74 research outputs found
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
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
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
Hysteresis from dynamically pinned sliding states
We report a surprising hysteretic behavior in the dynamics of a simple
one-dimensional nonlinear model inspired by the tribological problem of two
sliding surfaces with a thin solid lubricant layer in between. In particular,
we consider the frictional dynamics of a harmonic chain confined between two
rigid incommensurate substrates which slide with a fixed relative velocity.
This system was previously found, by explicit solution of the equations of
motion, to possess plateaus in parameter space exhibiting a remarkable
quantization of the chain center-of-mass velocity (dynamic pinning) solely
determined by the interface incommensurability. Starting now from this
quantized sliding state, in the underdamped regime of motion and in analogy to
what ordinarily happens for static friction, the dynamics exhibits a large
hysteresis under the action of an additional external driving force F_ext. A
critical threshold value F_c of the adiabatically applied force F_ext is
required in order to alter the robust dynamics of the plateau attractor. When
the applied force is decreased and removed, the system can jump to intermediate
sliding regimes (a sort of ``dynamic'' stick-slip motion) and eventually
returns to the quantized sliding state at a much lower value of F_ext. On the
contrary no hysteretic behavior is observed as a function of the external
driving velocity.Comment: 12 pages, 5 figures, ECOSS 200
AFM Dissipation Topography of Soliton Superstructures in Adsorbed Overlayers
In the atomic force microscope, the nanoscale force topography of even
complex surface superstructures is extracted by the changing vibration
frequency of a scanning tip. An alternative dissipation topography with similar
or even better contrast has been demonstrated recently by mapping the
(x,y)-dependent tip damping but the detailed damping mechanism is still
unknown. Here we identify two different tip dissipation mechanisms: local
mechanical softness and hysteresis. Motivated by recent data, we describe both
of them in a onedimensional model of Moire' superstructures of incommensurate
overlayers. Local softness at "soliton" defects yields a dissipation contrast
that can be much larger than the corresponding density or corrugation contrast.
At realistically low vibration frequencies, however, a much stronger and more
effective dissipation is caused by the tip-induced nonlinear jumping of the
soliton, naturally developing bistability and hysteresis. Signatures of this
mechanism are proposed for experimental identification.Comment: 5 pages, 5 figures, Phys Rev B 81, 045417 (2010
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