87 research outputs found
A multiscale Molecular Dynamics approach to Contact Mechanics
The friction and adhesion between elastic bodies are strongly influenced by
the roughness of the surfaces in contact. Here we develop a multiscale
molecular dynamics approach to contact mechanics, which can be used also when
the surfaces have roughness on many different length-scales, e.g., for self
affine fractal surfaces. As an illustration we consider the contact between
randomly rough surfaces, and show that the contact area varies linearly with
the load for small load. We also analyze the contact morphology and the
pressure distribution at different magnification, both with and without
adhesion. The calculations are compared with analytical contact mechanics
models based on continuum mechanics.Comment: Format Revtex4, two columns, 13 pages, 19 pictures. Submitted for
publication in the European Physical Journal E. Third revision with minimal
changes: Corrected a few mistypin
Nanodroplets on rough hydrophilic and hydrophobic surfaces
We present results of Molecular Dynamics (MD) calculations on the behavior of
liquid nanodroplets on rough hydrophobic and hydrophilic solid surfaces. On
hydrophobic surfaces, the contact angle for nanodroplets depends strongly on
the root mean square roughness amplitude, but it is nearly independent of the
fractal dimension of the surface. Since increasing the fractal dimension
increases the short-wavelength roughness, while the long-wavelength roughness
is almost unchanged, we conclude that for hydrophobic interactions the
short-wavelength (atomistic) roughness is not very important. We show that the
nanodroplet is in a Cassie-like state. For rough hydrophobic surfaces, there is
no contact angle hysteresis due to strong thermal fluctuations, which occur at
the liquid-solid interface on the nanoscale. On hydrophilic surfaces, however,
there is strong contact angle hysteresis due to higher energy barrier. These
findings may be very important for the development of artificially biomimetic
superhydrophobic surfaces.Comment: 15 pages, 25 figures. Minimal changes with respect to the previous
one. A few small improvements, references updated, added the reference to the
published paper. Previous work on the same subject: arXiv:cond-mat/060405
Electronic friction and liquid-flow-induced voltage in nanotubes
A recent exciting experiment by Ghosh et al. reported that the flow of an
ion-containing liquid such as water through bundles of single-walled carbon
nanotubes induces a voltage in the nanotubes that grows logarithmically with
the flow velocity v0. We propose an explanation for this observation. Assuming
that the liquid molecules nearest the nanotube form a 2D solid-like monolayer
pinned through the adsorbed ions to the nanotubes, the monolayer sliding will
occur by elastic loading followed by local yield (stick-slip). The drifting
adsorbed ions produce a voltage in the nanotube through electronic friction
against free electrons inside the nanotube. Thermally excited jumps over
force-biased barriers, well-known in stick-slip, can explain the logarithmic
voltage growth with flow velocity. We estimate the short circuit current and
the internal resistance of the nanotube voltage generator.Comment: 8 pages, 3 figures; published on PRB
(http://link.aps.org/abstract/PRB/v69/e235410) and on the Virtual Journal of
Nanoscale Science and Technology (http://www.vjnano.org, July 14, 2002, Vol.
10, Iss. 2
Sealing is at the Origin of Rubber Slipping on Wet Roads
Loss of braking power and rubber skidding on a wet road is still an open
physics problem, since neither the hydrodynamical effects nor the loss of
surface adhesion that are sometimes blamed really manage to explain the 20-30%
observed loss of low speed tire-road friction. Here we advance a novel
mechanism based on sealing of water-filled substrate pools by the rubber. The
sealed-in water effectively smoothens the substrate, thus reducing the
viscoelastic dissipation in bulk rubber induced by surface asperities, well
established as a major friction contribution. Starting with the measured
spectrum of asperities one can calculate the water-smoothened spectrum and from
that the predicted friction reduction, which is of the right magnitude. The
theory is directly supported by fresh tire-asphalt friction data.Comment: 5 pages, 4 figures. Published on Nature Materials (November 7th 2004
Impact of molecular structure on the lubricant squeeze-out between curved surfaces with long range elasticity
The properties of butane (C4H10) lubricants confined between two approaching
solids are investigated by a model that accounts for the curvature and elastic
properties of the solid surfaces. We consider the linear n-butane and the
branched iso-butane. For the linear molecule, well defined molecular layers
develop in the lubricant film when the width is of the order of a few atomic
diameters. The branched iso-butane forms more disordered structures which
permit it to stay liquid-like at smaller surface separations. During squeezing
the solvation forces show oscillations corresponding to the width of a
molecule. At low speeds (< 0.1 m/s) the last layers of iso-butane are squeezed
out before those of n-butane. Since the (interfacial) squeezing velocity in
most practical applications is very low when the lubricant layer has molecular
thickness, one expects n-butane to be a better boundary lubricant than
iso-butane. N-butane possessing a slightly lower viscosity at high pressures,
our result refutes the view that squeeze out should be harder for higher
viscosities, on the other hand our results are consistent with wear experiments
in which n-butane were shown to protect steel surfaces better than iso-butane.Comment: 7 pages, 10 figures, format revtex. Submitted to J. Chem. Phy
Influence of surface roughness on superhydrophobicity
Superhydrophobic surfaces, with liquid contact angle theta greater than 150
degree, have important practical applications ranging from self-cleaning window
glasses, paints, and fabrics to low-friction surfaces. Many biological
surfaces, such as the lotus leaf, have hierarchically structured surface
roughness which is optimized for superhydrophobicity through natural selection.
Here we present a molecular dynamics study of liquid droplets in contact with
self-affine fractal surfaces. Our results indicate that the contact angle for
nanodroplets depends strongly on the root-mean-square surface roughness
amplitude but is nearly independent of the fractal dimension D_f of the
surface.Comment: 5 Pages, 6 figures. Minimal changes with respect to the previous
versio
How do liquids confined at the nanoscale influence adhesion?
Liquids play an important role in adhesion and sliding friction. They behave
as lubricants in human bodies especially in the joints. However, in many
biological attachment systems they acts like adhesives, e.g. facilitating
insects to move on ceilings or vertical walls. Here we use molecular dynamics
to study how liquids confined at the nanoscale influence the adhesion between
solid bodies with smooth and rough surfaces. We show that a monolayer of liquid
may strongly affect the adhesion.Comment: 5 pages, 9 color figures. Some figures are in Postscript Level 3
format. Minimal changes with respect to the previous version. Added doi and
reference to the published article also inside the pape
Determinants of inappropriate acute pain management in old people unable to communicate verbally in the emergency department
Objectives: Poor pain management is relevant among individuals unable to communicate verbally (UCV). Analgesia may be due to three determinants: patients' status, physician's characteristics and pain etiology. Our aim is to investigate the association between prescription of ED pain treatment and these determinants. Materials and Methods: An observational prospective study including UCV patients was conducted. Severity of pain was evaluated by ALGOPLUS Scale and a score P â¥Â 2 out of 5 on the pain scale was retained as the threshold for the presence of acute pain in elderly UCV patients. Results: Our data showed that only 31,9% of UCV patients received a pharmacological treatment. The presence of the caregiver would influence the rate of therapy administration [OR 6,19 (95% CI 2,6â14,75)]. The presence of leg pain [OR 0,32 (95% CI 0,12â0,86)] and head pain [OR 0,29 (95% CI 0,10â0,84)] were less likely associated to receive analgesia. Pain related to trauma [OR 4.82 (95% CI 1.17 to 19.78)] and youngest physicians [OR 1.08 (95% CI 1.001 to 1.18)] were variables associated with the administration of drugs opiates. Discussion: Older UCV patients presenting to the ED with pain are at high risk of inadequate analgesia. Providers should always suspect presence of pain and an increasing need for behavioural pain evaluation is necessary for a complete assessment. Conclusions: Presence of a caregiver influences a more appropriate pain management in these patients. Staff training on pain management could result in better assessment, treatment, and interaction with caregivers. Keywords: Emergency department, Pain, Oligoanalgesi
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