3,098 research outputs found
Wall-liquid and wall-crystal interfacial free energies via thermodynamic integration: A molecular dynamics simulation study
A method is proposed to compute the interfacial free energy of a
Lennard-Jones system in contact with a structured wall by molecular dynamics
simulation. Both the bulk liquid and bulk face-centered-cubic crystal phase
along the (111) orientation are considered. Our approach is based on a
thermodynamic integration scheme where first the bulk Lennard-Jones system is
reversibly transformed to a state where it interacts with a structureless flat
wall. In a second step, the flat structureless wall is reversibly transformed
into an atomistic wall with crystalline structure. The dependence of the
interfacial free energy on various parameters such as the wall potential, the
density and orientation of the wall is investigated. The conditions are
indicated under which a Lennard-Jones crystal partially wets a flat wall.Comment: 15 pages, 11 figure
Curvature Dependence of Surface Free Energy of Liquid Drops and Bubbles: A Simulation Study
We study the excess free energy due to phase coexistence of fluids by Monte
Carlo simulations using successive umbrella sampling in finite LxLxL boxes with
periodic boundary conditions. Both the vapor-liquid phase coexistence of a
simple Lennard-Jones fluid and the coexistence between A-rich and B-rich phases
of a symmetric binary (AB) Lennard-Jones mixture are studied, varying the
density rho in the simple fluid or the relative concentration x_A of A in the
binary mixture, respectively. The character of phase coexistence changes from a
spherical droplet (or bubble) of the minority phase (near the coexistence
curve) to a cylindrical droplet (or bubble) and finally (in the center of the
miscibility gap) to a slab-like configuration of two parallel flat interfaces.
Extending the analysis of M. Schrader, P. Virnau, and K. Binder [Phys. Rev. E
79, 061104 (2009)], we extract the surface free energy gamma (R) of both
spherical and cylindrical droplets and bubbles in the vapor-liquid case, and
present evidence that for R -> Infinity the leading order (Tolman) correction
for droplets has sign opposite to the case of bubbles, consistent with the
Tolman length being independent on the sign of curvature. For the symmetric
binary mixture the expected non-existence of the Tolman length is confirmed. In
all cases {and for a range of radii} R relevant for nucleation theory, gamma(R)
deviates strongly from gamma (Infinity) which can be accounted for by a term of
order gamma(Infinity)/gamma(R)-1 ~ 1/R^2. Our results for the simple
Lennard-Jones fluid are also compared to results from density functional theory
and we find qualitative agreement in the behavior of gamma(R) as well as in the
sign and magnitude of the Tolman length.Comment: 25 pages, submitted to J. Chem. Phy
Convection in galaxy-cluster plasmas driven by active galactic nuclei and cosmic-ray buoyancy
Turbulent heating may play an important role in galaxy-cluster plasmas, but
if turbulent heating is to balance radiative cooling in a quasi-steady state,
some mechanism must set the turbulent velocity to the required value. This
paper explores one possible regulating mechanism associated with an active
galactic nucleus at cluster center. A steady-state model for the intracluster
medium is presented in which radiative cooling is balanced by a combination of
turbulent heating and thermal conduction. The turbulence is generated by
convection driven by the buoyancy of cosmic rays produced by a central radio
source. The cosmic-ray luminosity is powered by the accretion of intracluster
plasma onto a central black hole. The model makes the rather extreme assumption
that the cosmic rays and thermal plasma are completely mixed. Although the
intracluster medium is convectively unstable near cluster center in the model
solutions, the specific entropy of the thermal plasma still increases outwards
because of the cosmic-ray modification to the stability criterion. The model
provides a self-consistent calculation of the turbulent velocity as a function
of position, but fails to reproduce the steep central density profiles observed
in clusters. The principal difficulty is that in order for the fully mixed
intracluster medium to become convectively unstable, the cosmic-ray pressure
must become comparable to or greater than the thermal pressure within the
convective region. The large cosmic-ray pressure gradient then provides much of
the support against gravity, reducing the thermal pressure gradient near
cluster center and decreasing the central plasma density gradient. A more
realistic AGN-feedback model of intracluster turbulence in which relativistic
and thermal plasmas are only partially mixed may have greater success.Comment: version 2: minor changes in wordin
Towards a Critique of Educative Violence: Walter Benjamin and ‘Second Education’
Although modern systems of mass education are typically defined in their opposition to violence, it has been argued that it is only through an insistent and critical focus upon violence that radical thought can be sustained. This article seeks to take up this challenge in relation to Walter Benjamin’s lesser-known writings on education. Benjamin retained throughout his life a deep suspicion about academic institutions and about the pedagogic, social and economic violence implicated in the idea of cultural transmission. He nonetheless remained committed to the possibility of another kind of revolutionary potential inherent to true education and, when he comes to speak of this in his Critique of Violence, it is remarkable that he describes it as manifesting an educative violence. This article argues that Benjamin’s philosophy works toward a critique of educative violence that results in a distinction between a ‘first’ and ‘second’ kind of education and asks whether destruction might have a positive role to play within pedagogical theories in contrast to current valorisations of creativity and productivity
Iron-catalyzed methylation using the borrowing hydrogen approach
A general iron-catalyzed methylation has been developed using methanol as a C1 building block. This borrowing hydrogen approach employs a Knölker-type (cyclopentadienone)iron carbonyl complex as catalyst (2 mol %) and exhibits a broad reaction scope. A variety of ketones, indoles, oxindoles, amines, and sulfonamides undergo mono- or dimethylation in excellent isolated yields (>60 examples, 79% average yield)
Low-Intrusion Techniques and Sensitive Information Management for Warhead Counting and Verification: FY2012 Annual Report
Progress in the second year of this project is described by the series of technical reports and manuscripts that make up the content of this report. These documents summarize successes in our goals to develop our robust image-hash templating and material-discrimination techniques and apply them to test image data
New constraints on dark energy from the observed growth of the most X-ray luminous galaxy clusters
We present constraints on the mean matter density, Omega_m, the normalization
of the density fluctuation power spectrum, sigma_8, and the dark-energy
equation-of-state parameter, w, obtained from measurements of the X-ray
luminosity function of the largest known galaxy clusters at redshifts z<0.7, as
compiled in the Massive Cluster Survey (MACS) and the local BCS and REFLEX
galaxy cluster samples. Our analysis employs an observed mass-luminosity
relation, calibrated by hydrodynamical simulations, including corrections for
non-thermal pressure support and accounting for the presence of intrinsic
scatter. Conservative allowances for all known systematic uncertainties are
included, as are standard priors on the Hubble constant and mean baryon
density. We find Omega_m=0.28 +0.11 -0.07 and sigma_8=0.78 +0.11 -0.13 for a
spatially flat, cosmological-constant model, and Omega_m=0.24 +0.15 -0.07,
sigma_8=0.85 +0.13 -0.20 and w=-1.4 +0.4 -0.7 for a flat, constant-w model.
Future work improving our understanding of redshift evolution and observational
biases affecting the mass--X-ray luminosity relation have the potential to
significantly tighten these constraints. Our results are consistent with those
from recent analyses of type Ia supernovae, cosmic microwave background
anisotropies, the X-ray gas mass fraction of relaxed galaxy clusters, baryon
acoustic oscillations and cosmic shear. Combining the new X-ray luminosity
function data with current supernova, cosmic microwave background and cluster
gas fraction data yields the improved constraints Omega_m=0.269 +- 0.016,
sigma_8=0.82 +- 0.03 and w=-1.02 +- 0.06. (Abridged)Comment: Submitted to MNRAS. 15 pages, 15 figures. v2: Improved modeling of
the mass-luminosity relation, including additional systematic allowances for
evolution in the scatter and non-thermal pressure support. Constraints are
somewhat weaker, but overall conclusions are unchanged
The infrared dust bubble N22: an expanding HII region and the star formation around it
Aims. To increase the observational samples of star formation around
expanding Hii regions, we analyzed the interstellar medium and star formation
around N22.
Methods. We used data extracted from the seven large-scale surveys from
infrared to radio wavelengths. In addition we used the JCMT observations of the
J = 3-2 line of 12CO emission data released on CADC and the 12CO J = 2-1 and J
=3-2 lines observed by the KOSMA 3 m telescope. We performed a multiwavelength
study of bubble N22.
Results. A molecular shell composed of several clumps agrees very well with
the border of N22, suggesting that its expansion is collecting the surrounding
material. The high integrated 12CO line intensity ratio (ranging from 0.7 to
1.14) implies that shocks have driven into the molecular clouds. We identify
eleven possible O-type stars inside the Hii region, five of which are located
in projection inside the cavity of the 20 cm radio continuum emission and are
probably the exciting-star candidates of N22. Twenty-nine YSOs (young stellar
objects) are distributed close to the dense cores of N22. We conclude that star
formation is indeed active around N22; the formation of most of YSOs may have
been triggered by the expanding of the Hii region. After comparing the
dynamical age of N22 and the fragmentation time of the molecular shell, we
suggest that radiation-driven compression of pre-existing dense clumps may be
ongoing.Comment: accepted in A&A 30/05/2012. arXiv admin note: text overlap with
arXiv:1010.5430 by other author
The high-speed X-ray camera on AXIS
AXIS is a Probe-class mission concept that will provide high-throughput,
high-spatial-resolution X-ray spectral imaging, enabling transformative studies
of high-energy astrophysical phenomena. To take advantage of the advanced
optics and avoid photon pile-up, the AXIS focal plane requires detectors with
readout rates at least 20 times faster than previous soft X-ray imaging
spectrometers flying aboard missions such as Chandra and Suzaku, while
retaining the low noise, excellent spectral performance, and low power
requirements of those instruments. We present the design of the AXIS high-speed
X-ray camera, which baselines large-format MIT Lincoln Laboratory CCDs
employing low-noise pJFET output amplifiers and a single-layer polysilicon gate
structure that allows fast, low-power clocking. These detectors are combined
with an integrated high-speed, low-noise ASIC readout chip from Stanford
University that provides better performance than conventional discrete
solutions at a fraction of their power consumption and footprint. Our
complementary front-end electronics concept employs state of the art digital
video waveform capture and advanced signal processing to deliver low noise at
high speed. We review the current performance of this technology, highlighting
recent improvements on prototype devices that achieve excellent noise
characteristics at the required readout rate. We present measurements of the
CCD spectral response across the AXIS energy band, augmenting lab measurements
with detector simulations that help us understand sources of charge loss and
evaluate the quality of the CCD backside passivation technique. We show that
our technology is on a path that will meet our requirements and enable AXIS to
achieve world-class science.Comment: 17 pages, 11 figures, submitted to Proceedings of SPIE Optics +
Photonics 202
PLA Logistics and Sustainment: PLA Conference 2022
The US Army War College People’s Liberation Army Conference (PLA) Conference was held March 31 to April 2, 2022, at Carlisle Barracks, Pennsylvania.
The conference focused on PLA logistics and sustainment. As the PLA continues to build and modernize its combat forces, it is important to examine if the capabilities meant to support combat operations are also being developed.
Specific topics included: 1) China’s national-level logistics, including how China mobilizes national resources for the military and how it provides joint logistics support to the PLA Theater Commands; 2) the logistics capabilities of the different PLA services, especially the Army, Navy, and Air Forces; 3) PLA logistics in China’s remote regions, such as airpower projection in the Western Theater Command along the Indian border; and, 4) the PLA’s ability to sustain overseas operations at its base in Djibouti.
Despite notable potential shortfalls and points of friction, the PLA has successfully sustained counterpiracy maritime operations for many years and conducted noncombatant evacuation operations well-distant from China. It is increasingly able to move forces across the vast distances of China and conduct large training exercises. Far more must be known about PLA sustainment and logistics before the hard questions about PLA operational reach and endurance can be answered.https://press.armywarcollege.edu/monographs/1954/thumbnail.jp
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