Search

Bicellar templates are used for controlling the growth of platinum, platinum metal nanodisks and nanowheels. The metal growth methods are used as a means of imaging and discovering structural features of other types surfactant assemblies and other templating materials.

We report on the density functional theory calculations of electrolyte solutions confined between charged plates. Experiments with the surface forces apparatus have shown that for charged cylinders in simple solvents the surface force is oscillatory at short range. It has been suggested that the total force be considered a superposition of the electrostatic repulsion, the solvation force of a pure solvent, and the van der Waals attraction. In this paper we use the Rosenfeld--Kierlik--Rosinberg (RKR) functional to study a three-component electrolyte solution and explicitly test the superposition approximation. We find that the superposition approximation is a good approximation when the surface charge is low and the salt concentration is high. We also find that a three-component electrolyte can display short-range equilibrium separations in the absence of van der Waals interactions. [ABSTRACT FROM AUTHOR]

Osmotic stress (OS) techniques have found wide application in the crystallization of proteins, the study of intermolecular forces between biocolloids, and in studies of the role of solvent in molecular recognition events. In these cases, the role of the solvent is not well understood. We present here a combined Monte Carlo simulation and smoothed density functional theory approach with which model colloid–solvent mixtures may be studied in a potential of mean force approximation. In particular, our simulations focus on parallel cube colloids in Lennard-Jones solvents. We have applied this approach to simulate some OS experiments in which the dependence of the intermolecular separation (h) of DNA strands in aqueous solutions on an applied osmotic pressure (Π) have been measured. Our simulations demonstrate that an underlying oscillatory solvation potential qualitatively produces the features of Π–h relationships observed in the experiments. In particular, increasing Π produces a transition to a more dense colloidal crystal. More importantly, the transition Π decreases with increasing temperature as observed in experiments. In addition, hydrostatic pressure pb and Π are shown to operate conversely in isothermal solvent controlled OS experiments. In addition to characterizing some key features of solvent controlled processes, these simulations reconcile OS experiments with surface forces apparatus (SFA) experiments, and suggest new experimental routes to studying intermolecular forces. [ABSTRACT FROM AUTHOR]

We present grand canonical and canonical simulation results for the adsorption hysteresis in cylindrical and slitlike pores. Simulations for cylindrical symmetry were carried out for a continuous potential model, whereas the lattice gas was used to investigate the slitlike pores. In particular, we have studied the effects of pore ends on pore filling and pore emptying mechanism. We find that while the pore filling is unaffected, pore emptying is very sensitive to the presence of pore ends. Our findings confirm earlier lattice gas density functional theory results and indicate that fluctuations do not basically alter the mean field picture. [ABSTRACT FROM AUTHOR]

We study the dynamic response of confined liquid films to shear with molecular dynamics simulations. Both the hydrostatic pressure and chemical potential are controlled during the shearing process. We introduce grand canonical molecular dynamics to allow for material fluctuations. [ABSTRACT FROM AUTHOR]

The electrorheological (ER) response is defined as the rapid and reversible change in the rheological properties of nonaqueous suspensions due to the application of large electric fields [∼O(1 kV/mm)]. Orders of magnitude increases in suspension viscosities at small shear rates are commonly observed, and are believed to be due to the field-induced formation of fibrous structures. A molecular dynamics-like simulation technique is developed to investigate the ER response at small shear rates. The suspensions are modeled as monodisperse suspensions of hard, dielectric spheres contained in a Newtonian fluid between parallel plate electrodes, and subjected to electrostatic and hydrodynamic forces. The results predict a dynamic yield stress with a concentration dependence that agrees well with experimental results. The magnitudes of the simulated stresses are smaller than the experimental values, a result of approximations in the model. This issue is addressed in the second paper of this series. [ABSTRACT FROM AUTHOR]

Electrorheological (ER) suspensions display dramatic changes in their rheological properties when subjected to large electric fields. In an accompanying paper, a molecular dynamics-like simulation technique was developed to investigate sheared ER suspensions. The results of that study predicted a dynamic yield stress that saturates at large concentrations of the disperse phase. Experimental results are presented here which confirm the predicted saturation of the yield stress. The experimentally determined yield stresses are larger than the values predicted by the simulations in the point–dipole limit. This discrepancy is accounted for by reconciling approximations made in the simulation model. Incorporation of multipole and multibody contributions to the electrostatic interaction between spheres through a perturbation analysis suggests that accurate predictions of yield stresses can be made. However, we find that the ER response is sensitive not only to the material dielectric properties, but also the details of the pair interactions between particles at vanishing separations. [ABSTRACT FROM AUTHOR]

We present a new method for performing molecular simulations of systems with planar interfaces in the presence of fluctuating walls. The technique is essentially the realization of a frictionless piston exerting an external potential on a fluid within a container. The simulation cell is constructed by placing walls at each of the two ends and allowing periodic boundary conditions in the other two directions. Instead of holding the walls rigid, as in a conventional NVT simulation, an external field is imposed on either one or both walls which are free to move. In this paper we focus on a dynamic implementation of the method. The simplest case is a constant external force which is equivalent to imposing a constant normal pressure. We test the method through comparisons with conventional NVT molecular dynamics simulations. The results of these calculations indicate that the new method is both practical and reliable. [ABSTRACT FROM AUTHOR]

A simulation method is developed to investigate structure formation in electrorheological suspensions. The suspension is treated as polarizable, spherical particles in a nonconducting medium, with the spheres subject to electric polarization forces due to an applied electric field and to hydrodynamic resistance due to their motion through the continuous phase. The fibrous structures obtained from these simulations are independent of electric field strength and continuous phase viscosity in agreement with experimental observation. We have also found that the details of the simulated structures are sensitive to the treatment of the short-range forces preventing particle overlap. When this force is represented by a form that accurately approximates a hard-sphere interaction, the simulated structures agree well with those obtained experimentally, both with respect to their appearance and the time scale for structure formation. [ABSTRACT FROM AUTHOR]

We present molecular dynamics computer simulation data for the pressure tensor components of planar wall–liquid interfaces, along two ‘‘orthogonal’’ approaches to complete drying states. The interfacial structure varies from incipient solid-like packing, at positive adsorption, to smooth behavior, close to complete drying. We pay particular attention to the accompanying changes in the grand potential density, or to be more precise, in the transverse component of the pressure tensor of Irving and Kirkwood. [ABSTRACT FROM AUTHOR]

We present molecular dynamics computer simulation results for adsorption hysteresis in a cylindrical pore. The adsorption is examined by inducing pore filling and emptying by changing the pore length at constant number of particles. By calculating the full density profile of the fluid in the pore, one can observe both processes. Hysteresis, overshooting the thermodynamic transition, is observed for both pore filling and emptying. That is, we observe metastable one-phase states on either side of the transition. The end of the gas branch (pore filling) is signaled by the growth of an unduloid, resulting in the formation of a biconcave lens of liquid, while the end of the liquid branch (pore emptying) occurs by the formation of a bubble. However, the latter appears to be an artifact of the lack of pore ends in the model. In the presence of pore ends, the bubble formation is expected to be preempted by the receding of the meniscus. [ABSTRACT FROM AUTHOR]

Investigates the role of surface roughness on solvation forces and solvation free energies. Introduction of roughness; Use of the Monte Carlo simulation in the generation of solvation interactions; Comparison of simulation data to a simple superposition approximation of smooth wall solvation interactions.

Investigates the role of solvent-particle and particle-particle interactions on the swelling of colloidal systems. Application of the density functional theory; Measurement of degree of surface solvophobicity; Dependence of phase diagram phase on the particle-particle interaction.

Determines the adhesive energy of solid mica and explores the limits of the Laplace-Kelvin equation in micropores. Measurement of surface forces by others in undersaturated vapors; Understanding of the underlying oscillatory solvation forces, adsorption and surface deformation; Saturation limit of the pull-off force.

In inhomogeneous systems the diffusion paths available to particles are often restricted by the topology of the system itself. In search of a realistic scheme to simulate such inhomogeneous systems, we developed a new method, of local control of the chemical potential. The local control method, which limits creation and destruction of particles to a control volume, can be successfully implemented in both Monte Carlo (MC) and molecular dynamics (MD) simulations of the grand canonical ensemble (GC). We present applications of local GCMC calculations of the one-dimensional lattice gas and hard rods confined by hard walls, as well as local GCMC and GCMD of a three-dimensional bulk Lennard-Jones fluid system. The simulation results compare successfully with analytical solutions and equation of state approximations. [ABSTRACT FROM AUTHOR]

We report grand canonical Monte Carlo simulations and nonlocal density functional theory results for layering transitions that take place in a smooth cylindrical pore with strong fluid–wall interactions. For a pore with a radius of seven molecular diameters and a temperature below the bulk fluid triple point temperature the adsorption isotherms exhibit three distinct layering transitions ending in capillary condensation. The growth of the first layering in the simulation appears supercritical with respect to the layering critical point, but the subsequent layers appear to be true (first-order) transitions. In addition to the layering transitions we also present evidence for a quasi-two-dimensional freezing transition. That is, we observed the first adsorbed layer undergo a transition to a frustrated crystalline state prior to the next layering transition. Finally, for an adsorption isotherm at the bulk triple point there is no evidence for any first-order transitions, indicating that for our system the critical temperature associated with each layering transition does not exceed the bulk triple point temperature. [ABSTRACT FROM AUTHOR]

We report a study of hard-sphere mixtures of different sizes near a hard wall using both the Monte Carlo method and density functional theory. The theory is based on a semiempirical free-energy functional for an inhomogeneous hard-sphere mixture and is similar to that developed by Tarazona for pure hard-sphere fluids. Comparison between the theoretical results and the simulations for the density profiles of both species and the mole fraction profile shows that the present theory is capable of describing the structure of hard-sphere mixtures against a hard wall up to a size ratio, R≡σ2/σ1, of about 3. For R values greater than 3 the theory gives some discrepancies for densities very close to the wall. [ABSTRACT FROM AUTHOR]

We present adsorption isotherms, phase diagrams, and density profiles for a Lennard-Jones fluid confined to a cylindrical pore. In particular, we concentrate on the gas–liquid transition in the pore (capillary condensation). We compare simulations for a series of radii and different temperatures with mean field density functional theory (MFT). Two forms of MFT are considered, the simple local density approximation (LDA) and Tarazona’s nonlocal or smoothed density approximation (SDA). We find that the SDA provides a quite accurate description of fluid structure in the pore and that it produces phase diagrams in good agreement with the simulation data. For larger radii and temperatures T/Tc>=0.6 the SDA shows steep rises in adsorption close to the transition. This strongly affects the shape of the coexistence curve in the T, ρ plane. Here ρ is defined as the average density inside the pore. This behavior is confirmed by the simulation. In contrast, LDA gives a poor representation of the fluid structure and this underlies the failure to reproduce the phase diagrams and adsorption isotherms found with SDA or simulation. For extremely small radii (R*approx. 1) the simulation adsorption isotherms are smooth, and for not too low a temperature they are accurately described by an approach which starts from the potential distribution theorem and uses perturbation theory for the true one-dimensional fluid. [ABSTRACT FROM AUTHOR]

We report on the thermodynamic properties of binary compound mixtures of model groups II–VI semiconductors. We use the recently introduced Stillinger–Weber Hamiltonian to model binary mixtures of CdTe and CdSe. We use molecular dynamics simulations to calculate the volume and enthalpy of mixing as a function of mole fraction. The lattice parameter of the mixture closely follows Vegard's law: a linear relation. This implies that the excess volume is a cubic function of mole fraction. A connection is made with hard sphere models of mixed fcc and zincblende structures. The potential energy exhibits a positive deviation from ideal soluton behaviour; the excess enthalpy is nearly independent of temperatures studied (300 and 533 K) and is well described by a simple cubic function of the mole fraction. Using a regular solution approach (combining non-ideal behaviour for the enthalpy with ideal solution behaviour for the entropy of mixing), we arrive at the Gibbs free energy of the mixture. The Gibbs free energy results indicate that the CdTe and CdSe mixtures exhibit phase separation. The upper consolute temperature is found to be 335 K. Finally, we provide the surface energy as a function of composition. It roughly follows ideal solution theory, but with a negative deviation (negative excess surface energy). This indicates that alloying increases the stability, even for nano-particles. [ABSTRACT FROM PUBLISHER]