observations revealed homogeneous nucleation and constant interface velocity growth of quasispherical crystallites in the bulk and heterogeneous nucleation of a crystalline sheet with lower growth velocity at the cell wall. Complementary global observations of the recrystallization transition made by measuring the time dependence of the suspension transparency (the fraction of transmitted laser light) determined c by fitting this curve to a model based on an extension of Avrami's theory of crystallization.     

Electric field dependence of phase equilibria of binary Stockmayer fluid mixtures

A perturbation theory based study of the effect of an external electric field on the phase equilibrium properties of binary Stockmayer fluids is presented. The dipole–dipole interaction and the applied field are treated as independent perturbations to a Lennard–Jones mixture, and the reference fluid is treated by the van der Waals one-fluid approximation. A third-order free energy expression in the electric field strength is established, and the dielectric constant is calculated for a needle-shaped sample parallel to the field direction. We present and discuss vapour–liquid and liquid–liquid equilibrium curves at a given temperature for some dipolar mixtures exposed to an electric field, including chlorodifluoromethane +?difluoromethane and acetonitrile +?methanol. A sufficiently high electric field may result in massive shifts of vapour pressures and critical or azeotropic points, and can considerably alter the properties of coexisting phases. The vapour pressure decreases with increasing field strength.     

Glass/Crystal Interfaces in Liquid-Phase Sintered Materials

This paper presents a review of our studies of interfaces in liquid-phase sintered materials. Recent studies of three different types of interface are discussed, namely, (i) the interface between the free surface of the crystal and the glass, (ii) the interface between the intergranular glass and the crystal and (iii) the interface between the crystallized glass and the substrate crystal. Model systems with relatively well known thermodynamic and crystallographic properties have been chosen. The relationship between the three types of interface is discussed. The dewetting of silicate liquids on free surfaces provides an opportunity to study directly the interface between the free surface and glass. Observations on polycrystalline samples and bicrystals give new understanding of the interplay between intergranular glass layers and the adjoining crystalline grains. Crystallization of the glass on single-crystal substrates directly gives information about the crystallized-glass/crystal interface.     

Direct calculation of the hard-sphere crystal /Melt interfacial free energy

We present a direct calculation by molecular-dynamics computer simulation of the crystal/melt interfacial free energy gamma for a system of hard spheres of diameter sigma. The calculation is performed by thermodynamic integration along a reversible path defined by cleaving, using specially constructed movable hard-sphere walls, separate bulk crystal, and fluid systems, which are then merged to form an interface. We find the interfacial free energy to be slightly anisotropic with gamma = 0.62+/-0.01, 0.64+/-0.01, and 0. 58+/-0.01k(B)T/sigma(2) for the (100), (110), and (111) fcc crystal/fluid interfaces, respectively. These values are consistent with earlier density functional calculations and recent experiments.     

Density functional theory of vapor to liquid heterogeneous nucleation: Lennard–Jones fluid on solid substrate

Density functional theory has been applied to investigate the vapor to liquid heterogeneous nucleation on a flat solid surface, by invoking a model free energy density functional along with an exponential density model. The effects of supersaturation of the vapor and the strength of the solid-fluid interaction on the nucleation barrier have been investigated for Lennard–Jones fluid with 12–6 fluid–fluid and 9–3 solid–fluid interaction model. The spinodal decomposition of vapor has been observed at higher supersaturation or at higher strength of the solid–fluid interaction. The shape, density profile and the free energy of formation of droplets of any arbitrary size have been obtained in this work.     

Molecular dynamics simulation of a graphite-supported copper nanocluster: thermodynamic properties and gas adsorption

Molecular dynamics simulations were conducted for a cubic Cu cluster supported on a graphite bilayer. The Sutten–Chen and Lennard–Jones potentials were used for metal–metal and metal–graphite interactions, respectively. Heating and cooling processes were performed by NVT simulations at different temperatures in the range 200 to 1800?K. The melting point was identified on the basis of caloric and heat capacity curves. The calculated melting point was 770?K, far below the bulk melting point of crystalline copper. Several phenomena such as the appearance of a hysteresis (irreversibility) in caloric curves, surface melting, and cluster-induced surface wetting were justified from the results. The simulation of cluster in the presence of gas atmosphere showed that the CO gas is adsorbed more than H₂ and it has a greater impact on the cluster's structure.     

Surface freezing on patterned substrates

We show that the structure of a substrate pattern drastically influences the nature of surface freezing. By using phenomenological theory and computer simulations of a hard sphere fluid next to a substrate formed by a periodic array of fixed spheres, we find that a pattern which is commensurate with the bulk crystal induces complete surface freezing through a cascade of layering transitions. A rhombic pattern, on the other hand, either generates a crystalline sheet which is unstable as a bulk phase or prohibits surface freezing completely.     

Alchemical molecular dynamics for inverse design

We present a molecular dynamics (MD) implementation of an extended statistical mechanical ensemble that includes ‘alchemical’ degrees of freedom describing particle attributes as thermodynamic variables. We demonstrate the use of this alchemical MD method in inverse design simulations of particles interacting via the Oscillating Pair Potential (OPP) and the Lennard–Jones–Gauss potential (LJG) – two general, previously studied models for which phase diagrams are known. We show that alchemical MD can quickly and efficiently optimise pair potentials for target structures within a specified design space in the low-temperature regime, where internal energy adequately represents the features of the alchemical free energy landscape. We show that alchemical MD can be also used to inversely design pair potentials to achieve target materials properties (here, bulk modulus) directly, without explicit knowledge of the structure–property relationship. Alchemical MD can easily be generalised and applied to any target materials properties or structures and used with any differentiable interaction potential.     

Spontaneous translation of nanodroplet over a heterogeneous surface due to thermal cycles: role of solid–liquid interfacial fluctuations

ABSTRACT

We study the molecular-scale features of the solid surface that result in the spontaneous motion of a nanodroplet due to the periodic variation of temperature. We first employ a thermodynamic model to predict the variation of solid–fluid interfacial properties that can result in the above motion. The model identifies a composite (surface couple) made of two surfaces that are characterised by a large difference between the entropic parts of the solid–liquid interfacial free energies. In order to understand the molecular-scale features of the two surfaces that may form a surface couple, we performed grand canonical Monte Carlo simulations of Lennard Jones fluid and crystalline surfaces made of Lennard Jones-like atoms. We then used the cumulant expansions of the perturbation formulas to divide the interfacial entropy into two parts: The one that is directly affected by the solid–fluid attraction (direct part), and the other (indirect part) that is indirectly affected by the solid–fluid attraction via the alteration of interfacial fluctuations. Our results indicate that two surfaces form a surface couple if the differences between their chemical natures lead to large differences in the indirect part of the interfacial entropy, while the direct part remains relatively unaffected.     

Molecular dynamic simulation of Copper and Platinum nanoparticles Poiseuille flow in a nanochannels

In this paper, simulation of Poiseuille flow within nanochannel containing Copper and Platinum particles has been performed using molecular dynamic (MD). In this simulation LAMMPS code is used to simulate three-dimensional Poiseuille flow. The atomic interaction is governed by the modified Lennard–Jones potential. To study the wall effects on the surface tension and density profile, we placed two solid walls, one at the bottom boundary and the other at the top boundary. For solid–liquid interactions, the modified Lennard–Jones potential function was used. Velocity profiles and distribution of temperature and density have been obtained, and agglutination of nanoparticles has been discussed. It has also shown that with more particles, less time is required for the particles to fuse or agglutinate. Also, we can conclude that the agglutination time in nanochannel with Copper particles is faster that in Platinum nanoparticles. Finally, it is demonstrated that using nanoparticles raises thermal conduction in the channel.     

Coarse-grained model for gold nanocrystals with an organic capping layer

We have developed a coarse-grained interaction potential between icosahedral nanocrystals and united CH _x or SH atoms that interact via Lennard–Jones interactions. This interaction potential can be used to efficiently compute thermodynamic and structural properties of alkyl-thiol capping layers adsorbed on gold nanocrystals.