74,913 research outputs found
Different Melting Behavior in Pentane and Heptane Monolayers on Graphite; Molecular Dynamics Simulations
Molecular dynamics simulations are utilized to study the melting transition
in pentane (C5H12) and heptane (C7H16), physisorbed onto the basal plane of
graphite at near-monolayer coverages. Through use of the newest, optimized
version of the anisotropic united-atom model (AUA4) to simulate both systems at
two separate coverages, this study provides evidence that the melting
transition for pentane and heptane monolayers are significantly different.
Specifically, this study proposes a very rapid transition from the solid
crystalline rectangular-centered (RC) phase to a fluid phase in pentane
monolayers, whereas heptane monolayers exhibit a slower transition that
involves a more gradual loss of RC order in the solid-fluid phase transition.
Through a study of the melting behavior, encompassing variations where the
formation of gauche defects in the alkyl chains are eliminated, this study
proposes that this gradual melting behavior for heptane monolayers is a result
of less orientational mobility of the heptane molecules in the solid RC phase,
as compared to the pentane molecules. This idea is supported through a study of
a nonane monolayer, which gives the gradual melting signature that heptane
monolayers also seem to indicate. The results of this work are compared to
previous experiment over pentane and heptane monolayers, and are found to be in
good agreement
The directional contact distance of two ellipsoids: Coarse-grained potentials for anisotropic interactions
Copyright @ 2005 American Institute of Physics.We obtain the distance of closest approach of the surfaces of two arbitrary ellipsoids valid at any orientation and separation measured along their intercenter vector. This directional distance is derived from the elliptic contact function. The geometric meaning behind this approach is clarified. An elliptic pair potential for modeling arbitrary mixtures of elliptic particles, whether hard or soft, is proposed based on this distance. Comparisons with Gay-Berne potentials are discussed. Analytic expressions for the forces and torques acting on the elliptic particles are given.This research has been supported by GlaxoSmith-Klin
Some comments on Monte Carlo and molecular dynamics methods
We highlight some links between molecular dynamics and Monte Carlo algorithms used to simulate condensed matter systems. Special attention is paid to the question of sampling the desired statistical ensemble
Novel crystal phase in suspensions of hard ellipsoids
We present a computer simulation study on the crystalline phases of hard
ellipsoids of revolution. For aspect ratios greater than or equal to 3 the
previously suggested stretched-fcc phase [D. Frenkel and B. M. Mulder, Mol.
Phys. 55, 1171 (1985)] is replaced by a novel crystalline phase. Its unit cell
contains two ellipsoids with unequal orientations. The lattice is simple
monoclinic. The angle of inclination of the lattice, beta, is a very soft
degree of freedom, while the two right angles are stiff. For one particular
value of beta, the close-packed version of this crystal is a specimen of the
family of superdense packings recently reported [Donev et al., Phys. Rev. Lett.
92, 255506 (2004)]. These results are relevant for studies of nucleation and
glassy dynamics of colloidal suspensions of ellipsoids.Comment: 4 pages, 4 figure
Behavior of rod-like polyelectrolytes near an oppositely charged surface
The behavior of highly charged short rod-like polyelectrolytes near
oppositely charged planar surfaces is investigated by means of Monte Carlo
simulations. A detailed microstructural study, including monomer and fluid
charge distribution, and chain orientation, is provided. The influence of chain
length, substrate's surface-charge-density and image forces is considered. Due
to the lower chain-entropy (compared to flexible chains), our simulation data
show that rod-like polyelectrolytes can, in general, better adsorb than
flexible ones do. Nonetheless, at low substrate-dielectric-constant, it is
found that repulsive image forces tend to significantly reduce this
discrepancy.Comment: Updated results - 19 pages - 10 figs - RevTex
Key mechanistic features of swelling and blistering of helium-ion-irradiated tungsten
Helium-ion-induced swelling and blistering of single-crystal tungsten is investigated using a Helium Ion Microscope for site-specific dose-controlled irradiation (at 25 keV) with analysis by Helium Ion Microscopy, Atomic Force Microscopy and Transmission Electron Microscopy (cross-sectioning by Focused Ion Beam milling). We show that the blister cavity forms at a depth close to the simulated helium peak and that nanobubbles coalesce to form nanocracks within the envelope of the ion stopping range, swelling the blister shell. These results provide the first direct experimental evidence for the interbubble fracture mechanism proposed in the framework of the gas pressure model for blister formation
An assessment of the newest magnetar-SNR associations
Anomalous X-ray Pulsars and Soft-Gamma Repeaters groups are magnetar
candidates featuring low characteristic ages ().
At least some of them they should still be associated with the remnants of the
explosive events in which they were born, giving clues to the type of events
leading to their birth and the physics behind the apparent high value of the
magnetar magnetic fields. To explain the high values of , a self-consistent
picture of field growth also suggests that energy injection into the SNR is
large and unavoidable, in contrast with the evolution of {\it conventional}
SNR. This modified dynamics, in turn, has important implications for the
proposed associations. We show that this scenario yields low ages for the new
candidates CXOU J171405.7-381031/CTB 37B and XMMU J173203.3-344518/G353.6-0.7,
and predicted values agree with recently found , giving support to
the overall picture.Comment: Contributed talk to the ASTRONS 2010 Conference, Cesme, Turkey, Aug.
2-6 201
Molecular diffusion and slip boundary conditions at smooth surfaces with periodic and random nanoscale textures
The influence of periodic and random surface textures on the flow structure
and effective slip length in Newtonian fluids is investigated by molecular
dynamics (MD) simulations. We consider a situation where the typical pattern
size is smaller than the channel height and the local boundary conditions at
wetting and nonwetting regions are characterized by finite slip lengths. In
case of anisotropic patterns, transverse flow profiles are reported for flows
over alternating stripes of different wettability when the shear flow direction
is misaligned with respect to the stripe orientation. The angular dependence of
the effective slip length obtained from MD simulations is in good agreement
with hydrodynamic predictions provided that the stripe width is larger than
several molecular diameters. We found that the longitudinal component of the
slip velocity along the shear flow direction is proportional to the interfacial
diffusion coefficient of fluid monomers in that direction at equilibrium. In
case of random textures, the effective slip length and the diffusion
coefficient of fluid monomers in the first layer near the heterogeneous surface
depend sensitively on the total area of wetting regions.Comment: 30 pages, 11 figure
Interoperability between Multimedia Collections for Content and Metadata-Based Searching
Artiste is a European project developing a cross-collection search system for art galleries and museums. It combines image content retrieval with text based retrieval and uses RDF mappings in order to integrate diverse databases. The test sites of the Louvre, Victoria and Albert Museum, Uffizi Gallery and National Gallery London provide their own database schema for existing metadata, avoiding the need for migration to a common schema. The system will accept a query based on one museumâs fields and convert them, through an RDF mapping into a form suitable for querying the other collections. The nature of some of the image processing algorithms means that the system can be slow for some computations, so the system is session-based to allow the user to return to the results later. The system has been built within a J2EE/EJB framework, using the Jboss Enterprise Application Server
Understanding Legislator Experiences of Family-Friendly Working Practices in Political Institutions
This is a post-peer-review, pre-copy edit version of an article published in Politics and Gender. © 2015, Cambridge University Press
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