85 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
Physics of Solid and Liquid Alkali Halide Surfaces Near the Melting Point
This paper presents a broad theoretical and simulation study of the high
temperature behavior of crystalline alkali halide surfaces typified by
NaCl(100), of the liquid NaCl surface near freezing, and of the very unusual
partial wetting of the solid surface by the melt. Simulations are conducted
using two-body rigid ion BMHFT potentials, with full treatment of long-range
Coulomb forces. After a preliminary check of the description of bulk NaCl
provided by these potentials, which seems generally good even at the melting
point, we carry out a new investigation of solid and liquid surfaces. Solid
NaCl(100) is found in this model to be very anharmonic and yet exceptionally
stable when hot. It is predicted by a thermodynamic integration calculation of
the surface free energy that NaCl(100) should be a well ordered, non-melting
surface, metastable even well above the melting point. By contrast, the
simulated liquid NaCl surface is found to exhibit large thermal fluctuations
and no layering order. In spite of that, it is shown to possess a relatively
large surface free energy. The latter is traced to a surface entropy deficit,
reflecting some kind of surface short range order. Finally, the solid-liquid
interface free energy is derived through Young's equation from direct
simulation of partial wetting of NaCl(100) by a liquid droplet. It is concluded
that three elements, namely the exceptional anharmonic stability of the solid
(100) surface, the molecular short range order at the liquid surface, and the
costly solid liquid interface, all conspire to cause the anomalously poor
wetting of the (100) surface by its own melt in the BMHFT model of NaCl -- and
most likely also in real alkali halide surfaces.Comment: modified version of JCP 123, 164701 15 pages, 25 figure
Melting and nonmelting of solid surfaces and nanosystems
We present an extensive but concise review of our present understanding,
largely based on theory and simulation work from our group, on the equilibrium
behavior of solid surfaces and nanosystems close to the bulk melting point. In
the first part we define phenomena, in particular surface melting and
nonmelting, and review some related theoretical approaches, from heuristic
theories to computer simulation. In the second part we describe the surface
melting/nonmelting behavior of several different classes of solids, ranging
from van der Waals crystals, to valence semiconductors, to ionic crystals and
metals. In the third part, we address special cases such as strained solids,
the defreezing of glass surfaces, and rotational surface melting. Next, we
digress briefly to surface layering of a liquid metal, possibly leading to
solid-like or hexatic two dimensional phases floating on the liquid. In the
final part, the relationship of surface melting to the premelting of
nanoclusters and nanowires is reviewed.Comment: 54 pages, 26 figure
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
Dynamical chiral symmetry breaking in sliding nanotubes
We discovered in simulations of sliding coaxial nanotubes an unanticipated
example of dynamical symmetry breaking taking place at the nanoscale. While
both nanotubes are perfectly left-right symmetric and nonchiral, a nonzero
angular momentum of phonon origin appears spontaneously at a series of critical
sliding velocities, in correspondence with large peaks of the sliding friction.
The non-linear equations governing this phenomenon resemble the rotational
instability of a forced string. However, several new elements, exquisitely
"nano" appear here, with the crucial involvement of Umklapp and of sliding
nanofriction.Comment: To appear in PR
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