469 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
Kink plateau dynamics in finite-size lubricant chains
We extend the study of velocity quantization phenomena recently found in the
classical motion of an idealized 1D model solid lubricant -- consisting of a
harmonic chain interposed between two periodic sliding potentials [Phys. Rev.
Lett. 97, 056101 (2006)]. This quantization is due to one slider rigidly
dragging the commensurate lattice of kinks that the chain forms with the other
slider. In this follow-up work we consider finite-size chains rather than
infinite chains. The finite size (i) permits the development of robust velocity
plateaus as a function of the lubricant stiffness, and (ii) allows an overall
chain-length re-adjustment which spontaneously promotes single-particle
periodic oscillations. These periodic oscillations replace the quasi-periodic
motion produced by general incommensurate periods of the sliders and the
lubricant in the infinite-size model. Possible consequences of these results
for some real systems are discussed.Comment: 12 pages, 5 figures, ECOSS 200
Influence of substrate potential shape on the dynamics of a sliding lubricant chain
We investigate the frictional sliding of an incommensurate chain of
interacting particles confined in between two nonlinear on-site substrate
potential profiles in relative motion. We focus here on the class of
Remoissenet-Peyrard parametrized potentials , whose shape can
be varied continuously as a function of , recovering the sine-Gordon
potential as particular case. The observed frictional dynamics of the system,
crucially dependent on the mutual ratios of the three periodicities in the
sandwich geometry, turns out to be significantly influenced also by the shape
of the substrate potential. Specifically, variations of the shape parameter
affects significantly and not trivially the existence and robustness of the
recently reported velocity quantization phenomena [Vanossi {\it et al.}, Phys.
Rev. Lett. 97, 056101 (2006)], where the chain center-of-mass velocity to the
externally imposed relative velocity of the sliders stays pinned to exact
"plateau" values for wide ranges of the dynamical parameters.Comment: 7 pages, 6 figure
Finite-temperature phase diagram and critical point of the Aubry pinned-sliding transition in a 2D monolayer
The Aubry unpinned--pinned transition in the sliding of two incommensurate
lattices occurs for increasing mutual interaction strength in one dimension
() and is of second order at , turning into a crossover at nonzero
temperatures. Yet, real incommensurate lattices come into contact in two
dimensions (), at finite temperature, generally developing a mutual
Novaco-McTague misalignment, conditions in which the existence of a sharp
transition is not clear. Using a model inspired by colloid monolayers in an
optical lattice as a test case, simulations show a sharp Aubry transition
between an unpinned and a pinned phase as a function of corrugation. Unlike
, the transition is now of first order, and, importantly, remains well
defined at . It is heavily structural, with a local rotation of moir\'e
pattern domains from the nonzero initial Novaco-McTague equilibrium angle to
nearly zero. In the temperature () -- corrugation strength () plane,
the thermodynamical coexistence line between the unpinned and the pinned phases
is strongly oblique, showing that the former has the largest entropy. This
first-order Aubry line terminates with a novel critical point , marked
by a susceptibility peak. The expected static sliding friction upswing between
the unpinned and the pinned phase decreases and disappears upon heating from
to . The experimental pursuit of this novel scenario is proposed.Comment: 9 pages, 9 figure
Friction Boosted by Equilibrium Misalignment of Incommensurate Two-Dimensional Colloid Monolayers
Colloidal 2D monolayers sliding in an optical lattice are of recent
importance as a frictional system. In the general case when the monolayer and
optical lattices are incommensurate, we predict two important novelties, one in
the static equilibrium structure, the other in the frictional behavior under
sliding. Structurally, realistic simulations show that the colloid layer should
possess in full equilibrium a small misalignment rotation angle relative to the
optical lattice, an effect so far unnoticed but visible in some published
experimental moir\'e patterns. Under sliding, this misalignment has the effect
of boosting the colloid monolayer friction by a considerable factor over the
hypothetical aligned case discussed so far. A frictional increase of similar
origin must generally affect other incommensurate adsorbed monolayers and
contacts, to be sought out case by case.Comment: 9 pages, 11 figures (including Supplemental Material
Modeling friction: From nanoscale to mesoscale
The physics of sliding friction is gaining impulse from nanoscale and
mesoscale experiments, simulations, and theoretical modeling. This Colloquium
reviews some recent developments in modeling and in atomistic simulation of
friction, covering open-ended directions, unconventional nanofrictional
systems, and unsolved problems.Comment: 26 pages, 14 figures, Rev. Mod. Phys. Colloquiu
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