52 research outputs found
Transport and Helfand moments in the Lennard-Jones fluid. II. Thermal Conductivity
The thermal conductivity is calculated with the Helfand-moment method in the
Lennard-Jones fluid near the triple point. The Helfand moment of thermal
conductivity is here derived for molecular dynamics with periodic boundary
conditions. Thermal conductivity is given by a generalized Einstein relation
with this Helfand moment. We compute thermal conductivity by this new method
and compare it with our own values obtained by the standard Green-Kubo method.
The agreement is excellent.Comment: Submitted to the Journal of Chemical Physic
Transport and Helfand moments in the Lennard-Jones fluid. I. Shear viscosity
We propose a new method, the Helfand-moment method, to compute the shear
viscosity by equilibrium molecular dynamics in periodic systems. In this
method, the shear viscosity is written as an Einstein-like relation in terms of
the variance of the so-called Helfand moment. This quantity, is modified in
order to satisfy systems with periodic boundary conditions usually considered
in molecular dynamics. We calculate the shear viscosity in the Lennard-Jones
fluid near the triple point thanks to this new technique. We show that the
results of the Helfand-moment method are in excellent agreement with the
results of the standard Green-Kubo method.Comment: Submitted to the Journal of Chemical Physic
Methods of calculation of a friction coefficient: Application to the nanotubes
In this work we develop theoretical and numerical methods of calculation of a
dynamic friction coefficient. The theoretical method is based on an adiabatic
approximation which allows us to express the dynamic friction coefficient in
terms of the time integral of the autocorrelation function of the force between
both sliding objects. The motion of the objects and the autocorrelation
function can be numerically calculated by molecular-dynamics simulations. We
have successfully applied these methods to the evaluation of the dynamic
friction coefficient of the relative motion of two concentric carbon nanotubes.
The dynamic friction coefficient is shown to increase with the temperature.Comment: 4 pages, 6 figure
Rotational dynamics and friction in double-walled carbon nanotubes
We report a study of the rotational dynamics in double-walled nanotubes using
molecular dynamics simulations and a simple analytical model reproducing very
well the observations. We show that the dynamic friction is linear in the
angular velocity for a wide range of values. The molecular dynamics simulations
show that for large enough systems the relaxation time takes a constant value
depending only on the interlayer spacing and temperature. Moreover, the
friction force increases linearly with contact area, and the relaxation time
decreases with the temperature with a power law of exponent .Comment: submitted to PR
Temperature dependence of the slip length in polymer melts at attractive surfaces
Using Couette and Poiseuille flow, we extract the temperature dependence of
the slip length, , from molecular dynamics simulations of a
coarse-grained polymer model in contact with an attractive, corrugated surface.
is dictated by the ratio of bulk viscosity and surface mobility. At
weakly attractive surfaces, a lubrication layer forms, is large and
increases upon cooling. Close to the glass transition temperature, , very
large slip lengths are observed. At a more attractive surface, a``stick y
surface layer" is build up, which gives rise to a small slip length. Upon cool
ing, decreases at high temperatures, passes through a minimum and
grows upon approaching . At strongly attractive surfaces, the Navier-slip
condit ion fails to describe Couette and Poiseuille flow simultaneously. The
simulation results are corroborated by a schematic, two-layer model suggesting
that the ob servations do not depend on the details of the computational model.Comment: submitted to Phys. Rev. Let
Statics and dynamics of a cylindrical droplet under an external body force
We study the rolling and sliding motion of droplets on a corrugated substrate
by Molecular Dynamics simulations. Droplets are driven by an external body
force (gravity) and we investigate the velocity profile and dissipation
mechanisms in the steady state. The cylindrical geometry allows us to consider
a large range of droplet sizes. The velocity of small droplets with a large
contact angle is dominated by the friction at the substrate and the velocity of
the center of mass scales like the square root of the droplet size. For large
droplets or small contact angles, however, viscous dissipation of the flow
inside the volume of the droplet dictates the center of mass velocity that
scales linearly with the size. We derive a simple analytical description
predicting the dependence of the center of mass velocity on droplet size and
the slip length at the substrate. In the limit of vanishing droplet velocity we
quantitatively compare our simulation results to the predictions and good
agreement without adjustable parameters is found.Comment: Submitted to the Journal of Chemical Physic
Depth dependent dynamics in the hydration shell of a protein
We study the dynamics of hydration water/protein association in folded
proteins, using lysozyme and myoglobin as examples. Extensive molecular
dynamics simulations are performed to identify underlying mechanisms of the
dynamical transition that corresponds to the onset of amplified atomic
fluctuations in proteins. The number of water molecules within a cutoff
distance of each residue scales linearly with protein depth index and is not
affected by the local dynamics of the backbone. Keeping track of the water
molecules within the cutoff sphere, we observe an effective residence time,
scaling inversely with depth index at physiological temperatures while the
diffusive escape is highly reduced below the transition. A depth independent
orientational memory loss is obtained for the average dipole vector of the
water molecules within the sphere when the protein is functional. While below
the transition temperature, the solvent is in a glassy state, acting as a solid
crust around the protein, inhibiting any large scale conformational
fluctuations. At the transition, most of the hydration shell unfreezes and
water molecules collectively make the protein more flexible.Comment: Journal of Chemical Physics in pres
Molecular transport and flow past hard and soft surfaces: Computer simulation of model systems
The properties of polymer liquids on hard and soft substrates are
investigated by molecular dynamics simulation of a coarse-grained bead-spring
model and dynamic single-chain-in-mean-field (SCMF) simulations of a soft,
coarse-grained polymer model. Hard, corrugated substrates are modelled by an
FCC Lennard-Jones solid while polymer brushes are investigated as a
prototypical example of a soft, deformable surface. From the molecular
simulation we extract the coarse-grained parameters that characterise the
equilibrium and flow properties of the liquid in contact with the substrate:
the surface and interface tensions, and the parameters of the hydrodynamic
boundary condition. The so-determined parameters enter a continuum description
like the Stokes equation or the lubrication approximation.Comment: 41 pages, 13 figure
Hindered rolling and friction anisotropy in supported carbon nanotubes
Carbon nanotubes (CNTs) are well known for their exceptional thermal,
mechanical and electrical properties. For many CNT applications it is of the
foremost importance to know their frictional properties. However, very little
is known about the frictional forces between an individual nanotube and a
substrate or tip. Here, we present a combined theoretical and experimental
study of the frictional forces encountered by a nanosize tip sliding on top of
a supported multiwall CNT along a direction parallel or transverse to the CNT
axis. Surprisingly, we find a higher friction coefficient in the transverse
direction compared with the parallel direction. This behaviour is explained by
a simulation showing that transverse friction elicits a soft 'hindered rolling'
of the tube and a frictional dissipation that is absent, or partially absent
for chiral CNTs, when the tip slides parallel to the CNT axis. Our findings can
help in developing better strategies for large-scale CNT assembling and sorting
on a surface.Comment: 8 pages, 5 figure
Using the nonlinear control of anaesthesia-induced hypersensitivity of EEG at burst suppression level to test the effects of radiofrequency radiation on brain function
Background
In this study, investigating the effects of mobile phone radiation on test animals, eleven pigs were anaesthetised to the level where burst-suppression pattern appears in the electroencephalogram (EEG). At this level of anaesthesia both human subjects and animals show high sensitivity to external stimuli which produce EEG bursts during suppression. The burst-suppression phenomenon represents a nonlinear control system, where low-amplitude EEG abruptly switches to very high amplitude bursts. This switching can be triggered by very minor stimuli and the phenomenon has been described as hypersensitivity. To test if also radio frequency (RF) stimulation can trigger this nonlinear control, the animals were exposed to pulse modulated signal of a GSM mobile phone at 890 MHz. In the first phase of the experiment electromagnetic field (EMF) stimulation was randomly switched on and off and the relation between EEG bursts and EMF stimulation onsets and endpoints were studied. In the second phase a continuous RF stimulation at 31 W/kg was applied for 10 minutes. The ECG, the EEG, and the subcutaneous temperature were recorded.
Results
No correlation between the exposure and the EEG burst occurrences was observed in phase I measurements. No significant changes were observed in the EEG activity of the pigs during phase II measurements although several EEG signal analysis methods were applied. The temperature measured subcutaneously from the pigs' head increased by 1.6°C and the heart rate by 14.2 bpm on the average during the 10 min exposure periods.
Conclusion
The hypothesis that RF radiation would produce sensory stimulation of somatosensory, auditory or visual system or directly affect the brain so as to produce EEG bursts during suppression was not confirmed.BioMed Central Open acces
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