2,055 research outputs found
Direct calculation of interfacial tensions from computer simulation: Results for freely jointed tangent hard sphere chains
We develop a methodology for the calculation of surface free energies based
on the probability distribution of a wandering interface. Using a simple
extension of the NpT sampling, we allow the interface area to randomly probe
the available space and evaluate the surface free energy from histogram
analysis and the corresponding average. The method is suitable for studying
systems with either continuous or discontinuous potentials, as it does not
require explicit evaluation of the virial. The proposed algorithm is compared
with known results for the surface tension of Lennard--Jones and Square Well
fluid, as well as for the interface tension of a bead--spring polymer model and
good agreement is found. We also calculate interfacial tensions of freely
jointed tangent hard sphere chains on athermal walls for a wide range of chain
lengths and densities. The results are compared with three different
theoretical approaches, Scaled Particle Theory, the Yu and Wu density
functional theory and an analytical approximation based on the latter approach.
Whereas SPT only yields qualitative results, the last two approaches are found
to yield very good agreement with simulations.Comment: 20 pages, 6 figures, Phys. Rev. E in press
Surface tension of electrolytes: Hydrophilic and hydrophobic ions near an interface
We calculate the ion distributions around an interface in fluid mixtures of
highly polar and less polar fluids (water and oil) for two and three ion
species. We take into account the solvation and image interactions between ions
and solvent. We show that hydrophilic and hydrophobic ions tend to undergo a
microphase separation at an interface, giving rise to an enlarged electric
double layer. We also derive a general expression for the surface tension of
electrolyte systems, which contains a negative electrostatic contribution
proportional to the square root of the bulk salt density. The amplitude of this
square-root term is small for hydrophilic ion pairs, but is much increased for
hydrophilic and hydrophobic ion pairs. For three ion species including
hydrophilic and hydrophobic ions, we calculate the ion distributions to explain
those obtained by x-ray reflectivity measurements.Comment: 8 figure
Role of the first coordination shell in determining the equilibrium structure and dynamics of simple liquids
The traditional view that the physical properties of a simple liquid are
determined primarily by its repulsive forces was recently challenged by
Berthier and Tarjus, who showed that in some cases ignoring the attractions
leads to large errors in the dynamics [L. Berthier and G. Tarjus, Phys. Rev.
Lett. 103, 170601 (2009); J. Chem. Phys. 134, 214503 (2011)]. We present
simulations of the standard Lennard-Jones liquid at several condensed-fluid
state points, including a fairly low density state and a very high density
state, as well as simulations of the Kob-Andersen binary Lennard-Jones mixture
at several temperatures. By varying the range of the forces, results for the
thermodynamics, dynamics, and structure show that the determining factor for
getting the correct statics and dynamics is not whether or not the attractive
forces {\it per se} are included in the simulations. What matters is whether or
not interactions are included from all particles within the first coordination
shell (FCS) - the attractive forces can thus be ignored, but only at extremely
high densities. The recognition of the importance of a local shell in condensed
fluids goes back to van der Waals; our results confirm this idea and thereby
the basic picture of the old hole- and cell theories for simple condensed
fluids
Viscous coalescence of droplets: a Lattice Boltzmann study
The coalescence of two resting liquid droplets in a saturated vapor phase is
investigated by Lattice Boltzmann simulations in two and three dimensions. We
find that, in the viscous regime, the bridge radius obeys a t^{1/2}-scaling law
in time with the characteristic time scale given by the viscous time. Our
results differ significantly from the predictions of existing analytical
theories of viscous coalescence as well as from experimental observations.
While the underlying reason for these deviations is presently unknown, a simple
scaling argument is given that describes our results well.Comment: 12 pages, 10 figures; as published in Phys. Fluid
Non-Equilibrium Surface Tension of the Vapour-Liquid Interface of Active Lennard-Jones Particles
We study a three-dimensional system of self-propelled Brownian particles
interacting via the Lennard-Jones potential. Using Brownian Dynamics
simulations in an elongated simulation box, we investigate the steady states of
vapour-liquid phase coexistence of active Lennard-Jones particles with planar
interfaces. We measure the normal and tangential components of the pressure
tensor along the direction perpendicular to the interface and verify mechanical
equilibrium of the two coexisting phases. In addition, we determine the
non-equilibrium interfacial tension by integrating the difference of the normal
and tangential component of the pressure tensor, and show that the surface
tension as a function of strength of particle attractions is well-fitted by
simple power laws. Finally, we measure the interfacial stiffness using
capillary wave theory and the equipartition theorem, and find a simple linear
relation between surface tension and interfacial stiffness with a
proportionality constant characterized by an effective temperature.Comment: 12 pages, 5 figures (Corrected typos and References
Solvent-free coarse-grained lipid model for large-scale simulations
A coarse-grained molecular model, which consists of a spherical particle and
an orientation vector, is proposed to simulate lipid membrane on a large length
scale. The solvent is implicitly represented by an effective attractive
interaction between particles. A bilayer structure is formed by
orientation-dependent (tilt and bending) potentials. In this model, the
membrane properties (bending rigidity, line tension of membrane edge, area
compression modulus, lateral diffusion coefficient, and flip-flop rate) can be
varied over broad ranges. The stability of the bilayer membrane is investigated
via droplet-vesicle transition. The rupture of the bilayer and worm-like
micelle formation can be induced by an increase in the spontaneous curvature of
the monolayer membrane.Comment: 13 pages, 19 figure
Capillary Waves in a Colloid-Polymer Interface
The structure and the statistical fluctuations of interfaces between
coexisting phases in the Asakura-Oosawa (AO) model for a colloid--polymer
mixture are analyzed by extensive Monte Carlo simulations. We make use of a
recently developed grand canonical cluster move with an additional constraint
stabilizing the existence of two interfaces in the (rectangular) box that is
simulated. Choosing very large systems, of size LxLxD with L=60 and D=120,
measured in units of the colloid radius, the spectrum of capillary wave-type
interfacial excitations is analyzed in detail. The local position of the
interface is defined in terms of a (local) Gibbs surface concept. For small
wavevectors capillary wave theory is verified quantitatively, while for larger
wavevectors pronounced deviations show up. For wavevectors that correspond to
the typical distance between colloids in the colloid-rich phase, the
interfacial fluctuations exhibit the same structure as observed in the bulk
structure factor. When one analyzes the data in terms of the concept of a
wavevector-dependent interfacial tension, a monotonous decrease of this
quantity with increasing wavevector is found. Limitations of our analysis are
critically discussed.Comment: 12 pages, 15 figure
Description of the fluctuating colloid-polymer interface
To describe the full spectrum of surface fluctuations of the interface
between phase-separated colloid-polymer mixtures from low scattering vector q
(classical capillary wave theory) to high q (bulk-like fluctuations), one must
take account of the interface's bending rigidity. We find that the bending
rigidity is negative and that on approach to the critical point it vanishes
proportionally to the interfacial tension. Both features are in agreement with
Monte Carlo simulations.Comment: 5 pages, 3 figures, 1 table. Accepted for publication in Phys. Rev.
Let
The Search for Signatures Of Transient Mass Loss in Active Stars
The habitability of an exoplanet depends on many factors. One such factor is
the impact of stellar eruptive events on nearby exoplanets. Currently this is
poorly constrained due to heavy reliance on solar scaling relationships and a
lack of experimental evidence. Potential impacts of Coronal Mass Ejections
(CMEs), which are a large eruption of magnetic field and plasma from a star,
are space weather and atmospheric stripping. A method for observing CMEs as
they travel though the stellar atmosphere is the type II radio burst, and the
new LOw Frequency ARray (LOFAR) provides a means for detection. We report on 15
hours of observation of YZ Canis Minoris (YZ CMi), a nearby M dwarf flare star,
taken in LOFAR's beam-formed observation mode for the purposes of measuring
transient frequency-dependent low frequency radio emission. The observations
utilized Low-Band Antenna (10-90 MHz) or High-Band Antenna (110-190 MHz) for
five three-hour observation periods. In this data set, there were no confirmed
type II events in this frequency range. We explore the range of parameter space
for type II bursts constrained by our observations Assuming the rate of shocks
is a lower limit to the rate at which CMEs occur, no detections in a total of
15 hours of observation places a limit of shocks/hr for YZ CMi due to the stochastic nature of the events and
limits of observational sensitivity. We propose a methodology to interpret
jointly observed flares and CMEs which will provide greater constraints to CMEs
and test the applicability of solar scaling relations
Undulation instabilities in the meniscus of smectic membranes
Using optical microscopy, phase shifting interferometry and atomic force
microscopy, we demonstrate the existence of undulated structures in the
meniscus of ferroelectric smectic-C* films. The meniscus is characterized by a
periodic undulation of the smectic-air interface, which manifests itself in a
striped pattern. The instability disappears in the untilted smectic-A phase.
The modulation amplitude and wavelength both depend on meniscus thickness. We
study the temperature evolution of the structure and propose a simple model
that accounts for the observed undulations.Comment: Submitted to PR
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