299 research outputs found
Simulations of the kinetic friction due to adsorbed surface layers
Simulations of the kinetic friction due to a layer of adsorbed molecules
between two crystalline surfaces are presented. The adsorbed layer naturally
produces friction that is consistent with Amontons' laws and insensitive to
parameters that are not controlled in experiments. The kinetic friction rises
logarithmically with velocity as in many experimental systems. Variations with
potential parameters and temperature follow variations in the static friction.
This correlation is understood through analogy with the Tomlinson model and the
trends are explained with a hard-sphere picture.Comment: 8 pages, 6 figures, submitted to Tribology Letter
Capillary Adhesion at the Nanometer Scale
Molecular dynamics simulations are used to study the capillary adhesion from
a nonvolatile liquid meniscus between a spherical tip and a flat substrate. The
atomic structure of the tip, the tip radius, the contact angles of the liquid
on the two surfaces, and the volume of the liquid bridge are varied. The
capillary force between the tip and substrate is calculated as a function of
their separation h. The force agrees with continuum predictions for h down to ~
5 to 10nm. At smaller h, the force tends to be less attractive than predicted
and has strong oscillations. This oscillatory component of the capillary force
is completely missed in the continuum theory, which only includes contributions
from the surface tension around the circumference of the meniscus and the
pressure difference over the cross section of the meniscus. The oscillation is
found to be due to molecular layering of the liquid confined in the narrow gap
between the tip and substrate. This effect is most pronounced for large tip
radii and/or smooth surfaces. The other two components considered by the
continuum theory are also identified. The surface tension term, as well as the
meniscus shape, is accurately described by the continuum prediction for h down
to ~ 1nm, but the capillary pressure term is always more positive than the
corresponding continuum result. This shift in the capillary pressure reduces
the average adhesion by a factor as large as 2 from its continuum value and is
found to be due to an anisotropy in the pressure tensor. The cross-sectional
component is consistent with the capillary pressure predicted by the continuum
theory (i.e., the Young-Laplace equation), but the normal pressure that
determines the capillary force is always more positive than the continuum
counterpart.Comment: 16 pages, 14 figure
Defining Contact at the Atomic Scale
Molecular dynamics simulations are used to study different definitions of
contact at the atomic scale. The roles of temperature, adhesive interactions
and atomic structure are studied for simple geometries. An elastic, crystalline
substrate contacts a rigid, atomically flat surface or a spherical tip. The
rigid surface is formed from a commensurate or incommensurate crystal or an
amorphous solid. Spherical tips are made by bending crystalline planes or
removing material outside a sphere. In continuum theory the fraction of
atomically flat surfaces that is in contact rises sharply from zero to unity
when a load is applied. This simple behavior is surprisingly difficult to
reproduce with atomic scale definitions of contact. Due to thermal
fluctuations, the number of atoms making contact at any instant rises linearly
with load over a wide range of loads. Pressures comparable to the ideal
hardness are needed to achieve full contact at typical temperatures. A simple
harmonic mean-field theory provides a quantitative description of this behavior
and explains why the instantaneous forces on atoms have a universal exponential
form. Contact areas are also obtained by counting the number of atoms with a
time-averaged repulsive force. For adhesive interactions, the resulting area is
nearly independent of temperature and averaging interval, but usually rises
from zero to unity over a range of pressures that is comparable to the ideal
hardness. The only exception is the case of two identical commensurate
surfaces. For nonadhesive surfaces, the mean pressure is repulsive if there is
any contact during the averaging interval . The associated area is
very sensitive to and grows monotonically. Similar complications are
encountered in defining contact areas for spherical tips.Comment: 18 pages, 11 figure
Mapping molecular models to continuum theories for partially miscible fluids
We map molecular dynamics simulations of fluid-fluid interfaces onto
mesoscale continuum theories for partially miscible fluids. Unlike most
previous work, we examine not only the interface order parameter and density
profiles, but also the stress. This allows a complete mapping from the length
scales of molecular dynamics simulations onto a mesoscale model suitable for a
lattice Boltzmann or other mesoscale simulation method. Typical assumptions of
mesoscale models, such as incompressibility, are found to fail at the
interface, and this has a significant impact on the surface tension. Spurious
velocities, found in a number of discrete models of curved interfaces, are
found to be minimized when the parameters of the mesoscopic model are made
consistent with molecular dynamics results. An improved mesoscale model is
given and demonstrated to produce results consistent with molecular dynamics
simulations for interfaces with widths down to near molecular size.Comment: 43 pages, 17 figures, RevTex
Computer Simulations of Friction, Lubrication and Wear
An overview of computer simulations of tribology is presented. The chapter
begins with a brief overview of simulation techniques and the special
requirements for simulations of tribological processes. Then simple
one-dimensional models of friction between crystalline surfaces are introduced
to illustrate general features of friction, such as the importance of
metastability and the effect of commensurability. Next two- and
three-dimensional studies of dry sliding between crystalline surfaces are
described, and compared to scanning probe experiments and measurements of the
friction on adsorbed monolayers. Lubrication is then discussed, starting from
thick films and describing the breakdown in bulk hydrodynamics as the thickness
of the lubricant decreases to molecular scales. Deviations from the usual
no-slip boundary condition are quantified and the onset of solid behavior in
molecularly thick films is described. The possibility that solidification of
thin layers of adventitious carbon is responsible for the prevalence of static
friction is explored. The final sections describe stick-slip motion,
tribochemical reactions, machining, and the evolution of microstructure in
sliding contacts.Comment: Review chapter for the Handbook of Modern Tribology edited by Bharat
Bhushan (CRC Press), 42 pages, 16 figure
Chain Ends and the Ultimate Strength of Polyethylene Fibers
We use large scale molecular dynamics (MD) simulations to determine the
tensile yield mechanism of orthorhombic polyethylene (PE) crystals with finite
chains spanning carbons in length. We find the yield stress
saturates for long chains at 6.3 GPa, agreeing well with
experiments. We show chains do not break but always yield by slip, after
nucleation of 1D dislocations at chain ends. Dislocations are accurately
described by a Frenkel-Kontorova model parametrized by the mechanical
properties of an ideal crystal. We compute a dislocation core size
\AA\ and determine the high and low strain rate limits of
. Our results suggest characterizing the 1D dislocations of polymer
crystals as an efficient method for numerically predicting the ultimate tensile
strength of aligned fibers
The effect of inertia on sheared disordered solids: Critical scaling of avalanches in two and three dimensions
Molecular dynamics simulations with varying damping are used to examine the
effects of inertia and spatial dimension on sheared disordered solids in the
athermal, quasistatic limit. In all cases the distribution of avalanche sizes
follows a power law over at least three orders of magnitude in dissipated
energy or stress drop. Scaling exponents are determined using finite-size
scaling for systems with thousands to millions of particles. Three distinct
universality classes are identified corresponding to overdamped and underdamped
limits, as well as a crossover damping that separates the two regimes. For each
universality class, the exponent describing the avalanche distributions is the
same in two and three dimensions. The spatial extent of plastic damage is
proportional to the energy dissipated in an avalanche. Both rise much more
rapidly with system size in the underdamped limit where inertia is important.
Inertia also lowers the mean energy of configurations sampled by the system and
leads to an excess of large events like that seen in earthquake distributions
for individual faults. The distribution of stress values during shear narrows
to zero with increasing system size and may provide useful information about
the size of elemental events in experimental systems. For overdamped and
crossover systems the stress variation scales inversely with the square root of
the system size. For underdamped systems the variation is determined by the
size of the largest events.Comment: 17 pages, 13 figure
Stretching of Proteins in the Entropic Limit
Mechanical stretching of six proteins is studied through molecular dynamics
simulations. The model is Go-like, with Lennard-Jones interactions at native
contacts. Low temperature unfolding scenarios are remarkably complex and
sensitive to small structural changes. Thermal fluctuations reduce the peak
forces and the number of metastable states during unfolding. The unfolding
pathways also simplify as temperature rises. In the entropic limit, all
proteins show a monotonic decrease of the extension where bonds rupture with
their separation along the backbone (contact order).Comment: RevTex, 5 pages, 5 figures, to appear in Phys. Rev.
Thermal Folding and Mechanical Unfolding Pathways of Protein Secondary Structures
Mechanical stretching of secondary structures is studied through molecular
dynamics simulations of a Go-like model. Force vs. displacement curves are
studied as a function of the stiffness and velocity of the pulling device. The
succession of stretching events, as measured by the order in which contacts are
ruptured, is compared to the sequencing of events during thermal folding and
unfolding. Opposite cross-correlations are found for an -helix and a
-hairpin structure. In a tandem of two -helices, the two
constituent helices unravel nearly simultaneously. A simple condition for
simultaneous vs. sequential unraveling of repeat units is presented.Comment: 12 pages, 17 figure
Thermal effects in stretching of Go-like models of titin and secondary structures
The effect of temperature on mechanical unfolding of proteins is studied
using a Go-like model with a realistic contact map and Lennard-Jones contact
interactions. The behavior of the I27 domain of titin and its serial repeats is
contrasted to that of simple secondary structures. In all cases thermal
fluctuations accelerate the unraveling process, decreasing the unfolding force
nearly linearly at low temperatures. However differences in bonding geometry
lead to different sensitivity to temperature and different changes in the
unfolding pattern. Due to its special native state geometry titin is much more
thermally and elastically stable than the secondary structures. At low
temperatures serial repeats of titin show a parallel unfolding of all domains
to an intermediate state, followed by serial unfolding of the domains. At high
temperatures all domains unfold simultaneously and the unfolding distance
decreases monotonically with the contact order, that is the sequence distance
between the amino acids that form the native contact.Comment: 38 pages, 17 figures, to appear in Protein
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