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Jeffrey R. Errington

Publications and source records attributed to Jeffrey R. Errington.

At least 19 recordsLinked to original sources
arXiv2014

Dimensionality and design of isotropic interactions that stabilize honeycomb, square, simple cubic, and diamond lattices

We use inverse methods of statistical mechanics and computer simulations to investigate whether an isotropic interaction designed to stabilize a given two-dimensional (2D) lattice will also favor an analogous three-dimensional (3D) structure, and vice versa. Specifically, we determine the 3D ordered lattices favored by isotropic potentials optimized to exhibit stable 2D honeycomb (or square) periodic structures, as well as the 2D ordered structures favored by isotropic interactions designed to stabilize 3D diamond (or simple cubic) lattices. We find a remarkable `transferability' of isotropic potentials designed to stabilize analogous morphologies in 2D and 3D, irrespective of the exact interaction form, and we discuss the basis of this cross-dimensional behavior. Our results suggest that the discovery of interactions that drive assembly into certain 3D periodic structures of interest can be assisted by less computationally intensive optimizations targeting the analogous 2D lattices.

cond-mat.mtrl-sci↗
arXiv2013

Predicting how nanoconfinement changes the relaxation time of a supercooled liquid

The properties of nanoconfined fluids can be strikingly different from those of bulk liquids. A basic unanswered question is whether the equilibrium and dynamic consequences of confinement are related to each other in a simple way. We study this question by simulation of a liquid comprising asymmetric dumbbell-shaped molecules, which can be deeply supercooled without crystallizing. We find that the dimensionless structural relaxation times - spanning six decades as a function of temperature, density, and degree of confinement - collapse when plotted versus excess entropy. The data also collapse when plotted versus excess isochoric heat capacity, a behaviour that follows from the existence of isomorphs in the bulk and confined states.

cond-mat.soft↗
arXiv2013

Phase Behavior of Materials with Isotropic Interactions Designed by Inverse Strategies to Favor Diamond and Simple Cubic Lattice Ground States

We use molecular simulation to construct equilibrium phase diagrams for two recently introduced model materials with isotropic, soft-repulsive pair interactions designed to favor diamond and simple cubic lattice ground states, respectively, over a wide range of densities [Jain et al., Soft Matter 9 14 (2013)]. We employ free energy based Monte Carlo simulation techniques to precisely trace the inter-crystal and fluid-crystal coexistence curves. We find that both model materials display rich polymorphic phase behavior featuring stable crystals corresponding to the target ground-state structures, as well as a variety of other crystalline (e.g., hexagonal and body-centered cubic) phases and multiple reentrant melting transitions.

cond-mat.mtrl-sci↗
arXiv2011

Enhancing tracer diffusivity by tuning interparticle interactions and coordination shell structure

This study uses a combination of stochastic optimization, statistical mechanical theory, and molecular simulation to test the extent to which the long-time dynamics of a single tracer particle can be enhanced by rationally modifying its interactions--and hence static correlations--with the other particles of a dense fluid. Specifically, a simulated annealing strategy is introduced that, when coupled with test-particle calculations from an accurate density functional theory, finds interactions that maximize either the tracer's partial molar excess entropy or a related pair-correlation measure (i.e., two quantities known to correlate with tracer diffusivity in other contexts). The optimized interactions have soft, Yukawa-like repulsions, which extend beyond the hard-sphere interaction and disrupt the coordination-shell cage structure surrounding the tracer. Molecular and Brownian dynamics simulations find that tracers with these additional soft repulsions can diffuse more than three times faster than bare hard spheres in a moderately supercooled fluid, despite the fact that the former appear considerably larger than the latter by conventional definitions of particle size.

cond-mat.soft↗
arXiv2011

Impact of surface roughness on diffusion of confined fluids

Using event-driven molecular dynamics simulations, we quantify how the self diffusivity of confined hard-sphere fluids depends on the nature of the confining boundaries. We explore systems with featureless confining boundaries that treat particle-boundary collisions in different ways and also various types of physically (i.e., geometrically) rough boundaries. We show that, for moderately dense fluids, the ratio of the self diffusivity of a rough wall system to that of an appropriate smooth-wall reference system is a linear function of the reciprocal wall separation, with the slope depending on the nature of the roughness. We also discuss some simple practical ways to use this information to predict confined hard-sphere fluid behavior in different rough-wall systems.

cond-mat.soft↗
arXiv2011

Implications of the effective one-component analysis of pair correlations in colloidal fluids with polydispersity

Partial pair-correlation functions of colloidal suspensions with continuous polydispersity can be challenging to characterize from optical microscopy or computer simulation data due to inadequate sampling. As a result, it is common to adopt an effective one-component description of the structure that ignores the differences between particle types. Unfortunately, whether this kind of simplified description preserves or averages out information important for understanding the behavior of the fluid depends on the degree of polydispersity and can be difficult to assess, especially when the corresponding multicomponent description of the pair correlations is unavailable for comparison. Here, we present a computer simulation study that examines the implications of adopting an effective one-component structural description of a polydisperse fluid. The square-well model that we investigate mimics key aspects of the experimental behavior of suspended colloids with short-range, polymer-mediated attractions. To characterize the partial pair-correlation functions and thermodynamic excess entropy of this system, we introduce a Monte Carlo sampling strategy appropriate for fluids with a large number of pseudo-components. The data from our simulations at high particle concentrations, as well as exact theoretical results for dilute systems, show how qualitatively different trends between structural order and particle attractions emerge from the multicomponent and effective one-component treatments, even with systems characterized by moderate polydispersity. We examine consequences of these differences for excess-entropy based scalings of shear viscosity, and we discuss how use of the multicomponent treatment reveals similarities between the corresponding dynamic scaling behaviors of attractive colloids and liquid water that the effective one-component analysis does not capture.

cond-mat.soft↗
arXiv2011

Mapping between long-time molecular and Brownian dynamics

We use computer simulations to test a simple idea for mapping between long-time self diffusivities obtained from molecular and Brownian dynamics. The strategy we explore is motivated by the behavior of fluids comprising particles that interact via inverse-power-law pair potentials, which serve as good reference models for dense atomic or colloidal materials. Based on our simulation data, we present an empirical expression that semi-quantitatively describes the "atomic" to "colloidal" diffusivity mapping for inverse-power-law fluids, but also for model complex fluids with considerably softer (star-polymer, Gaussian-core, or Hertzian) interactions. As we show, the anomalous structural and dynamic properties of these latter ultrasoft systems pose problems for other strategies designed to relate Newtonian and Brownian dynamics of hard-sphere-like particles.

cond-mat.soft↗
arXiv2011

Generalizing Rosenfeld's excess-entropy scaling to predict long-time diffusivity in dense fluids of Brownian particles: From hard to ultrasoft interactions

Computer simulations are used to test whether a recently introduced generalization of Rosenfeld's excess-entropy scaling method for estimating transport coefficients in systems obeying molecular dynamics can be extended to predict long-time diffusivities in fluids of particles undergoing Brownian dynamics in the absence of interparticle hydrodynamic forces. Model fluids with inverse-power-law, Gaussian-core, and Hertzian pair interactions are considered. Within the generalized Rosenfeld scaling method, long-time diffusivities of ultrasoft Gaussian-core and Hertzian particle fluids, which display anomalous trends with increasing density, are predicted (to within 20%) based on knowledge of interparticle interactions, excess entropy, and scaling behavior of simpler inverse-power-law fluids.

cond-mat.soft↗
arXiv2009

Generalized Rosenfeld scalings for tracer diffusivities in not-so-simple fluids: Mixtures and soft particles

Rosenfeld [Phys. Rev. A 15, 2545 (1977)] noticed that casting transport coefficients of simple monatomic, equilibrium fluids in specific dimensionless forms makes them approximately single-valued functions of excess entropy. This has predictive value because, while the transport coefficients of dense fluids are difficult to estimate from first principles, excess entropy can often be accurately predicted from liquid-state theory. Here, we use molecular simulations to investigate whether Rosenfeld's observation is a special case of a more general scaling law relating mobility of particles in mixtures to excess entropy. Specifically, we study tracer diffusivities, static structure, and thermodynamic properties of a variety of one- and two-component model fluid systems with either additive or non-additive interactions of the hard-sphere or Gaussian-core form. The results of the simulations demonstrate that the effects of mixture concentration and composition, particle-size asymmetry and additivity, and strength of the interparticle interactions in these fluids are consistent with an empirical scaling law relating the excess entropy to a new dimensionless (generalized Rosenfeld) form of tracer diffusivity, which we introduce here. The dimensionless form of the tracer diffusivity follows from knowledge of the intermolecular potential and the transport / thermodynamic behavior of fluids in the dilute limit. The generalized Rosenfeld scaling requires less information, and provides more accurate predictions, than either Enskog theory or scalings based on the pair-correlation contribution to the excess entropy. As we show, however, it also suffers from some limitations, especially for systems that exhibit significant decoupling of individual component tracer diffusivities.

cond-mat.stat-mech↗
arXiv2009

Composition and concentration anomalies for structure and dynamics of Gaussian-core mixtures

We report molecular dynamics simulation results for two-component fluid mixtures of Gaussian-core particles, focusing on how tracer diffusivities and static pair correlations depend on temperature, particle concentration, and composition. At low particle concentrations, these systems behave like simple atomic mixtures. However, for intermediate concentrations, the single-particle dynamics of the two species largely decouple, giving rise to the following anomalous trends. Increasing either the concentration of the fluid (at fixed composition) or the mole fraction of the larger particles (at fixed particle concentration) enhances the tracer diffusivity of the larger particles, but decreases that of the smaller particles. In fact, at sufficiently high particle concentrations, the larger particles exhibit higher mobility than the smaller particles. Each of these dynamic behaviors is accompanied by a corresponding structural trend that characterizes how either concentration or composition affects the strength of the static pair correlations. Specifically, the dynamic trends observed here are consistent with a single empirical scaling law that relates an appropriately normalized tracer diffusivity to its pair-correlation contribution to the excess entropy.

cond-mat.soft↗
arXiv2009

Anomalous structure and dynamics of the Gaussian-core fluid

It is known that there are thermodynamic states for which the Gaussian-core (GC) fluid displays anomalous properties such as expansion upon isobaric cooling (density anomaly) and increased single-particle mobility upon isothermal compression (self-diffusivity anomaly). We investigate how temperature and density affect its short-range translational structural order, as characterized by the two-body excess entropy. We find that there is a wide range of conditions for which the short-range translational order of the GC fluid decreases upon isothermal compression (structural order anomaly). The origin of the structural anomaly is qualitatively similar to that of other anomalous fluids and is connected to how compression affects static correlations at different length scales. We find that the self-diffusivity of the GC fluid obeys a scaling relationship with the two-body excess entropy that is very similar to the one observed for a variety of simple liquids. One consequence of this relationship is that the state points for which structural, self-diffusivity, and density anomalies of the Gaussian-core fluid occur appear as cascading regions on the temperature-density plane, a phenomenon observed earlier for models of waterlike fluids. There are, however, key differences between the anomalies of GC and waterlike fluids, and we discuss how those can be qualitatively understood by considering the respective interparticle potentials of these models. Finally, we note that the self-diffusivity of the Gaussian-core fluid obeys different scaling laws depending on whether the two-body or total excess entropy is considered. This finding, which deserves more comprehensive future study, appears to underscore the significance of higher-body correlations for the behavior of fluids with bounded interactions.

cond-mat.soft↗
arXiv2009

Available states and available space: Static properties that predict dynamics of confined fluids

Although density functional theory provides reliable predictions for the static properties of simple fluids under confinement, a theory of comparative accuracy for the transport coefficients has yet to emerge. Nonetheless, there is evidence that knowledge of how confinement modifies static behavior can aid in forecasting dynamics. Specifically, molecular simulation studies have shown that the relationship between excess entropy and self diffusivity of a bulk equilibrium fluid changes only modestly when the fluid is isothermally confined, indicating that knowledge of the former might allow semi-quantitative predictions of the latter. Do other static measures, such as those that characterize free or available volume, also strongly correlate with single-particle dynamics of confined fluids? Here, we study this issue for both the single-component hard-sphere fluid and hard-sphere mixtures. Specifically, we use molecular simulations and fundamental measure theory to study these systems at approximately $10^3$ equilibrium state points. We examine three different confining geometries (slit pore, square channel, and cylindrical pore) and the effects of packing fraction and particle-boundary interactions. Although density fails to predict some key qualitative trends for the dynamics of confined fluids, we find that a new generalized measure of available volume for inhomogeneous fluids strongly correlates with the self diffusivity across a wide parameter space in these systems, approximately independent of the degree of confinement. An important consequence, which we demonstrate here, is that density functional theory predictions of this static property can be used together with knowledge of bulk fluid behavior to estimate the diffusion coefficient of confined fluids under equilibrium conditions.

cond-mat.stat-mech↗
arXiv2008

Tuning density profiles and mobility of inhomogeneous fluids

Density profiles are the most common measure of inhomogeneous structure in confined fluids, but their connection to transport coefficients is poorly understood. We explore via simulation how tuning particle-wall interactions to flatten or enhance the particle layering of a model confined fluid impacts its self-diffusivity, viscosity, and entropy. Interestingly, interactions that eliminate particle layering significantly reduce confined fluid mobility, whereas those that enhance layering can have the opposite effect. Excess entropy helps to understand and predict these trends.

cond-mat.stat-mech↗
arXiv2008

Layering and position-dependent diffusive dynamics of confined fluids

We study the diffusive dynamics of a hard-sphere fluid confined between parallel smooth hard walls. The position-dependent diffusion coefficient normal to the walls is larger in regions of high local packing density. High density regions also have the largest available volume, consistent with the fast local diffusivity. Indeed, local and global diffusivities as a function of the Widom insertion probability approximately collapse onto a master curve. Parallel and average normal diffusivities are strongly coupled at high densities and deviate from bulk fluid behavior.

cond-mat.soft↗
arXiv2007

Calculation of surface tension via area sampling

We examine the performance of several molecular simulation techniques aimed at evaluation of the surface tension through its thermodynamic definition. For all methods explored, the surface tension is calculated by approximating the change in Helmholtz free energy associated with a change in interfacial area through simulation of a liquid slab at constant particle number, volume, and temperature. The methods explored fall within three general classes: free-energy perturbation, the Bennett acceptance-ratio scheme, and the expanded ensemble technique. Calculations are performed for both the truncated Lennard-Jones and square-well fluids at select temperatures spaced along their respective liquid-vapor saturation lines. Overall, we find that Bennett and expanded ensemble approaches provide the best combination of accuracy and precision. All of the methods, when applied using sufficiently small area perturbation, generate equivalent results for the Lennard-Jones fluid. However, single-stage free-energy-perturbation methods and the closely related test-area technique recently introduced by Gloor et al. [J. Chem. Phys. 123, 134703 (2005)], generate surface tension values for the square-well fluid that are not consistent with those obtained from the more robust expanded ensemble and Bennett approaches, regardless of the size of the area perturbation. Single-stage perturbation methods fail also for the Lennard-Jones system when applied using large area perturbations. Here an analysis of phase-space overlap produces a quantitative explanation of the observed inaccuracy, and shows that the satisfactory results obtained in these cases from the test-area method arise from a cancellation of errors that cannot be expected in general.

cond-mat.stat-mech↗
arXiv2007

Cofinement, entropy, and single-particle dynamics of equilibrium hard-sphere mixtures

We use discontinuous molecular dynamics and grand-canonical transition-matrix Monte Carlo simulations to explore how confinement between parallel hard walls modifies the relationships between packing fraction, self-diffusivity, partial molar excess entropy, and total excess entropy for binary hard-sphere mixtures. To accomplish this, we introduce an efficient algorithm to calculate partial molar excess entropies from the transition-matrix Monte Carlo simulation data. We find that the species-dependent self-diffusivities of confined fluids are very similar to those of the bulk mixture if compared at the same, appropriately defined, packing fraction up to intermediate values, but then deviate negatively from the bulk behavior at higher packing fractions. On the other hand, the relationships between self-diffusivity and partial molar excess entropy (or total excess entropy) observed in the bulk fluid are preserved under confinement even at relatively high packing fractions and for different mixture compositions. This suggests that the partial molar excess entropy, calculable from classical density functional theories of inhomogeneous fluids, can be used to predict some of the nontrivial dynamical behaviors of fluid mixtures in confined environments.

cond-mat.soft↗
arXiv2007

Does confining the hard-sphere fluid between hard walls change its average properties?

We use grand canonical transition-matrix Monte Carlo and discontinuous molecular dynamics simulations to generate precise thermodynamic and kinetic data for the equilibrium hard-sphere fluid confined between smooth hard walls. These simulations show that the pronounced inhomogeneous structuring of the fluid normal to the confining walls, often the primary focus of density functional theory studies, has a negligible effect on many of its average properties over a surprisingly broad range of conditions. We present one consequence of this insensitivity to confinement: a simple analytical equation relating the average density of the confined fluid to that of the bulk fluid with equal activity. Nontrivial implications of confinement for average fluid properties do emerge in this system, but only when the fluid is both (i) dense and (ii) confined to a gap smaller than approximately three particle diameters. For this limited set of conditions, we find that "in-phase" oscillatory deviations in excess entropy and self-diffusivity (relative to the behavior of the bulk fluid at the same average density) occur as a function of gap size. These paired thermodynamic/kinetic deviations from bulk behavior appear to reflect the geometric packing frustration that arises when the confined space cannot naturally accommodate an integer number of particle layers.

cond-mat.soft↗