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Designing coatings and films that can protect surfaces is important in a wide variety of applications from corrosion prevention to anti-fouling. These systems are challenging from a modeling perspective because they are invariably multicomponent, which quickly leads to an expansive design space. At a minimum, the system has a substrate, a film (often composed of a polymeric material), a ubiquitous carrier solvent, which may be either a vapor or liquid phase, and one or more contaminants. Each component has an impact on the effectiveness of coating. This paper focuses on films that are used as a barrier to surface contamination, but the results also extend to surface coatings that are designed to extract a low density species from the fluid phase as in liquid chromatography. A coarse-grained model is developed using Yukawa potentials that encompasses both repulsive and attractive interactions among the species. Classical density functional theory calculations are presented to show how contaminant adsorption is controlled by the molecular forces in the system. Two specific vectors through the parameter space are considered to address likely experimental manipulations that change either the solvent or the polymer in a system. We find that all the adsorption results can be unified by considering an appropriate combination of molecular parameters. As a result, these calculations provide a link between molecular interactions and film performance and may serve to guide the rational design of films. [ABSTRACT FROM AUTHOR]

We investigate diffusion in fluids near surfaces that may be coated with polymer films. We first consider diffusion in hard sphere fluids near a planar hard wall. We specifically consider color diffusion, where hard spheres are labeled A or B but are otherwise identical in all respects. In this inhomogeneous fluid, we consider a surface reaction–diffusion problem. At the left wall, a particle of species A is converted to one of species B upon a wall collision. At the opposing wall, the reverse reaction takes place: B → A. Using molecular dynamics simulation, we study the steady state of this system. We demonstrate that in the homogeneous region, a diffusing particle is subject to an equilibrium oscillatory force, the solvation force, that arises from the interfacial structuring of the fluid at the wall. For the hard sphere/hard wall system, the solvation force can be determined in various ways. We use the solvation force [the potential of mean force (PMF)] to solve the continuum diffusion equation. This provides an adequate and accurate description of the reaction–diffusion problem. The analysis is then extended to consider both color diffusion in the presence of a slowly varying one-body field such as gravity and a more applied problem of diffusion of free species through a surface film consisting of tethered chains. In both cases, the PMF experienced by the free particles is affected, but the diffusion problem can be treated in the same way as for the simpler hard sphere color diffusion case. [ABSTRACT FROM AUTHOR]

We study the interaction between a flat surface and a contaminant solution. The surface is protected by a grafted polymer layer. Our primary interest is to better understand and elucidate the effect of simple molecular interactions on the contamination and decontamination of the surface through molecular diffusion. These interactions manifest themselves in the potential of mean force that the contaminant molecule experiences as it diffuses across the grafted polymer layer. For simplicity, we consider that all interactions are of the hard-sphere type. The size of the contaminant molecule is the same as that of the solvent as well as the individual polymer segment. Despite these simplifications, the analysis offers important physical insights and a qualitative description of the contamination and decontamination processes. [ABSTRACT FROM AUTHOR]

In this paper, we show that multiaxial fields can induce time-averaged, noncentrosymmetric interactions between particles having polarization anisotropy, yet the multiaxial field itself does not exert either a force or a torque on an isolated particle. These induced interactions lead to particle assemblies whose energy is strongly dependent on both the translational and orientational degrees of freedom of the system. The situation is similar to a collection of permanent dipoles, but the symmetry of the time-averaged interaction is quite distinct, and the scale of the system energy can be dynamically controlled by the magnitude of the applied multiaxial field. In this paper, the case of polarizable rods is considered in detail, and it is suggested that collections of rods embedded in spheres can be used to create a material with a dynamically tunable magnetic permeability or dielectric permittivity. We report on Monte Carlo simulations performed to investigate the behavior of assemblies of both multiaxial-field induced dipoles and permanent dipoles arranged onto two-dimensional lattices. The ground state of the induced dipoles is an orientational soft mode of aligned dipoles, whereas that of the permanent dipoles is a vortex state. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations and a stochastic Fokker-Planck equation based approach are used to illuminate how position-dependent solvent mobility near one or more tracer particle(s) is affected when tracer-solvent interactions are rationally modified to affect corresponding solvation structure. For tracers in a dense hard-sphere fluid, we compare two types of tracer-solvent interactions: (1) a hard-sphere-like interaction, and (2) a soft repulsion extending beyond the hard core designed via statistical mechanical theory to enhance tracer mobility at infinite dilution by suppressing coordination-shell structure [Carmer et al., Soft Matter 8, 4083-4089 (2012)]. For the latter case, we show that the mobility of surrounding solvent particles is also increased by addition of the soft repulsive interaction, which helps to rationalize the mechanism underlying the tracer's enhanced diffusivity. However, if multiple tracer surfaces are in closer proximity (as at higher tracer concentrations), similar interactions that disrupt local solvation structure instead suppress the position-dependent solvent dynamics. [ABSTRACT FROM AUTHOR]

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.

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.

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]

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]

The interface between a doped semiconductor material and electrolyte solution is of considerable fundamental interest, and is relevant to systems of practical importance. Both adjacent domains contain mobile charges, which respond to potential variations. This is exploited to design electronic and optoelectronic sensors, and other enabling semiconductor colloidal materials. We show that the charge mobility in both phases leads to a new type of interaction between semiconductor colloids suspended in aqueous electrolyte solutions. This interaction is due to the electrostatic response of the semiconductor interior to disturbances in the external field upon the approach of two particles. The electrostatic repulsion between two charged colloids is reduced from the one governed by the charged groups present at the particles surfaces. This type of interaction is unique to semiconductor particles and may have a substantial effect on the suspension dynamics and stability. [ABSTRACT FROM AUTHOR]

Novel platinum nanowheels were synthesized by the reduction of aqueous platinum complex with ascorbic acid in the presence of disk-like bicelles. The platinum nanowheels possess thickened centers and flared edges that are connected by dendritic platinum nanosheets. This structural complexity can be attributed to the inhomogeneous micro-environment of the templating bicelles consisting of a central bi-layer region and a high curvature rim. The formation mechanism of the nanowheels was investigated by imaging nanostructures at different stages of the reaction. The templating bicelles were also imaged by TEM with the aid of negative staining. The variation of reaction parameters including platinum concentration, temperature, and total concentration of surfactants (CTAB + FC7) led to other types of platinum nanostructures, such as circular dendritic nanosheets with a tunable diameter and rectangular dendritic nanosheets. Interestingly, under irradiation by a TEM electron beam, the dendritic nanosheet portion of the nanowheels transforms into a metastable holey sheet. In addition, the platinum nanowheels have an electrochemical active surface area comparable to that of ETEK platinum black and thus are expected to have potential applications in catalysis. [ABSTRACT FROM AUTHOR]

We study fluid flow in the vicinity of textured and superhydrophobically coated surfaces with characteristic texture sizes on the order of 10 μm. Both for droplets moving down an inclined surface and for an external flow near the surface (hydrofoil), there is evidence of appreciable drag reduction in the presence of surface texture combined with superhydrophobic coating. On textured inclined surfaces, the drops roll faster than on a coated untextured surface at the same angle. The highest drop velocities are achieved on surfaces with irregular textures with characteristic feature size ∼8 μm. Application of the same texture and coating to the surface of a hydrofoil in a water tunnel results in drag reduction on the order of 10% or higher. This behavior is explained by the reduction of the contact area between the surface and the fluid, which can be interpreted in terms of changing the macroscopic boundary condition to allow nonzero slip velocity. [ABSTRACT FROM AUTHOR]