4,135 research outputs found
Contact of Single Asperities with Varying Adhesion: Comparing Continuum Mechanics to Atomistic Simulations
Atomistic simulations are used to test the equations of continuum contact
mechanics in nanometer scale contacts. Nominally spherical tips, made by
bending crystals or cutting crystalline or amorphous solids, are pressed into a
flat, elastic substrate. The normal displacement, contact radius, stress
distribution, friction and lateral stiffness are examined as a function of load
and adhesion. The atomic scale roughness present on any tip made of discrete
atoms is shown to have profound effects on the results. Contact areas, local
stresses, and the work of adhesion change by factors of two to four, and the
friction and lateral stiffness vary by orders of magnitude. The microscopic
factors responsible for these changes are discussed. The results are also used
to test methods for analyzing experimental data with continuum theory to
determine information, such as contact area, that can not be measured directly
in nanometer scale contacts. Even when the data appear to be fit by continuum
theory, extracted quantities can differ substantially from their true values
Drifting diffusion on a circle as continuous limit of a multiurn Ehrenfest model
We study the continuous limit of a multibox Erhenfest urn model proposed
before by the authors. The evolution of the resulting continuous system is
governed by a differential equation, which describes a diffusion process on a
circle with a nonzero drifting velocity. The short time behavior of this
diffusion process is obtained directly by solving the equation, while the long
time behavior is derived using the Poisson summation formula. They reproduce
the previous results in the large (number of boxes) limit. We also discuss
the connection between this diffusion equation and the Schrdinger
equation of some quantum mechanical problems.Comment: 4 pages prevtex4 file, 1 eps figur
Bayesian linear mixed models with polygenic effects
We considered Bayesian estimation of polygenic effects, in
particular heritability in relation to a class of linear mixed models
implemented in R. Our approach is applicable to
both family-based and population-based studies in human genetics
with which a genetic relationship matrix can be derived either from
family structure or genome-wide data. Using a simulated and a real
data, we demonstrate our implementation of the models in the generic
statistical software systems JAGS and
Stan as well as several R
packages. In doing so, we have not only provided facilities in
R linking standalone programs such as GCTA
and other packages in R but also addressed
some technical issues in the analysis. Our experience with a host of
general and special software systems will facilitate investigation
into more complex models for both human and nonhuman genetics
QCD Matter Thermalization at RHIC and LHC
Employing the perturbative QCD inspired parton cascade, we investigate
kinetic and chemical equilibration of the partonic matter created in central
heavy ion collisions at RHIC and LHC energies. Two types of initial conditions
are chosen. One is generated by the model of wounded nucleons using the PYTHIA
event generator and Glauber geometry. Another is considered as a color glass
condensate. We show that kinetic equilibration is almost independent on the
chosen initial conditions, whereas there is a sensitive dependence for chemical
equilibration. The time scale of thermalization lies between 1 and 1.5 fm/c.
The final parton transverse energy obtained from BAMPS calculations is compared
with the RHIC data and is estimated for the LHC energy.Comment: 8 pages, 10 figures, plenary talk at International Conference on
Strangeness in Quark Matter 2008, Beijing, China, October 6-10, 200
Effects of Dehydration on Freezing Characteristics and Survival in Liquid Nitrogen of Three Recalcitrant Seeds
The recalcitrant seeds rambutan( Nephelium lappaceum). durian (Durio zibethinus) and cempedak
(Artocarpus inleger) have a high critical moisture content (below which ·rapid loss of viability occurs of
27.0%, 26.0% and 37.9%,respectively. The critical moisture for embroys were higher at 39.0%
for rambutan, 53.9% for durian and 43.2% for Cempedak. Differential Thermal analysis of the embroyos
confirmed that their threshhold moistures (below which there is no freezable water) were lower than their critical
moistureS. The Threshhold moistures for rambutan, durian and cempedak embryos were approximately 30%,
32% and 33% respectively. It is suggested that unsuccessful attempts at cryopreservation of embroyos of
'recalcitrant seeds in the past maybe due to the absence of safe window between the high critical moisture
content and the threshhold moisture. This results in freezing injury at the higher moistures and dehydration
injury' at the lower moistures. Potential techniques to overcome this and improve cryopreservation of recalcitrant
seed embryos are discussed
Fluid Flows of Mixed Regimes in Porous Media
In porous media, there are three known regimes of fluid flows, namely,
pre-Darcy, Darcy and post-Darcy. Because of their different natures, these are
usually treated separately in literature. To study complex flows when all three
regimes may be present in different portions of a same domain, we use a single
equation of motion to unify them. Several scenarios and models are then
considered for slightly compressible fluids. A nonlinear parabolic equation for
the pressure is derived, which is degenerate when the pressure gradient is
either small or large. We estimate the pressure and its gradient for all time
in terms of initial and boundary data. We also obtain their particular bounds
for large time which depend on the asymptotic behavior of the boundary data but
not on the initial one. Moreover, the continuous dependence of the solutions on
initial and boundary data, and the structural stability for the equation are
established.Comment: 33 page
Nonlocal Phases of Local Quantum Mechanical Wavefunctions in Static and Time-Dependent Aharonov-Bohm Experiments
We show that the standard Dirac phase factor is not the only solution of the
gauge transformation equations. The full form of a general gauge function (that
connects systems that move in different sets of scalar and vector potentials),
apart from Dirac phases also contains terms of classical fields that act
nonlocally (in spacetime) on the local solutions of the time-dependent
Schr\"odinger equation: the phases of wavefunctions in the Schr\"odinger
picture are affected nonlocally by spatially and temporally remote magnetic and
electric fields, in ways that are fully explored. These contributions go beyond
the usual Aharonov-Bohm effects (magnetic or electric). (i) Application to
cases of particles passing through static magnetic or electric fields leads to
cancellations of Aharonov-Bohm phases at the observation point; these are
linked to behaviors at the semiclassical level (to the old Werner & Brill
experimental observations, or their "electric analogs" - or to recent reports
of Batelaan & Tonomura) but are shown to be far more general (true not only for
narrow wavepackets but also for completely delocalized quantum states). By
using these cancellations, certain previously unnoticed sign-errors in the
literature are corrected. (ii) Application to time-dependent situations
provides a remedy for erroneous results in the literature (on improper uses of
Dirac phase factors) and leads to phases that contain an Aharonov-Bohm part and
a field-nonlocal part: their competition is shown to recover Relativistic
Causality in earlier "paradoxes" (such as the van Kampen thought-experiment),
while a more general consideration indicates that the temporal nonlocalities
found here demonstrate in part a causal propagation of phases of quantum
mechanical wavefunctions in the Schr\"odinger picture. This may open a direct
way to address time-dependent double-slit experiments and the associated causal
issuesComment: 49 pages, 1 figure, presented in Conferences "50 years of the
Aharonov-Bohm effect and 25 years of the Berry's phase" (Tel Aviv and
Bristol), published in Journ. Phys. A. Compared to the published paper, this
version has 17 additional lines after eqn.(14) for maximum clarity, and the
Abstract has been slightly modified and reduced from the published 2035
characters to the required 1920 character
Fire and flooding resilience in Mimosa L. of the Brazilian Pantanal
PosterThe Pantanal ecoregion, in south-central Brazil, is one of the largest foodplains in the world. Cycles of fooding and periodic fre have been determinant to the diversity of species in the ecoregion. In the fora of Pantanal, Leguminosae is one of the most rich families, and Mimosa is its most diversifed genus. Species of this genus are widely distributed and occur in the capões, sandy ridges and monominant stands (e.g. Cambarazal, Carandazal and Paratudal), in addition to native grasslands and pasture areas. In the Cerrado ecoregion, adaptations to fre have been recognized in some species of Mimosa. However, there is no data in the Pantanal ecoregion about which species of Mimosa are resilient to fre and/or fooding. This study aimed to investigate morphological adaptation in Mimosa in terms of resilience to fre and fooding. We verifed 45 specifc and infraspecifc taxa of Mimosa occurring in the Brazilian Pantanal, by means of the revision of literature, herbarium data, personal observations, plant collections and feld notesInstituto de Recursos BiológicosFil: Nogueira, Luan. Universidade Federal de Mato Grosso do Sul, BrasilFil: Morales, Matias. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Recursos Biológicos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Morón. Facultad de Agronomía y Ciencias Agroalimentarias; ArgentinaFil: Sartori, Angela Lucia. Universidade Federal de Mato Grosso do Sul, Brasi
Contact and Friction of Nano-Asperities: Effects of Adsorbed Monolayers
Molecular dynamics simulations are used to study contact between a rigid,
nonadhesive, spherical tip with radius of order 30nm and a flat elastic
substrate covered with a fluid monolayer of adsorbed chain molecules. Previous
studies of bare surfaces showed that the atomic scale deviations from a sphere
that are present on any tip constructed from discrete atoms lead to significant
deviations from continuum theory and dramatic variability in friction forces.
Introducing an adsorbed monolayer leads to larger deviations from continuum
theory, but decreases the variations between tips with different atomic
structure. Although the film is fluid, it remains in the contact and behaves
qualitatively like a thin elastic coating except for certain tips at high
loads. Measures of the contact area based on the moments or outer limits of the
pressure distribution and on counting contacting atoms are compared. The number
of tip atoms making contact in a time interval grows as a power of the interval
when the film is present and logarithmically with the interval for bare
surfaces. Friction is measured by displacing the tip at a constant velocity or
pulling the tip with a spring. Both static and kinetic friction rise linearly
with load at small loads. Transitions in the state of the film lead to
nonlinear behavior at large loads. The friction is less clearly correlated with
contact area than load.Comment: RevTex4, 17 pages, 13 figure
- …