117 research outputs found
Comment on "Melting of Isolated Tin Nanoparticles"
Comment on the paper of T.Bachels, H. J. G\"{u}ntherodt and R.Sch\"{a}fer :
"Melting of Isolated Tin Nanoparticles".Comment: Accepted for publication in Phys. Rev. Let
Realistic simulations of Au(100): Grand Canonical Monte Carlo and Molecular Dynamics
The large surface density changes associated with the (100) noble metals
surface hex-reconstruction suggest the use of non-particle conserving
simulation methods. We present an example of a surface Grand Canonical Monte
Carlo applied to the transformation of a square non reconstructed surface to
the hexagonally covered low temperature stable Au(100). On the other hand,
classical Molecular Dynamics allows to investigate microscopic details of the
reconstruction dynamics, and we show, as an example, retraction of a step and
its interplay with the surface reconstruction/deconstruction mechanism.Comment: 9 pages, 5 figures, accepted for publication in Surf. Rev. and
Letters (ICSOS-6
DOF phase separation of the Lennard-Jones fcc(111) surface
Recent lattice model calculations have suggested that a full-layered crystal
surface may undergo, under canonical (particle-conserving) conditions, a
preroughening-driven two-dimensional phase separation into two disordered flat
(DOF) regions, of opposite order parameter. We have carried out extensive
classical molecular dynamics (MD) simulations of the Lennard-Jones fcc(111)
surface, to check whether these predictions are relevant or not for a realistic
continuous system. Very long simulation times, a grid of temperatures from
(2/3)Tm to Tm, and unusually large system sizes are employed to ensure full
equilibrium and good statistics. By examining layer-by-layer occupancies,
height fluctuations, sublattice order parameter and X-ray structure factors, we
find a clear anomaly at ~0.83Tm. The anomaly is distinct from roughening (whose
incipiency is also detected at ~0.94Tm), and is seen to be consistent with the
preroughening plus phase separation scenario.Comment: REVTeX, 8 pages, 4 figures; new figure showing simulation snapshots
added; reference updated and other minor change
The development of Integrated Real Time Control to optimise storm water management for the combined sewer system of Rome
Increasing urbanisation and intensification of human activities are common
trends all over the world. The higher portion of impermeable urban surfaces
often leads to well known effects on storm water runoff and its polluting
potential for receiving waters. Despite the variety of structural solutions and
management practices proposed to mitigate the operational and environmental
impact of urban runoff, their application on existing drainage systems can often
be either ineffective at a metropolitan scale or unfeasible for a densely urbanised
territory. Among all the proposed alternatives, the real time control (RTC) of
drainage systems is proving more and more promising to dynamically regulate
the system capacity in response to intense rainfall. The combined sewer network
of Rome, historically built with high-capacity pipes to collect storm water from
both urban and natural catchments, holds significant potential for RTC of online
storage and combined sewer overflows, to optimise the global drainage capacity
and reduce the impact of discharges on local river quality. To assess the real
benefits, the potential limits and the feasibility of such a system for the city
sewers, a pilot study has been conducted on a 3,000 hectare sub-catchment. It
involved the development of a fast-response hydrodynamic simulation tool for
the sewer network, the definition and evaluation of RTC strategies and the
implementation of an environmental integrated telemetry system. As described
here, the study has highlighted significant margins for the optimisation of the
global network capacity without any major interventions on the physical assets,
as well as some critical issues to solve for a fully operational RTC application
Sediment Transport in Sewers: The Cesarina Combined Sewer Network
The polluting effects of storm water runoff on the receiving waterbodies
represent an increasingly relevant problem in developing urban areas. In
combined sewer pipes, transiting flood waves cause the alternation of sediment
erosion and deposition of the solid material transported by the flow. Combined
sewer deposit, mainly generated as an effect of such phenomena during the dry
weather period between two rain events, is generally a mix of sand and highly
polluting materials. Accumulation of sediments along a combined sewer network
is often the cause of dysfunctions in the drainage system itself and negative
impacts on the quality of receiving waters, due to the resuspension and overflow
of pollutants. Both aspects have been investigated for the combined sewer of
Rome thanks to an experimental catchment of about 2800 ha in the Cesarina –
S. Basilio area. Based on the simulations conducted, structural solutions were
proposed and evaluated, aimed at reducing the operational and environmental
problems related to sewer sediment. The results show noticeable margins for the
optimisation of the whole sewer system and for the reduction of its
environmental impact
Multiple scattering in random mechanical systems and diffusion approximation
This paper is concerned with stochastic processes that model multiple (or
iterated) scattering in classical mechanical systems of billiard type, defined
below. From a given (deterministic) system of billiard type, a random process
with transition probabilities operator P is introduced by assuming that some of
the dynamical variables are random with prescribed probability distributions.
Of particular interest are systems with weak scattering, which are associated
to parametric families of operators P_h, depending on a geometric or mechanical
parameter h, that approaches the identity as h goes to 0. It is shown that (P_h
-I)/h converges for small h to a second order elliptic differential operator L
on compactly supported functions and that the Markov chain process associated
to P_h converges to a diffusion with infinitesimal generator L. Both P_h and L
are selfadjoint (densely) defined on the space L2(H,{\eta}) of
square-integrable functions over the (lower) half-space H in R^m, where {\eta}
is a stationary measure. This measure's density is either (post-collision)
Maxwell-Boltzmann distribution or Knudsen cosine law, and the random processes
with infinitesimal generator L respectively correspond to what we call MB
diffusion and (generalized) Legendre diffusion. Concrete examples of simple
mechanical systems are given and illustrated by numerically simulating the
random processes.Comment: 34 pages, 13 figure
Deformation of a free interface pierced by a tilted cylinder
We investigate the interaction between an infinite cylinder and a free
fluid-fluid interface governed only by its surface tension. We study the
deformation of an initially flat interface when it is deformed by the presence
of a cylindrical object, tilted at an arbitrary angle, that the interface
"totally wets". Our simulations predict all significant quantities such as the
interface shape, the position of the contact line, and the force exerted by the
interface on the cylinder. These results are compared with an experimental
study of the penetration of a soap film by a cylindrical liquid jet. This
dynamic situation exhibits all the characteristics of a totally wetting
interface. We show that whatever the inclination, the force is always
perpendicular to the plane of the interface, and its amplitude diverges as the
inclination angle increases. Such results should bring new insights in both
fluid and solid mechanics, from animal locomotion to surface
micro-processing.-processing.Comment: 6 pages, 5 figure
Orbital frustration at the origin of the magnetic behavior in LiNiO2
We report on the ESR, magnetization and magnetic susceptibility measurements
performed over a large temperature range, from 1.5 to 750 K, on high-quality
stoichiometric LiNiO2. We find that this compound displays two distinct
temperature regions where its magnetic behavior is anomalous. With the help of
a statistical model based on the Kugel'-Khomskii Hamiltonian, we show that
below T_of ~ 400 K, an orbitally-frustrated state characteristic of the
triangular lattice is established. This then gives a solution to the
long-standing controversial problem of the magnetic behavior in LiNiO2.Comment: 5 pages, 5 figures, RevTex, accepted in PR
Measuring kinetic coefficients by molecular dynamics simulation of zone melting
Molecular dynamics simulations are performed to measure the kinetic
coefficient at the solid-liquid interface in pure gold. Results are obtained
for the (111), (100) and (110) orientations. Both Au(100) and Au(110) are in
reasonable agreement with the law proposed for collision-limited growth. For
Au(111), stacking fault domains form, as first reported by Burke, Broughton and
Gilmer [J. Chem. Phys. {\bf 89}, 1030 (1988)]. The consequence on the kinetics
of this interface is dramatic: the measured kinetic coefficient is three times
smaller than that predicted by collision-limited growth. Finally,
crystallization and melting are found to be always asymmetrical but here again
the effect is much more pronounced for the (111) orientation.Comment: 8 pages, 9 figures (for fig. 8 : [email protected]). Accepted for
publication in Phys. Rev.
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