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ABSTRACT: 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 $V_{\rm RP}(x,s)$, whose shape can
be varied continuously as a function of $s$, 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 $s$
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.
06/2013;
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ABSTRACT: Stick-slip -- the sequence of mechanical instabilities through which a slider
advances on a solid substrate -- is pervasive throughout sliding friction, from
nano to geological scales. Here we suggest that trapped cold ions in an optical
lattice can also be of help in understanding stick-slip friction, and also the
way friction changes when one of the sliders undergoes structural transitions.
For that scope, we simulated the dynamical properties of a 101-ions chain,
driven to slide back and forth by a slowly oscillating electric field in an
incommensurate periodic "corrugation" potential of increasing magnitude U0. We
found the chain sliding to switch, as U0 increases and before the Aubry
transition, from a smooth-sliding regime with low dissipation to a stick-slip
regime with high dissipation. In the stick-slip regime the onset of overall
sliding is preceded by precursor events consisting of partial slips of few ions
only, leading to partial depinning of the chain, a nutshell remnant of
precursor events at the onset of motion also observed in macroscopic sliders.
Seeking to identify the possible effects on friction of a structural
transition, we reduced the trapping potential aspect ratio until the ion chain
shape turned from linear to zigzag. Dynamic friction was found to rise at the
transition, reflecting the opening of newer dissipation channels.
03/2013;
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ABSTRACT: 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 that 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 F(s) when the two lattices are commensurate and pinned. The frictional work directly extracted from particles' velocities can be analyzed 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 unique friction study instrument.
Proceedings of the National Academy of Sciences 09/2012; 109(41):16429-33. · 9.68 Impact Factor
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Nature Material 12/2011; 11(2):97-8. · 32.84 Impact Factor
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ABSTRACT: The physics of sliding friction is gaining impulse from nano 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.
12/2011;
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Physical Review Letters 11/2011; 107(20):209402. · 7.37 Impact Factor
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ABSTRACT: 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.
06/2011;
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ABSTRACT: The effects of a displacive structural phase transition on sliding friction
are in principle accessible to nanoscale tools such as the Atomic Force
Microscopy, yet they are still surprisingly unexplored. We present model
simulations demonstrating and clarifying the mechanism and potential impact of
these effects. A structural order parameter inside the material will yield a
contribution to stick-slip friction that is nonmonotonic as temperature crosses
the phase transition, peaking at the critical Tc where critical fluctuations
are strongest, and the sliding-induced order parameter local flips from one
value to another more numerous. Accordingly, the friction below Tc is larger
when the order parameter orientation is such that flips are more effectively
triggered by the slider. The observability of these effects and their use for
friction control are discussed, for future application to sliding on the
surface of and ferro- or antiferro-distortive materials.
05/2011;
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ABSTRACT: Sliding friction between crystal lattices and the physics of cold ion traps
are so far non-overlapping fields. Two sliding lattices may either stick and
show static friction or slip with dynamic friction; cold ions are known to form
static chains, helices, or clusters, depending on trapping conditions. Here we
show, based on simulations, that much could be learnt about friction by
sliding, via e.g. an electric field, the trapped ion chains over a periodic
corrugated potential. Unlike infinite chains where, according to theory, the
classic Aubry transition to free sliding may take place, trapped chains are
always pinned. Nonetheless we find that a properly defined static friction
still vanishes Aubry-like at a symmetric-asymmetric structural transition,
ubiquitous for decreasing corrugation in both straight and zig-zag trapped
chains. Dynamic friction can also be addressed by ringdown oscillations of the
ion trap. Long theorized static and dynamic one dimensional friction phenomena
could thus become exquisitely accessible in future cold ion tribology.
05/2011;
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ABSTRACT: Non-equilibrium molecular dynamics simulations, of crucial importance in sliding friction, are hampered by arbitrariness and uncertainties in the way Joule heat is removed. We implement in a realistic frictional simulation a parameter-free, non-markovian, stochastic dynamics, which, as expected from theory, absorbs Joule heat precisely as a semi-infinite harmonic substrate would. Simulating stick-slip friction of a slider over a 2D Lennard-Jones solid, we compare our virtually exact frictional results with approximate ones from commonly adopted empirical dissipation schemes. While the latter are generally in serious error, we show that the exact results can be closely reproduced by a viscous Langevin dissipation at the boundary layer, once the back-reflected frictional energy is variationally optimized.
08/2010;
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ABSTRACT: Sliding parts in nanosystems such as nanoelectromechanical systems and nanomotors increasingly involve large speeds, and rotations as well as translations of the moving surfaces; yet, the physics of high-speed nanoscale friction is so far unexplored. Here, by simulating the motion of drifting and of kicked Au clusters on graphite--a workhorse system of experimental relevance--we demonstrate and characterize a new 'ballistic' friction regime at high speed, separate from drift at low speed. The temperature dependence of the cluster slip distance and time, measuring friction, is opposite in these two regimes, consistent with theory. Crucial to both regimes is the interplay of rotations and translations, shown to be correlated in slow drift but anticorrelated in fast sliding. Despite these differences, we find the velocity dependence of ballistic friction to be, like drift, viscous.
Nature Material 08/2010; 9(8):634-7. · 32.84 Impact Factor
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ABSTRACT: Wearless friction force experiments [Science 309, 1354 (2005)10.1126/science.1113239] have recently demonstrated that tribological response in quasicrystals could be related to the exotic atomic structure of the bulk material. Here, by numerical simulations, we address the origin of the experimentally observed friction anisotropy on a twofold decagonal quasicrystal surface. We predict the distinct stick-slip patterns in the lateral force along the periodic and quasiperiodic directions, specifically exploring the temperature dependence that rules the transitions between single and multiple-slip regimes of motion.
Physical Review Letters 02/2010; 104(7):074302. · 7.37 Impact Factor
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ABSTRACT: Design and synthesis of an HDAC inhibitor and its merger with three tubulin binders to create releasable conjugate compounds is described. The biological evaluation includes: (a) in vitro reactivity with glutathione, (b) antiproliferative activity, (c) cell cycle analysis and (d) quantification of protein acetylation. The cellular pharmacology study indicated that the HDAC-inhibitor-drug conjugates retained antimitotic and proapoptotic activity with a reduced potency.
Bioorganic & medicinal chemistry letters 11/2009; 19(22):6358-63. · 2.65 Impact Factor
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ABSTRACT: In the framework of Langevin dynamics, we demonstrate clear evidence of the peculiar quantized sliding state, previously found in a simple one-dimensional boundary lubricated model [A. Vanossi et al., Phys. Rev. Lett. 97, 056101 (2006)], for a substantially less idealized two-dimensional description of a confined multilayer solid lubricant under shear. This dynamical state, marked by a nontrivial "quantized" ratio of the averaged lubricant center-of-mass velocity to the externally imposed sliding speed, is recovered, and shown to be robust against the effects of thermal fluctuations, quenched disorder in the confining substrates, and over a wide range of loading forces. The lubricant softness, setting the width of the propagating solitonic structures, is found to play a major role in promoting in-registry commensurate regions beneficial to this quantized sliding. By evaluating the force instantaneously exerted on the top plate, we find that this quantized sliding represents a dynamical "pinned" state, characterized by significantly low values of the kinetic friction. While the quantized sliding occurs due to solitons being driven gently, the transition to ordinary unpinned sliding regimes can involve lubricant melting due to large shear-induced Joule heating, for example at large speed.
The Journal of chemical physics 11/2009; 131(17):174711. · 3.09 Impact Factor
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ABSTRACT: Mechanical vibrations are known to affect frictional sliding and the associated stick-slip patterns causing sometimes a drastic reduction of the friction force. This issue is relevant for applications in nanotribology and to understand earthquake triggering by small dynamic perturbations. We study the dynamics of repulsive particles confined between a horizontally driven top plate and a vertically oscillating bottom plate. Our numerical results show a suppression of the high dissipative stick-slip regime in a well-defined range of frequencies that depends on the vibrating amplitude, the normal applied load, the system inertia and the damping constant. We propose a theoretical explanation of the numerical results and derive a phase diagram indicating the region of parameter space where friction is suppressed. Our results allow to define better strategies for the mechanical control of friction.
Physical Review Letters 08/2009; 103(8):085502. · 7.37 Impact Factor
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ABSTRACT: 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)
07/2009;
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ABSTRACT: We introduce a family of molecular rotors that may convert light or chemical energy into directed translational motion along surfaces. The dependencies of diffusion coefficient and drift velocity of the rotating molecule on the magnitude of external torque, symmetry of surface potential, and temperature have been investigated. Our simulations show that the rotation-translation coupling could be very effective, and the molecule may move by approximately one surface lattice spacing per complete rotation. We have found that the unidirectionality of the rotary motion is not required to produce efficient directed sliding; this effect can be achieved by applying a time-periodic, or even randomly, oscillating torque which induces alternating molecular reorientations but does not generate complete rotations.
Physical Review E 03/2009; 79(2 Pt 1):021108. · 2.26 Impact Factor
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ABSTRACT: Interfacial friction is one of the oldest problems in physics and chemistry, and certainly one of the most important from a practical point of view. Everyday operations on a broad range of scales, from nanometer and up, depend upon the smooth and satisfactory functioning of countless tribological systems. Friction imposes serious constraints and limitations on the performance and lifetime of micro-machines and, undoubtedly, will impose even more severe constraints on the emerging technology of nano-machines. Standard lubrication techniques used for large objects are expected to be less effective in the nano-world. Novel methods for control and manipulation are therefore needed. What has been missing is a molecular level understanding of processes occurring between and close to interacting surfaces to help understand, and later manipulate friction. Friction is intimately related to both adhesion and wear, and all three require an understanding of highly non-equilibrium processes occurring at the molecular level to determine what happens at the macroscopic level. Due to its practical importance and the relevance to basic scientific questions there has been major increase in activity in the study of interfacial friction on the microscopic level during the last decade. Intriguing structural and dynamical features have been observed experimentally. These observations have motivated theoretical efforts, both numerical and analytical. This special issue focusses primarily on discussion of microscopic mechanisms of friction and adhesion at the nanoscale level. The contributions cover many important aspects of frictional behaviour, including the origin of stick-slip motion, the dependence of measured forces on the material properties, effects of thermal fluctuations, surface roughness and instabilities in boundary lubricants on both static and kinetic friction. An important problem that has been raised in this issue, and which has still to be resolved, concerns the possibility of controlling frictional response. The ability to control and manipulate frictional forces is extremely important for a variety of applications. These include magnetic storage and recording systems, miniature motors, and more. This special issue aims to provide an overview of current theoretical and experimental works on nanotribology and possible applications. In selecting the papers we have tried to maintain a balance between new results and review-like aspects, so that the present issue is self-contained and, we hope, readily accessible to non-specialists in the field. We believe that the particular appeal of this collection of papers also lies in the fusion of both experiment and theory, thus providing the connection to reality of the sometimes demanding, mathematically inclined contributions. Profound thanks go to all our colleagues and friends who have contributed to this special issue. Each has made an effort not only to present recent results in a clear and lucid way, but also to provide an introductory review that helps the reader to understand the different topics.
Journal of Physics Condensed Matter 08/2008; 20(35):350301. · 2.55 Impact Factor
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ABSTRACT: Within the idealized scheme of a 1-dimensional Frenkel-Kontorova-like model,
a special "quantized" sliding state was found for a solid lubricant confined
between two periodic layers [PRL 97, 056101 (2006)]. This state, characterized
by a nontrivial geometrically fixed ratio of the mean lubricant drift velocity
and the externally imposed translational velocity v_ext, was understood
as due to the kinks (or solitons), formed by the lubricant due to
incommensuracy with one of the substrates, pinning to the other sliding
substrate. A quantized sliding state of the same nature is demonstrated here
for a substantially less idealized 2-dimensional model, where atoms are allowed
to move perpendicularly to the sliding direction and interact via Lennard-Jones
potentials. Clear evidence for quantized sliding at finite temperature is
provided, even with a confined solid lubricant composed of multiple (up to 6)
lubricant layers. Characteristic backward lubricant motion produced by the
presence of "anti-kinks" is also shown in this more realistic context.
04/2008;
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ABSTRACT: 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.
10/2007;
