4,976 research outputs found
Vapor-liquid surface tension of strong short-range Yukawa fluid
The thermodynamic properties of strong short-range attractive Yukawa fluids,
k=10, 9, 8, and 7, are determined by combining the slab technique with the
standard and the replica exchange Monte Carlo (REMC) methods. A good agreement
was found among the coexistence curves of these systems calculated by REMC and
those previously reported in the literature. However, REMC allows exploring the
coexistence at lower temperatures, where dynamics turns glassy. To obtain the
surface tension we employed, for both methods, a procedure that yields the
pressure tensor components for discontinuous potentials. The surface tension
results obtained by the standard MC and REMC techniques are in good agreement.Comment: 6 pages, 4 figure
Phase coexistence of cluster crystals: beyond the Gibbs phase rule
We report a study of the phase behavior of multiple-occupancy crystals
through simulation. We argue that in order to reproduce the equilibrium
behavior of such crystals it is essential to treat the number of lattice sites
as a constraining thermodynamic variable. The resulting free-energy
calculations thus differ considerably from schemes used for single-occupancy
lattices. Using our approach, we obtain the phase diagram and the bulk modulus
for a generalized exponential model that forms cluster crystals at high
densities. We compare the simulation results with existing theoretical
predictions. We also identify two types of density fluctuations that can lead
to two sound modes and evaluate the corresponding elastic constants.Comment: 4 pages, 3 figure
Bulk inhomogeneous phases of anisotropic particles: A fundamental measure functional study of the restricted orientations model
The phase diagram of prolate and oblate particles in the restricted
orientations approximation (Zwanzig model) is calculated. Transitions to
different inhomogeneous phases (smectic, columnar, oriented, or plastic solid)
are studied through minimization of the fundamental measure functional (FMF) of
hard parallelepipeds. The study of parallel hard cubes (PHC's) as a particular
case is also included motivated by recent simulations of this system. As a
result a rich phase behavior is obtained which include, apart from the usual
liquid crystal phases, a very peculiar phase (called here discotic smectic)
which was already found in the only existing simulation of the model, and which
turns out to be stable because of the restrictions imposed on the orientations.
The phase diagram is compared at a qualitative level with simulation results of
other anisotropic particle systems.Comment: 11 pages, 10 figure
Inversion of Sequence of Diffusion and Density Anomalies in Core-Softened Systems
In this paper we present a simulation study of water-like anomalies in
core-softened system introduced in our previous publications. We investigate
the anomalous regions for a system with the same functional form of the
potential but with different parameters and show that the order of the region
of anomalous diffusion and the region of density anomaly is inverted with
increasing the width of the repulsive shoulder.Comment: 8 pages, 10 figure
The 3-graviton vertex function in thermal quantum gravity
The high temperature limit of the 3-graviton vertex function is studied in
thermal quantum gravity, to one loop order. The leading () contributions
arising from internal gravitons are calculated and shown to be twice the ones
associated with internal scalar particles, in correspondence with the two
helicity states of the graviton. The gauge invariance of this result follows in
consequence of the Ward and Weyl identities obeyed by the thermal loops, which
are verified explicitly.Comment: 19 pages, plain TeX, IFUSP/P-100
Core-Softened System With Attraction: Trajectory Dependence of Anomalous Behavior
In the present article we carry out a molecular dynamics study of the
core-softened system and show that the existence of the water-like anomalies in
this system depends on the trajectory in space along which the
behavior of the system is studied. For example, diffusion and structural
anomalies are visible along isotherms as a function of density, but disappears
along the isochores and isobars as a function of temperature. On the other
hand, the diffusion anomaly may be seen along adiabats as a function of
temperature, density and pressure. It should be noted that it may be no
signature of a particular anomaly along a particular trajectory, but the
anomalous region for that particular anomaly can be defined when all possible
trajectories in the same space are examined (for example, signature of
diffusion anomaly is evident through the crossing of different isochors.
However, there is no signature of diffusion anomaly along a particular
isochor). We also analyze the applicability of the Rosenfeld entropy scaling
relations to this system in the regions with the water-like anomalies. It is
shown that the validity of the Rosenfeld scaling relation for the diffusion
coefficient also depends on the trajectory in the space along which
the kinetic coefficients and the excess entropy are calculated.Comment: 16 pages, 21 figures. arXiv admin note: this contains much of the
content of arXiv:1010.416
Kinetics of Phase Separation in Fluids: A Molecular Dynamics Study
We present results from extensive 3-d molecular dynamics (MD) simulations of
phase separation kinetics in fluids. A coarse-graining procedure is used to
obtain state-of-the-art MD results. We observe an extended period of temporally
linear growth in the viscous hydrodynamic regime. The morphological similarity
of coarsening in fluids and solids is also quantified. The velocity field is
characterized by the presence of monopole-like defects, which yield a
generalized Porod tail in the corresponding structure factor.Comment: 4 pages, 4 figure
Heterogeneous condensation of the Lennard-Jones vapor onto a nanoscale seed particle
The heterogeneous condensation of a Lennard-Jones vapor onto a nanoscale seed
particle is studied using molecular dynamics simulations. Measuring the
nucleation rate and the height of the free energy barrier using the mean first
passage time method shows that the presence of a weakly interacting seed has
little effect on the work of forming very small cluster embryos but accelerates
the rate by lowering the barrier for larger clusters. We suggest that this
results from a competition between the energetic and entropic features of
cluster formation in the bulk and at the heterogeneity. As the interaction is
increased, the free energy of formation is reduced for all cluster sizes. We
also develop a simple phenomenological model of film formation on a small seed
that captures the general features of the nucleation process for small
heterogeneities. A comparison of our simulation results with the model shows
that heterogeneous classical nucleation theory provides a good estimate of the
critical size of the film but significantly over-estimates the size of the
barrier.Comment: 9 pages, 10 figures, In Print J. Chem. Phy
On the size and shape of excluded volume polymers confined between parallel plates
A number of recent experiments have provided detailed observations of the
configurations of long DNA strands under nano-to-micrometer sized confinement.
We therefore revisit the problem of an excluded volume polymer chain confined
between two parallel plates with varying plate separation. We show that the
non-monotonic behavior of the overall size of the chain as a function of
plate-separation, seen in computer simulations and reproduced by earlier
theories, can already be predicted on the basis of scaling arguments. However,
the behavior of the size in a plane parallel to the plates, a quantity observed
in recent experiments, is predicted to be monotonic, in contrast to the
experimental findings. We analyze this problem in depth with a mean-field
approach that maps the confined polymer onto an anisotropic Gaussian chain,
which allows the size of the polymer to be determined separately in the
confined and unconfined directions. The theory allows the analytical
construction of a smooth cross-over between the small plate-separation de
Gennes regime and the large plate-separation Flory regime. The results show
good agreement with Langevin dynamics simulations, and confirm the scaling
predictions.Comment: 15 pages, 3 figure
Shear flow pumping in open microfluidic systems
We propose to drive open microfluidic systems by shear in a covering fluid
layer, e.g., oil covering water-filled chemical channels. The advantages as
compared to other means of pumping are simpler forcing and prevention of
evaporation of volatile components. We calculate the expected throughput for
straight channels and show that devices can be built with off-the-shelf
technology. Molecular dynamics simulations suggest that this concept is
scalable down to the nanoscale.Comment: 4 pages, 4 figure, submitted to Phys. Rev. Let
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