1,644 research outputs found
Dependence of kinetic friction on velocity: Master equation approach
We investigate the velocity dependence of kinetic friction with a model which
makes minimal assumptions on the actual mechanism of friction so that it can be
applied at many scales provided the system involves multi-contact friction.
Using a recently developed master equation approach we investigate the
influence of two concurrent processes. First, at a nonzero temperature thermal
fluctuations allow an activated breaking of contacts which are still below the
threshold. As a result, the friction force monotonically increases with
velocity. Second, the aging of contacts leads to a decrease of the friction
force with velocity. Aging effects include two aspects: the delay in contact
formation and aging of a contact itself, i.e., the change of its
characteristics with the duration of stationary contact. All these processes
are considered simultaneously with the master equation approach, giving a
complete dependence of the kinetic friction force on the driving velocity and
system temperature, provided the interface parameters are known
Rack-and-pinion effects in molecular rolling friction
Rolling lubrication with spherical molecules working as 'nanobearings' has
failed experimentally so far, without a full understanding of the physics
involved and of the reasons why. Past model simulations and common sense have
shown that molecules can only roll when they are not too closely packed to jam.
The same type of model simulations now shows in addition that molecular rolling
friction can develop deep minima once the molecule's peripheral 'pitch' can
match the substrate periodicity, much as ordinary cogwheels do in a
rack-and-pinion system. When the pinion-rack matching is bad, the driven
molecular rolling becomes discontinuous and noisy, whence energy is dissipated
and friction is large. This suggests experiments to be conducted by varying the
rack-and-pinion matching. That could be pursued not only by changing molecules
and substrates, but also by applying different sliding directions within the
same system, or by applying pressure, to change the effective matching.Comment: 5 figure
Relaxation Tribometry: A Generic Method to Identify the Nature of Contact Forces
Recent years have witnessed the development of so-called relaxation
tribometers, the free oscillation of which is altered by the presence of
frictional stresses within the contact. So far, analysis of such oscillations
has been restricted to the shape of their decaying envelope, to identify in
particular solid or viscous friction components. Here, we present a more
general expression of the forces possibly acting within the contact , and
retain six possible, physically relevant terms. Two of them, which had never
been proposed in the context of relaxation tribometry, only affect the
oscillation frequency, not the amplitude of the signal. We demonstrate that
each of those six terms has a unique signature in the time-evolution of the
oscillation, which allows efficient identification of their respective weights
in any experimental signal. We illustrate our methodology on a PDMS
sphere/glass plate torsional contact
The role of aging in a minimal model of earthquakes
International audienceWe introduce a simple model of earthquakes, inspired by the spring-block models, but describing contacts at a meso-scale. A single contact point synthesizes many rock contacts so that these ''macro-contacts'' can have an internal dynamics, described by a stochastic process, that leads to an evolution of their breaking threshold. This aging process leads to the Gutenberg-Richter law, which relates the probability of occurrence of earthquakes to their magnitude. An analytical approach is used to determine the range of magnitudes in which this law applies
Modeling friction on a mesoscale: Master equation for the earthquake-like model
The earthquake-like model with a continuous distribution of static thresholds
is used to describe the properties of solid friction. The evolution of the
model is reduced to a master equation which can be solved analytically. This
approach naturally describes stick-slip and smooth sliding regimes of
tribological systems within a framework which separates the calculation of the
friction force from the studies of the properties of the contacts.Comment: Accepted for publication by Physical Review Letter
Comment on "Soliton ratchets induced by excitation of internal modes"
Very recently Willis et al. [Phys. Rev. E {\bf 69}, 056612 (2004)] have used
a collective variable theory to explain the appearance of a nonzero energy
current in an ac driven, damped sine-Gordon equation. In this comment, we prove
rigorously that the time-averaged energy current in an ac driven nonlinear
Klein-Gordon system is strictly zero.Comment: 3 pages, 1 figure, Submitted to Phys. Rev.
Substrate-induced pairing of Si ad-dimers on the Si(100)surface
The interaction between Si ad-dimers on the Si(100) surface has been studied by total-energy calculations with a three-particle Stillinger-Weber potential. We have found a strong attractive interaction between neighboring Si ad-dimers located in neighboring on-top and deep-channel positions in adjacent substrate dimer rows. This should result in a four-atomic block consisting of two dimers as an important elementary object of the Si(100) kinetics
Dynamical transitions in correlated driven diffusion in a periodic potential
The diffusion of a two-dimensional array of particles driven by a constant
force in the presence of a periodic external potential exhibits a hierarchy of
dynamical phase transitions when the driving force is varied. This behavior can
be explained by a simple phenomenological approach which reduces the system of
strongly interacting particles to weakly interacting quasi-particles (kinks).
The richness of the strongly coupled system is however not lost because,
contrary to a single-Brownian particle, the array shows an hysteretic behavior
even at non-zero temperature. The present investigation can be viewed as a
first step toward understanding nanotribology.Comment: 4 pages, 3 pictures, revtex to appear in Phys Rev. Let
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