7,824 research outputs found
Slip boundary conditions for shear flow of polymer melts past atomically flat surfaces
Molecular dynamics simulations are carried out to investigate the dynamic
behavior of the slip length in thin polymer films confined between atomically
smooth thermal surfaces. For weak wall-fluid interactions, the shear rate
dependence of the slip length acquires a distinct local minimum followed by a
rapid growth at higher shear rates. With increasing fluid density, the position
of the local minimum is shifted to lower shear rates. We found that the ratio
of the shear viscosity to the slip length, which defines the friction
coefficient at the liquid/solid interface, undergoes a transition from a nearly
constant value to the power law decay as a function of the slip velocity. In a
wide range of shear rates and fluid densities, the friction coefficient is
determined by the product of the value of surface induced peak in the structure
factor and the contact density of the first fluid layer near the solid wall.Comment: 27 pages, 11 figure
The supernova remnant CTB 37B and its associated magnetar CXOU J171405.7-381031: evidence for a magnetar-driven remnant
We discuss in this Letter the association of the candidate magnetar CXOU
J171405.7-381031 with the supernova remnant CTB 37B. The recent detection of
the period derivative of the object allowed an estimation of a young
characteristic age of only . This value is too small to be
compatible even with the minimum radius of the remnant , the value
corresponding to the {\it lower} limit of the estimated distance of , unless the true distance happens to be even smaller than the lower
limit. We argue that a consistent scenario for the remnant origin, in which the
latter is powered by the energy injected by a young magnetar, is indeed more
accurate to explain the young age, and points out to its non-standard (i.e.
magnetar-driven) nature.Comment: 6 pp., 1 figure, to appear in RAA Letter
Molecular Dynamics Simulation of Compressible Fluid Flow in Two-Dimensional Channels
We study compressible fluid flow in narrow two-dimensional channels using a
novel molecular dynamics simulation method. In the simulation area, an upstream
source is maintained at constant density and temperature while a downstream
reservoir is kept at vacuum. The channel is sufficiently long in the direction
of the flow that the finite length has little effect on the properties of the
fluid in the central region. The simulated system is represented by an
efficient data structure, whose internal elements are created and manipulated
dynamically in a layered fashion. Consequently the code is highly efficient and
manifests completely linear performance in simulations of large systems. We
obtain the steady-state velocity, temperature, and density distributions in the
system. The velocity distribution across the channel is very nearly a quadratic
function of the distance from the center of the channel and reveals velocity
slip at the boundaries; the temperature distribution is only approximately a
quartic function of this distance from the center to the channel. The density
distribution across the channel is non-uniform. We attribute this
non-uniformity to the relatively high Mach number, approximately 0.5, in the
fluid flow. An equation for the density distribution based on simple
compressibility arguments is proposed; its predictions agree well with the
simulation results. Validity of the concept of local dynamic temperature and
the variation of the temperature along the channel are discussed.Comment: 16 pages (in latex) + 8 figures (in a single ps file). Submitted to
the Physical Review
Anomalous density dependence of static friction in sand
We measured experimentally the static friction force on the surface of
a glass rod immersed in dry sand. We observed that is extremely sensitive
to the closeness of packing of grains. A linear increase of the grain-density
yields to an exponentially increasing friction force. We also report on a novel
periodicity of during gradual pulling out of the rod. Our observations
demonstrate the central role of grain bridges and arches in the macroscopic
properties of granular packings.Comment: plain tex, 6 pages, to appear in Phys.Rev.
Inter-species variation in colour perception
Inter-species variation in colour perception poses a serious problem for the view that colours are mind-independent properties. Given that colour perception varies so drastically across species, which species perceives colours as they really are? In this paper, I argue that all do. Specifically, I argue that members of different species perceive properties that are determinates of different, mutually compatible, determinables. This is an instance of a general selectionist strategy for dealing with cases of perceptual variation. According to selectionist views, objects simultaneously instantiate a plurality of colours, all of them genuinely mind-independent, and subjects select from amongst this plurality which colours they perceive. I contrast selectionist views with relationalist views that deny the mind-independence of colour, and consider some general objections to this strategy
SCUBA and Spitzer observations of the Taurus molecular cloud - pulling the bull's tail
We present continuum data from the Submillimetre Common-User Bolometer Array
(SCUBA) on the James Clerk Maxwell Telescope (JCMT), and the Mid-Infrared
Photometer for Spitzer (MIPS) on the Spitzer Space Telescope, at submillimetre
and infrared wavelengths respectively. We study the Taurus molecular cloud 1
(TMC1), and in particular the region of the Taurus Molecular Ring (TMR). In the
continuum data we see no real evidence for a ring, but rather we see one side
of it only, appearing as a filament. We name the filament `the bull's tail'.
The filament is seen in emission at 850, 450 and 160um, and in absorption at
70um. We compare the data with archive data from the Infra-Red Astronomical
Satellite (IRAS) at 12, 25, 60, 100um, in which the filament is also seen in
absorption. We find that the emission from the filament consists of two
components: a narrow, cold (~8K), central core; and a broader, slightly warmer
(~12K), shoulder of emission. We use a radiative transfer code to model the
filament's appearance, either in emission or absorption, simultaneously at each
of the different wavelengths. Our best fit model uses a Plummer-like density
profile and a homogeneous interstellar dust grain population. Unlike previous
work on a similar, but different filament in Taurus, we require no grain
coagulation to explain our data.Comment: 10 pages, 9 Figures, Accepted by MNRA
Slip behavior in liquid films on surfaces of patterned wettability: Comparison between continuum and molecular dynamics simulations
We investigate the behavior of the slip length in Newtonian liquids subject
to planar shear bounded by substrates with mixed boundary conditions. The upper
wall, consisting of a homogenous surface of finite or vanishing slip, moves at
a constant speed parallel to a lower stationary wall, whose surface is
patterned with an array of stripes representing alternating regions of no-shear
and finite or no-slip. Velocity fields and effective slip lengths are computed
both from molecular dynamics (MD) simulations and solution of the Stokes
equation for flow configurations either parallel or perpendicular to the
stripes. Excellent agreement between the hydrodynamic and MD results is
obtained when the normalized width of the slip regions, , where is the (fluid) molecular diameter characterizing the
Lennard-Jones interaction. In this regime, the effective slip length increases
monotonically with to a saturation value. For and transverse flow configurations, the non-uniform interaction
potential at the lower wall constitutes a rough surface whose molecular scale
corrugations strongly reduce the effective slip length below the hydrodynamic
results. The translational symmetry for longitudinal flow eliminates the
influence of molecular scale roughness; however, the reduced molecular ordering
above the wetting regions of finite slip for small values of
increases the value of the effective slip length far above the hydrodynamic
predictions. The strong inverse correlation between the effective slip length
and the liquid structure factor representative of the first fluid layer near
the patterned wall illustrates the influence of molecular ordering effects on
slip in non-inertial flows.Comment: 12 pages, 10 figures Web reference added for animations:
http://www.egr.msu.edu/~priezjev/bubble/bubble.htm
The physical and psychological health benefits of positive emotional writing:Investigating the moderating role of Type D (distressed) personality
Objectives
Type D personality is associated with psychological and physical illâhealth. However, there has been limited investigation of the role of Type D personality in interventions designed to enhance wellâbeing. This study investigated associations between Type D personality and the efficacy of positive emotional writing for reducing stress, anxiety, and physical symptoms.
Design
A betweenâsubjects longitudinal design was employed.
Method
Participants (N = 71, Mage = 28.2, SDage = 12.4) completed selfâreport measures of Type D personality, physical symptoms, perceived stress, and trait anxiety, before completing either (1) positive emotional writing or (2) a nonâemotive control writing task, for 20 min per day over three consecutive days. State anxiety was measured immediately before and after each writing session, and selfâreport questionnaires were again administered 4 weeks postâwriting.
Results
Participants in the positive emotional writing condition showed significantly greater reductions in (1) state anxiety and (2) both trait anxiety and perceived stress over the 4âweek followâup period, compared to the control group. While these effects were not moderated by Type D personality, a decrease in trait anxiety was particularly evident in participants who reported both high levels of social inhibition and low negative affectivity. Linguistic analysis of the writing diaries showed that Type D personality was positively associated with swear word use, but not any other linguistic categories.
Conclusion
These findings support the efficacy of positive emotional writing for alleviating stress and anxiety, but not perceived physical symptoms. Swearing may be a coping strategy employed by high Type D individuals
Boundary conditions at a fluid - solid interface
We study the boundary conditions at a fluid-solid interface using molecular
dynamics simulations covering a broad range of fluid-solid interactions and
fluid densities, and both simple and chain-molecule fluids. The slip length is
shown to be independent of the type of flow, but rather is related to the fluid
organization near the solid, as governed by the fluid-solid molecular
interactions.Comment: REVtex, to appear in Physical Review Letter
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