198 research outputs found
Lubricated friction between incommensurate substrates
This paper is part of a study of the frictional dynamics of a confined solid
lubricant film - modelled as a one-dimensional chain of interacting particles
confined between two ideally incommensurate substrates, one of which is driven
relative to the other through an attached spring moving at constant velocity.
This model system is characterized by three inherent length scales; depending
on the precise choice of incommensurability among them it displays a strikingly
different tribological behavior. Contrary to two length-scale systems such as
the standard Frenkel-Kontorova (FK) model, for large chain stiffness one finds
that here the most favorable (lowest friction) sliding regime is achieved by
chain-substrate incommensurabilities belonging to the class of non-quadratic
irrational numbers (e.g., the spiral mean). The well-known golden mean
(quadratic) incommensurability which slides best in the standard FK model shows
instead higher kinetic-friction values. The underlying reason lies in the
pinning properties of the lattice of solitons formed by the chain with the
substrate having the closest periodicity, with the other slider.Comment: 14 pagine latex - elsart, including 4 figures, submitted to Tribology
Internationa
Static and dynamic friction in sliding colloidal monolayers
In a pioneer experiment, Bohlein et al. realized the controlled sliding of
two-dimensional colloidal crystals over laser-generated periodic or
quasi-periodic potentials. Here we present realistic simulations and arguments
which besides reproducing the main experimentally observed features, give a
first theoretical demonstration of the potential impact of colloid sliding in
nanotribology. The free motion of solitons and antisolitons in the sliding of
hard incommensurate crystals is contrasted with the soliton-antisoliton pair
nucleation at the large static friction threshold Fs when the two lattices are
commensurate and pinned. The frictional work directly extracted from particles'
velocities can be analysed as a function of classic tribological parameters,
including speed, spacing and amplitude of the periodic potential (representing
respectively the mismatch of the sliding interface, and the corrugation, or
"load"). These and other features suggestive of further experiments and
insights promote colloid sliding to a novel friction study instrument.Comment: in print in the Proceedings of the National Academy of Sciences
U.S.A. This v2 is identical to v1, but includes ancillary material. A few
figures were undersampled due to size limits: those in v1 are far sharpe
Static friction on the fly: velocity depinning transitions of lubricants in motion
The dragging velocity of a model solid lubricant confined between sliding
periodic substrates exhibits a phase transition between two regimes,
respectively with quantized and with continuous lubricant center-of-mass
velocity. The transition, occurring for increasing external driving force F_ext
acting on the lubricant, displays a large hysteresis, and has the features of
depinning transitions in static friction, only taking place on the fly.
Although different in nature, this phenomenon appears isomorphic to a static
Aubry depinning transition in a Frenkel-Kontorova model, the role of particles
now taken by the moving kinks of the lubricant-substrate interface. We suggest
a possible realization in 2D optical lattice experiments.Comment: 5 pages, 4 figures, revtex, in print in Phys. Rev. Let
Optimal Energy Dissipation in Sliding Friction Simulations
Non-equilibrium molecular dynamics simulations, of crucial importance in
sliding friction, are hampered by arbitrariness and uncertainties in the
removal of the frictionally generated Joule heat. Building upon general
pre-existing formulation, we implement a fully microscopic dissipation approach
which, based on a parameter-free, non-Markovian, stochastic dynamics, absorbs
Joule heat equivalently to a semi-infinite solid and harmonic substrate. As a
test case, we investigate the stick-slip friction of a slider over a
two-dimensional Lennard-Jones solid, comparing our virtually exact frictional
results with approximate ones from commonly adopted dissipation schemes.
Remarkably, the exact results can be closely reproduced by a standard Langevin
dissipation scheme, once its parameters are determined according to a general
and self-standing variational procedure
Experimental observation of the Aubry transition in two-dimensional colloidal monolayers
The possibility to achieve entirely frictionless, i.e. superlubric, sliding
between solids, holds enormous potential for the operation of mechanical
devices. At small length scales, where mechanical contacts are well-defined,
Aubry predicted a transition from a superlubric to a pinned state when the
mechanical load is increased. Evidence for this intriguing Aubry transition
(AT), which should occur in one dimension (1D) and at zero temperature, was
recently obtained in few-atom chains. Here, we experimentally and theoretically
demonstrate the occurrence of the AT in an extended two-dimensional (2D) system
at room temperature using a colloidal monolayer on an optical lattice. Unlike
the continuous nature of the AT in 1D, we observe a first-order transition in
2D leading to a coexistence regime of pinned and unpinned areas. Our data
demonstrate that the original concept of Aubry does not only survive in 2D but
is relevant for the design of nanoscopic machines and devices at ambient
temperature.Comment: 12 pages including 4 figures + 9 pages supplemental informatio
Controlling Microscopic Friction through Mechanical Oscillations
We study in detail the recent suggestions by Tshiprut et al. [Phys. Rev.
Lett. 95, 016101 (2005)] to tune tribological properties at the nanoscale by
subjecting a substrate to periodic mechanical oscillations. We show that both
in stick-slip and sliding regimes of motion friction can be tuned and reduced
by controlling the frequency and amplitude of the imposed substrate lateral
excitations. We demonstrate that the mechanisms of oscillation-induced
reduction of friction are different for stick-slip and sliding dynamics. In the
first regime the effect results from a giant enhancement of surface diffusion,
while in the second regime it is due to the interplay between washboard and
oscillation frequencies that leads to the occurrence of parametric resonances.
Moreover we show that for particular set of parameters it is possible to
sustain the motion with the only oscillations
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