2,858 research outputs found
Efficient multiple time scale molecular dynamics: using colored noise thermostats to stabilize resonances
Multiple time scale molecular dynamics enhances computational efficiency by
updating slow motions less frequently than fast motions. However, in practice
the largest outer time step possible is limited not by the physical forces but
by resonances between the fast and slow modes. In this paper we show that this
problem can be alleviated by using a simple colored noise thermostatting scheme
which selectively targets the high frequency modes in the system. For two
sample problems, flexible water and solvated alanine dipeptide, we demonstrate
that this allows the use of large outer time steps while still obtaining
accurate sampling and minimizing the perturbation of the dynamics. Furthermore,
this approach is shown to be comparable to constraining fast motions, thus
providing an alternative to molecular dynamics with constraints.Comment: accepted for publication by the Journal of Chemical Physic
Magnetic domain fluctuations in an antiferromagnetic film observed with coherent resonant soft x-ray scattering
We report the direct observation of slow fluctuations of helical
antiferromagnetic domains in an ultra-thin holmium film using coherent resonant
magnetic x-ray scattering. We observe a gradual increase of the fluctuations in
the speckle pattern with increasing temperature, while at the same time a
static contribution to the speckle pattern remains. This finding indicates that
domain-wall fluctuations occur over a large range of time scales. We ascribe
this non-ergodic behavior to the strong dependence of the fluctuation rate on
the local thickness of the film.Comment: to appear in Phys. Rev. Let
Multiangle static and dynamic light scattering in the intermediate scattering angle range
We describe a light scattering apparatus based on a novel optical scheme
covering the scattering angle range 0.5\dg \le \theta \le 25\dg, an
intermediate regime at the frontier between wide angle and small angle setups
that is difficult to access by existing instruments. Our apparatus uses
standard, readily available optomechanical components. Thanks to the use of a
charge-coupled device detector, both static and dynamic light scattering can be
performed simultaneously at several scattering angles. We demonstrate the
capabilities of our apparatus by measuring the scattering profile of a variety
of samples and the Brownian dynamics of a dilute colloidal suspension
Speckle visibility spectroscopy and variable granular fluidization
We introduce a dynamic light scattering technique capable of resolving motion
that changes systematically, and rapidly, with time. It is based on the
visibility of a speckle pattern for a given exposure duration. Applying this to
a vibrated layer of glass beads, we measure the granular temperature and its
variation with phase in the oscillation cycle. We observe several transitions
involving jammed states, where the grains are at rest during some portion of
the cycle. We also observe a two-step decay of the temperature on approach to
jamming.Comment: 4 pages, 4 figures, experimen
Aneurisma na artéria aorta em caprinos.
Resumo: De 726 caprinos necropsiados, dois (0,28%) apresentaram aneurisma na arteria aorta, sendo um macho com idade de 48 meses e uma femea com cerca de 30 meses. Os animais eram oriundos de um rebanho mantido em regime semi-extensivo com pastejo em caatinga nativa e pernoite em aprisco. Macroscopicamente observou-se palidez das mucosas externas e das visceras. Nas cavidades toracica e abdominal estavam presentes grandes coagulos resultantes da ruptura das partes afetadas das arterias. As lesoes das arterias aorta toracica e abdominal foram carcterizadas pela dilatacao das paredes formando estruturas saculares com presenca de inumeros coagulos e tecidos necroticos de aproximadamente 10cm a 20cm de comprimento. Histologicamente, observou-se a destruicao do tecido endotelial, necrose e infiltracao de leucocitos das camadas intima, media e muscular da arteria. Acredita-se que a causa primaria dos aneurismas detectados esteja relacionado com os processos inflamatorios presentes em outros orgaos. [Aneurism in the aorta artery in goats]. Abstract: Two (0.28%) of 726 necropsyed goats presented aneurism In the aorta artery, one being a male with 48 months and the other, a female about 30 months of age. The animais carne from a herd in semi-extensive regime, grazing in caatinga vegetation and spending the nights in appropriate pan. Paleness in external mucosa and viscera was observed macroscopically. Large coagula were presented in the thoracic and abdominal cavities, dueto ruptura ol the aneurismatic parts of the blood vessels. The lesions of thoracic and abdominal aortas were characterized by wali dilations forming saccular struc-tures, with presenco of many coagula and necrotic tissues of about 10cm to 20cm of length. Histo-logically, a destruetion In the endotelial tissues, nacrosis and infiltration of leucocytes in the intimate, median, and muscular Iayers were observed. It is believed that the primary cause of the aneurisms detected in this study would be related to teh inflamatory processes of other organs
Monitoramento por OPG e cultura de fezes de ovinos de uma propriedade rural na região de Bagé (2007-2009).
bitstream/item/55798/1/CT38.pd
Dynamic correlations in stochastic rotation dynamics
The dynamic structure factor, vorticity and entropy density dynamic
correlation functions are measured for Stochastic Rotation Dynamics (SRD), a
particle based algorithm for fluctuating fluids. This allows us to obtain
unbiased values for the longitudinal transport coefficients such as thermal
diffusivity and bulk viscosity. The results are in good agreement with earlier
numerical and theoretical results, and it is shown for the first time that the
bulk viscosity is indeed zero for this algorithm. In addition, corrections to
the self-diffusion coefficient and shear viscosity arising from the breakdown
of the molecular chaos approximation at small mean free paths are analyzed. In
addition to deriving the form of the leading correlation corrections to these
transport coefficients, the probabilities that two and three particles remain
collision partners for consecutive time steps are derived analytically in the
limit of small mean free path. The results of this paper verify that we have an
excellent understanding of the SRD algorithm at the kinetic level and that
analytic expressions for the transport coefficients derived elsewhere do indeed
provide a very accurate description of the SRD fluid.Comment: 33 pages including 16 figure
Length scale dependence of dynamical heterogeneity in a colloidal fractal gel
We use time-resolved dynamic light scattering to investigate the slow
dynamics of a colloidal gel. The final decay of the average intensity
autocorrelation function is well described by , with and
decreasing from 1.5 to 1 with increasing . We show that the dynamics is not
due to a continuous ballistic process, as proposed in previous works, but
rather to rare, intermittent rearrangements. We quantify the dynamical
fluctuations resulting from intermittency by means of the variance
of the instantaneous autocorrelation function, the analogous of
the dynamical susceptibility studied in glass formers. The amplitude
of is found to grow linearly with . We propose a simple --yet
general-- model of intermittent dynamics that accounts for the dependence
of both the average correlation functions and .Comment: Revised and improved, to appear in Europhys. Let
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