57,827 research outputs found

### On the nonlocal viscosity kernel of mixtures

In this report we investigate the multiscale hydrodynamical response of a
liquid as a function of mixture composition. This is done via a series of
molecular dynamics simulations where the wave vector dependent viscosity kernel
is computed for three mixtures each with 7-15 different compositions. We
observe that the nonlocal viscosity kernel is dependent on composition for
simple atomic mixtures for all the wave vectors studied here, however, for a
model polymer melt mixture the kernel is independent of composition for large
wave vectors. The deviation from ideal mixing is also studied. Here it is shown
that a Lennard-Jones mixture follows the ideal mixing rule surprisingly well
for a large range of wave vectors, whereas for both the Kob-Andersen mixture
and the polymer melt large deviations are found. Furthermore, for the polymer
melt the deviation is wave vector dependent such that there exists a critical
length scale at which the ideal mixing goes from under-estimating to
over-estimating the viscosity

### An improved 2.5 GHz electron pump: single-electron transport through shallow-etched point contacts driven by surface acoustic waves

We present an experimental study of a 2.5 GHz electron pump based on the
quantized acoustoelectric current driven by surface acoustic waves (SAWs)
through a shallow-etched point contact in a GaAs/AlGaAs heterostructure. At low
temperatures and with an additional counter-propagating SAW beam, up to n = 20
current plateaus at I=nef could be resolved, where n is an integer, e the
electron charge, and f the SAW frequency. In the best case the accuracy of the
first plateau at 0.40 nA was estimated to be dI/I = +/- 25 ppm over 0.25 mV in
gate voltage, which is better than previous results.Comment: 11 pages, 4 figure

### How `sticky' are short-range square-well fluids?

The aim of this work is to investigate to what extent the structural
properties of a short-range square-well (SW) fluid of range $\lambda$ at a
given packing fraction and reduced temperature can be represented by those of a
sticky-hard-sphere (SHS) fluid at the same packing fraction and an effective
stickiness parameter $\tau$. Such an equivalence cannot hold for the radial
distribution function since this function has a delta singularity at contact in
the SHS case, while it has a jump discontinuity at $r=\lambda$ in the SW case.
Therefore, the equivalence is explored with the cavity function $y(r)$.
Optimization of the agreement between y_{\sw} and y_{\shs} to first order
in density suggests the choice for $\tau$. We have performed Monte Carlo (MC)
simulations of the SW fluid for $\lambda=1.05$, 1.02, and 1.01 at several
densities and temperatures $T^*$ such that $\tau=0.13$, 0.2, and 0.5. The
resulting cavity functions have been compared with MC data of SHS fluids
obtained by Miller and Frenkel [J. Phys: Cond. Matter 16, S4901 (2004)].
Although, at given values of $\eta$ and $\tau$, some local discrepancies
between y_{\sw} and y_{\shs} exist (especially for $\lambda=1.05$), the SW
data converge smoothly toward the SHS values as $\lambda-1$ decreases. The
approximate mapping y_{\sw}\to y_{\shs} is exploited to estimate the internal
energy and structure factor of the SW fluid from those of the SHS fluid. Taking
for y_{\shs} the solution of the Percus--Yevick equation as well as the
rational-function approximation, the radial distribution function $g(r)$ of the
SW fluid is theoretically estimated and a good agreement with our MC
simulations is found. Finally, a similar study is carried out for short-range
SW fluid mixtures.Comment: 14 pages, including 3 tables and 14 figures; v2: typo in Eq. (5.1)
corrected, Fig. 14 redone, to be published in JC

### Origin of the Universal Roughness Exponent of Brittle Fracture Surfaces: Correlated Percolation in the Damage Zone

We suggest that the observed large-scale universal roughness of brittle
fracture surfaces is due to the fracture process being a correlated percolation
process in a self-generated quadratic damage gradient. We use the quasi-static
two-dimensional fuse model as a paradigm of a fracture model. We measure for
this model, that exhibits a correlated percolation process, the correlation
length exponent nu approximately equal to 1.35 and conjecture it to be equal to
that of uncorrelated percolation, 4/3. We then show that the roughness exponent
in the fuse model is zeta = 2 nu/(1+2 nu)= 8/11. This is in accordance with the
numerical value zeta=0.75. As for three-dimensional brittle fractures, a
mean-field theory gives nu=2, leading to zeta=4/5 in full accordance with the
universally observed value zeta =0.80.Comment: 4 pages RevTeX

### Dielectric response of a polar fluid trapped in a spherical nanocavity

We present extensive Molecular Dynamics simulation results for the structure,
static and dynamical response of a droplet of 1000 soft spheres carrying
extended dipoles and confined to spherical cavities of radii $R=2.5$, 3, and 4
nm embedded in a dielectric continuum of permittivity $\epsilon' \geq 1$. The
polarisation of the external medium by the charge distribution inside the
cavity is accounted for by appropriate image charges. We focus on the influence
of the external permittivity $\epsilon'$ on the static and dynamic properties
of the confined fluid. The density profile and local orientational order
parameter of the dipoles turn out to be remarkably insensitive to $\epsilon'$.
Permittivity profiles $\epsilon(r)$ inside the spherical cavity are calculated
from a generalised Kirkwood formula. These profiles oscillate in phase with the
density profiles and go to a ``bulk'' value $\epsilon_b$ away from the
confining surface; $\epsilon_b$ is only weakly dependent on $\epsilon'$, except
for $\epsilon' = 1$ (vacuum), and is strongly reduced compared to the
permittivity of a uniform (bulk) fluid under comparable thermodynamic
conditions.
The dynamic relaxation of the total dipole moment of the sample is found to
be strongly dependent on $\epsilon'$, and to exhibit oscillatory behaviour when
$\epsilon'=1$; the relaxation is an order of magnitude faster than in the bulk.
The complex frequency-dependent permittivity $\epsilon(\omega)$ is sensitive to
$\epsilon'$ at low frequencies, and the zero frequency limit
$\epsilon(\omega=0)$ is systematically lower than the ``bulk'' value
$\epsilon_b$ of the static primitivity.Comment: 12 pages including 17 figure

### Time scale for the onset of Fickian diffusion in supercooled liquids

We propose a quantitative measure of a time scale on which Fickian diffusion
sets in for supercooled liquids and use Brownian Dynamics computer simulations
to determine the temperature dependence of this onset time in a Lennard-Jones
binary mixture. The time for the onset of Fickian diffusion ranges between 6.5
and 31 times the $\alpha$ relaxation time (the $\alpha$ relaxation time is the
characteristic relaxation time of the incoherent intermediate scattering
function). The onset time increases faster with decreasing temperature than the
$\alpha$ relaxation time. Mean squared displacement at the onset time increases
with decreasing temperature

### Three-body interactions in complex fluids: virial coefficients from simulation finite-size effects

A simulation technique is described for quantifying the contribution of
three-body interactions to the thermodynamical properties of coarse-grained
representations of complex fluids. The method is based on comparing the third
virial coefficient $B_3$ for a complex fluid with that of an approximate
coarse-grained model described by a pair potential. To obtain $B_3$ we
introduce a new technique which expresses its value in terms of the measured
volume-dependent asymptote of a certain structural function. The strategy is
applicable to both Molecular Dynamics and Monte Carlo simulation. Its utility
is illustrated via measurements of three-body effects in models of star polymer
and highly size-asymmetrical colloid-polymer mixtures.Comment: 13 pages, 8 figure

### Coarse-graining diblock copolymer solutions: a macromolecular version of the Widom-Rowlinson model

We propose a systematic coarse-grained representation of block copolymers,
whereby each block is reduced to a single ``soft blob'' and effective intra- as
well as intermolecular interactions act between centres of mass of the blocks.
The coarse-graining approach is applied to simple athermal lattice models of
symmetric AB diblock copolymers, in particular to a Widom-Rowlinson-like model
where blocks of the same species behave as ideal polymers (i.e. freely
interpenetrate), while blocks of opposite species are mutually avoiding walks.
This incompatibility drives microphase separation for copolymer solutions in
the semi-dilute regime. An appropriate, consistent inversion procedure is used
to extract effective inter- and intramolecular potentials from Monte Carlo
results for the pair distribution functions of the block centres of mass in the
infinite dilution limit.Comment: To be published in mol.phys(2005

### The role of binaries in the enrichment of the early Galactic halo. I. r-process-enhanced metal-poor stars

The detailed chemical composition of most metal-poor halo stars has been
found to be highly uniform, but a minority of stars exhibit dramatic
enhancements in their abundances of heavy neutron-capture elements and/or of
carbon. The key question for Galactic chemical evolution models is whether
these peculiarities reflect the composition of the natal clouds, or if they are
due to later mass transfer of processed material from a binary companion. If
the former case applies, the observed excess of certain elements was implanted
within selected clouds in the early ISM from a production site at interstellar
distances. Our aim is to determine the frequency and orbital properties of
binaries among these chemically peculiar stars. This information provides the
basis for deciding whether mass transfer from a binary companion is necessary
and sufficient to explain their unusual compositions. This paper discusses our
study of a sample of 17 moderately (r-I) and highly (r-II) r-process-element
enhanced VMP and EMP stars. High-resolution, low signal-to-noise spectra of the
stars were obtained at roughly monthly intervals over 8 years with the FIES
spectrograph at the Nordic Optical Telescope. From these spectra, radial
velocities with an accuracy of ~100 m/s were determined by cross-correlation
against an optimized template. 14 of the programme stars exhibit no significant
RV variation over this period, while 3 are binaries with orbits of typical
eccentricity for their periods, resulting in a normal binary frequency of
~18+-6% for the sample. Our results confirm our preliminary conclusion from
2011, based on partial data, that the chemical peculiarity of the r-I and r-II
stars is not caused by any putative binary companions. Instead, it was
imprinted on the natal molecular clouds of these stars by an external, distant
source. Models of the ISM in early galaxies should account for such mechanisms.Comment: 14 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic

### Fourth virial coefficients of asymmetric nonadditive hard-disc mixtures

The fourth virial coefficient of asymmetric nonadditive binary mixtures of
hard disks is computed with a standard Monte Carlo method. Wide ranges of size
ratio ($0.05\leq q\leq 0.95$) and nonadditivity ($-0.5\leq \Delta\leq 0.5$) are
covered. A comparison is made between the numerical results and those that
follow from some theoretical developments. The possible use of these data in
the derivation of new equations of state for these mixtures is illustrated by
considering a rescaled virial expansion truncated to fourth order. The
numerical results obtained using this equation of state are compared with Monte
Carlo simulation data in the case of a size ratio $q=0.7$ and two
nonadditivities $\Delta=\pm 0.2$.Comment: 9 pages, 7 figures; v2: section on equation of state added; tables
moved to supplementary material
(http://jcp.aip.org/resource/1/jcpsa6/v136/i18/p184505_s1#artObjSF

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