35 research outputs found
Diffusion in simple fluids
Computed self diffusion coefficients for the Lennard-Jones and hard sphere fluids are related by
Dej = DNs(aB) exp (--e/2kB T)
where σB=σLJ(2/[1+ii(1+2kBT/ε)])1/6, the effective hard sphere diameter, is the (average) distance of closest approach in collisions between molecules which interact with the positive part of the LJ potential, and the Arrhenius term reflects the influence of the negative part. σLJ and ε are the size and well depth parameters. Measured diffusion coefficients of the halomethane liquids are reproduced by the equation over wide ranges of temperature and density and do not reveal any influence of the inelastic effects associated with molecular anisotropy
Waterlike thermodynamic anomalies in a repulsive-step potential system
We report a computer-simulation study of the equilibrium phase diagram of a
three-dimensional system of particles with a repulsive step potential. The
phase diagram is obtained using free-energy calculations. At low temperatures,
we observe a number of distinct crystal phases. We show that at certain values
of the potential parameters the system exhibits the water-like thermodynamic
anomalies: density anomaly and diffusion anomaly. The anomalies disappear with
increasing the repulsive step width: their locations move to the region inside
the crystalline phase.Comment: 6 pages, 5 figure
Density anomaly in a competing interactions lattice gas model
We study a very simple model of a short-range attraction and an outer shell
repulsion as a test system for demixing phase transition and density anomaly.
The phase-diagram is obtained by applying mean field analysis and Monte Carlo
simulations to a two dimensional lattice gas with nearest-neighbors attraction
and next-nearest-neighbors repulsion (the outer shell). Two liquid phases and
density anomaly are found.
The coexistence line between these two liquid phases meets a critical line
between the fluid and the low density liquid at a tricritical point. The line
of maximum density emerges in the vicinity of the tricritical point, close to
the demixing transition
Instantaneous Normal Mode Analysis of Supercooled Water
We use the instantaneous normal mode approach to provide a description of the
local curvature of the potential energy surface of a model for water. We focus
on the region of the phase diagram in which the dynamics may be described by
the mode-coupling theory. We find, surprisingly, that the diffusion constant
depends mainly on the fraction of directions in configuration space connecting
different local minima, supporting the conjecture that the dynamics are
controlled by the geometric properties of configuration space. Furthermore, we
find an unexpected relation between the number of basins accessed in
equilibrium and the connectivity between them.Comment: 5 pages, 4 figure
Boundary lubrication properties of materials with expansive freezing
We have performed molecular dynamics simulations of solid-solid contacts
lubricated by a model fluid displaying many of the properties of water,
particularly its expansive freezing. Near the region where expansive freezing
occurs, the lubricating film remains fluid, and the friction force decreases
linearly as the shear velocity is reduced. No sign of stick-slip motion is
observed even at the lowest velocities. We give a simple interpretation of
these results, and suggest that in general good boundary lubrication properties
will be found in the family of materials with expansive freezing.Comment: Version to appear in Phys. Rev. Let
Supercooled confined water and the Mode Coupling crossover temperature
We present a Molecular Dynamics study of the single particle dynamics of
supercooled water confined in a silica pore. Two dynamical regimes are found:
close to the hydrophilic substrate molecules are below the Mode Coupling
crossover temperature, , already at ambient temperature. The water closer
to the center of the pore (free water) approaches upon supercooling as
predicted by Mode Coupling Theories. For free water the crossover temperature
and crossover exponent are extracted from power-law fits to both the
diffusion coefficient and the relaxation time of the late region.Comment: To be published, Phys. Rev. Lett., 4 pages, 3 figures, revTeX, minor
changes in the figures, references added, changes in the tex
Pressure dependence of the self-diffusion in liquid trifluoromethane
A simple modification of a high resolution N.M.R. spectrometer is presented, permitting determination of self-diffusion coefficients at pressures up to 200 MPa in the temperature range between 140 K and 450 K. The self-diffusion coefficients of CHF3 are determined between 142 K and 250 K. The activation energies derived from the data are [EaD]p = const = (6·1 ± 0·4) kJ mol-1, [EaD]V = const = (3·5 ± 0·4) kJ mol-1. Except for the lowest temperatures, compressed CHF3 appears to behave as a normal, rough hard-sphere fluid