162 research outputs found
Model for the free-volume distributions of equilibrium fluids
We introduce and test via molecular simulation a simple model for predicting
the manner in which interparticle interactions and thermodynamic conditions
impact the single-particle free-volume distributions of equilibrium fluids. The
model suggests a scaling relationship for the density-dependent behavior of the
hard-sphere system. It also predicts how the second virial coefficients of
fluids with short-range attractions affect their free-volume distributions.Comment: 7 pages, 5 figure
Structural anomalies of fluids: Origins in second and higher coordination shells
Compressing or cooling a fluid typically enhances its static interparticle correlations. However, there are notable exceptions. Isothermal compression can reduce the translational order of fluids that exhibit anomalous waterlike trends in their thermodynamic and transport properties, while isochoric cooling (or strengthening of attractive interactions) can have a similar effect on fluids of particles with short-range attractions. Recent simulation studies by Yan [Phys. Rev. E 76, 051201 (2007)] on the former type of system and Krekelberg [J. Chem. Phys. 127, 044502 (2007)] on the latter provide examples where such structural anomalies can be related to specific changes in second and more distant coordination shells of the radial distribution function. Here, we confirm the generality of this microscopic picture through analysis, via molecular simulation and integral equation theory, of coordination shell contributions to the two-body excess entropy for several related model fluids which incorporate different levels of molecular resolution. The results suggest that integral equation theory can be an effective and computationally inexpensive tool for assessing, based on the pair potential alone, whether new model systems are good candidates for exhibiting structural (and hence thermodynamic and transport) anomalies.Chemical Engineerin
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.Comment: 5 pages, 3 figure
Web Applet For Predicting Structure And Thermodynamics Of Complex Fluids
Based on a recently introduced analytical strategy [Hollingshead et al., J. Chem. Phys. 139, 161102 (2013)], we present a web applet that can quickly and semi-quantitatively estimate the equilibrium radial distribution function and related thermodynamic properties of a fluid from knowledge of its pair interaction. We describe the applet's features and present two (of many possible) examples of how it can be used to illustrate concepts of interest for introductory statistical mechanics courses: the transition from ideal gas-like behavior to correlated-liquid behavior with increasing density, and the tradeoff between dominant length scales with changing temperature in a system with ramp-shaped repulsions. The latter type of interaction qualitatively captures distinctive thermodynamic properties of liquid water, because its energetic bias toward locally open structures mimics that of water's hydrogen-bond network. (C) 2015 American Association of Physics Teachers.Chemical Engineerin
Origin and Detection of Microstructural Clustering in Fluids with Spatial-Range Competitive Interactions
Fluids with competing short-range attractions and long-range repulsions mimic
dispersions of charge-stabilized colloids that can display equilibrium
structures with intermediate range order (IRO), including particle clusters.
Using simulations and analytical theory, we demonstrate how to detect cluster
formation in such systems from the static structure factor and elucidate links
to macrophase separation in purely attractive reference fluids. We find that
clusters emerge when the thermal correlation length encoded in the IRO peak of
the structure factor exceeds the characteristic lengthscale of interparticle
repulsions. We also identify qualitative differences between the dynamics of
systems that form amorphous versus micro-crystalline clusters.Comment: 6 pages, 5 figure
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
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.Comment: 5 pages, 4 figure
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