Andrea Vanossi

Scuola Internazionale Superiore di Studi Avanzati di Trieste, Trst, Friuli Venezia Giulia, Italy

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Publications (78)388.28 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We investigate theoretically the possibility to observe dynamical mode locking, in the form of Shapiro steps, when a time-periodic potential or force modulation is applied to a two-dimensional (2D) lattice of colloidal particles that are dragged by an external force over an optically generated periodic potential. Here we present realistic molecular dynamics simulations of a 2D experimental setup, where the colloid sliding is realized through the motion of soliton lines between locally commensurate patches or domains, and where the Shapiro steps are predicted and analyzed. Interestingly, the jump between one step and the next is seen to correspond to a fixed number of colloids jumping from one patch to the next, across the soliton line boundary, during each AC cycle. In addition to ordinary "integer" steps, coinciding here with the synchronous rigid advancement of the whole colloid monolayer, our main prediction is the existence of additional smaller "subharmonic" steps due to localized solitonic regions of incommensurate layers executing synchronized slips, while the majority of the colloids remains pinned to a potential minimum. The current availability and wide parameter tunability of colloid monolayers makes these predictions potentially easy to access in an experimentally rich 2D geometrical configuration.
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    Rosario Capozza · Andrea Benassi · Andrea Vanossi · Erio Tosatti
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    ABSTRACT: Recent measurements suggest the possibility to exploit ionic liquids (ILs) as smart lubricants for nano-contacts, tuning their tribological and rheological properties by charging the sliding interfaces. Following our earlier theoretical study of charging effects on nanoscale confinement and squeezout of a model IL, we present here molecular dynamics simulations of the frictional and lubrication properties of that model under charging conditions.First we describe the case when two equally charged plates slide while being held together to a confinement distance of a few molecular layers.The shear sliding stress is found to rise as the number of IL layers decreases stepwise. However the shear stress shows, within each given number of layers, only a weak dependence upon the precise value of the normal load, a result in agreement with data extracted from recent experiments.We subsequently describe the case of opposite charging of the sliding plates, and follow the shear stress when the charging is slowly and adiabatically reversed in the course of time, under fixed load. Despite the fixed load, the number and structure of the confined IL layers changes with changing charge, and that in turn drives strong friction variations. The latter involve first of all charging-induced freezing of the IL film, followed by a discharging-induced melting, both made possible by the nanoscale confinement. Another mechanism for charging-induced frictional changes is a shift of the plane of maximum shear from mid-film to the plate-film interface, and viceversa. While these occurrences and results invariably depend upon the parameters of the model IL and upon its specific interaction with the plates, the present study helps identifying a variety of possible behavior, obtained under very simple assumptions, while connecting it to an underlying equilibrium thermodynamics picture.
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    ABSTRACT: Two-dimensional (2D) crystalline colloidal monolayers sliding over a laser-induced optical lattice recently emerged as a new tool for the study of friction between ideal crystal surfaces. Here we focus in particular on static friction, the minimal sliding force necessary to depin one lattice from the other. If the colloid and the optical lattices are mutually commensurate, the colloid sliding is always pinned by static friction; but when they are incommensurate the presence or absence of pinning can be expected to depend upon the system parameters. If a 2D analogy to the mathematically established Aubry transition of one-dimensional systems were to hold, an increasing periodic corrugation strength $U_0$ should turn an initially free-sliding monolayer into a pinned state through a well-defined dynamical phase transition. We address this problem by the simulated sliding of a realistic model 2D colloidal lattice, confirming the existence of a clear and sharp superlubric-pinned transition for increasing corrugation strength. Unlike the 1D Aubry transition which is continuous, the 2D transition exhibits a definite first-order character. With no change of symmetry, the transition entails a structural character, with a sudden increase of the colloid-colloid interaction energy, accompanied by a compensating downward jump of the colloid-corrugation energy. The transition value for the corrugation amplitude $U_0$ depends upon the misalignment angle $\theta$ between the optical and the colloidal lattices, superlubricity surviving until larger corrugations for angles away from the energetically favored orientation, which is itself generally slightly misaligned, as shown in recent work. The observability of the superlubric-pinned colloid transition is proposed and discussed.
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    ABSTRACT: The inertial sliding of physisorbed submonolayer islands on crystal surfaces contains unexpected information on the exceptionally smooth sliding state associated with incommensurate superlubricity and on the mechanisms of its disappearance. Here, in a joint quartz crystal microbalance and molecular dynamics simulation case study of Xe on Cu(111), we show how superlubricity emerges in the large size limit of naturally incommensurate Xe islands. As coverage approaches a full monolayer, theory also predicts an abrupt adhesion-driven two-dimensional density compression on the order of several per cent, implying a hysteretic jump from superlubric free islands to a pressurized commensurate immobile monolayer. This scenario is fully supported by the quartz crystal microbalance data, which show remarkably large slip times with increasing submonolayer coverage, signalling superlubricity, followed by a dramatic drop to zero for the dense commensurate monolayer. Careful analysis of this variety of island sliding phenomena will be essential in future applications of friction at crystal/adsorbate interfaces.
    Nature Nanotechnology 05/2015; DOI:10.1038/nnano.2015.106 · 33.27 Impact Factor
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    Davide Mandelli · Andrea Vanossi · Nicola Manini · Erio Tosatti
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    ABSTRACT: 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.
    Physical Review Letters 03/2015; 114(10). DOI:10.1103/PhysRevLett.114.108302 · 7.51 Impact Factor
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    Ming Ma · Andrea Benassi · Andrea Vanossi · Michael Urbakh
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    ABSTRACT: Since the demonstration of super-low friction (superlubricity) in graphite at nanoscale, one of the main challenges in the field of nano- and micro-mechanics was to scale this phenomenon up. A key question to be addressed is to what extent superlubricity could persist, and what mechanisms could lead to its failure. Here, using an edge-driven Frenkel-Kontorova model, we establish a connection between the critical length above which superlubricity disappears and both intrinsic material properties and experimental parameters. A striking boost in dissipated energy with chain length emerges abruptly due to a high-friction stick-slip mechanism caused by deformation of the slider leading to a local commensuration with the substrate lattice. We derived a parameter-free analytical model for the critical length that is in excellent agreement with our numerical simulations. Our results provide a new perspective on friction and nano-manipulation and can serve as a theoretical basis for designing nano-devices with super-low friction, such as carbon nanotubes.
    Physical Review Letters 01/2015; 114(5). DOI:10.1103/PhysRevLett.114.055501 · 7.51 Impact Factor
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    R. Capozza · A. Vanossi · A. Benassi · E. Tosatti
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    ABSTRACT: Electrical charging of parallel plates confining a model ionic liquid down to nanoscale distances yields a variety of charge-induced changes in the structural features of the confined film. That includes even-odd switching of the structural layering and charging-induced solidification and melting, with important changes of local ordering between and within layers, and of squeezout behavior. By means of molecular dynamics simulations, we explore this variety of phenomena in the simplest charged Lennard-Jones coarse-grained model including or excluding the effect a neutral tail giving an anisotropic shape to one of the model ions. Using these models and open conditions permitting the flow of ions in and out of the interplate gap, we simulate the liquid squeezout to obtain the distance dependent structure and forces between the plates during their adiabatic appraoch under load. Simulations at fixed applied force illustrate an effective electrical pumping of the ionic liquid, from a thick nearly solid film that withstands the interplate pressure for high plate charge to complete squeezout following melting near zero charge. Effective enthalpy curves obtained by integration of interplate forces versus distance show the local minima that correspond to layering, and predict the switching between one minimum and another under squeezing and charging.
    The Journal of Chemical Physics 12/2014; 142(6). DOI:10.1063/1.4907747 · 3.12 Impact Factor
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    ABSTRACT: The static friction preventing the free sliding of nanosized rare gas solid islands physisorbed on incommensurate crystalline surfaces is not completely understood. Simulations modeled on Kr/Pb(111) highlights the importance and the scaling behavior of the island's edge contribution to static friction.
    Nanoscale 12/2014; 7(5). DOI:10.1039/C4NR06521B · 7.39 Impact Factor
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    ABSTRACT: The surface of a crystal made of roughly spherical molecules exposes, above its bulk rotational phase transition at T= Tr, a carpet of freely rotating molecules, possibly functioning as “nanobearings” in sliding friction. We explored by extensive molecular dynamics simulations the frictional and adhesion changes experienced by a sliding C60 flake on the surface of the prototype system C60 fullerite. At fixed flake orientation both quantities exhibit only a modest frictional drop of order 20% across the transition. However, adhesion and friction drop by a factor of 2 as the flake breaks its perfect angular alignment with the C60 surface lattice suggesting an entropy-driven aligned-misaligned switch during pull-off at Tr. The results can be of relevance for sliding Kr islands, where very little frictional differences were observed at Tr, but also to the sliding of C60 -coated tip, where a remarkable factor 2 drop has been reported.
    Nanoscale 09/2014; DOI:10.1039/C4NR04641B · 7.39 Impact Factor
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    ABSTRACT: Recent highly idealized model studies of lubricated nanofriction for two crystalline sliding surfaces with an interposed thin solid crystalline lubricant layer showed that the overall relative velocity of the lubricant $v_{\rm lub} / v_{\rm slider}$ depends only on the ratio of the lattice spacings, and retains a strictly constant value even when system parameters are varied within a wide range. This peculiar "quantized" dynamical locking was understood as due to the sliding-induced motion of misfit dislocations, or soliton structures. So far, the practical relevance of this concept to realistic sliding three dimensional crystals has not been demonstrated. In this work, by means of classical molecular dynamics simulations and theoretical considerations, we realize a realistic three-dimensional crystal-lubricant-crystal geometry. Results show that the flux of lubricant particles associated to the advancing soliton lines gives rise here too to a quantized velocity ratio. Moreover, depending on the interface lattice spacing mismatch, both forward and backward quantized motion of the lubricant is predicted. The persistence under realistic conditions of the dynamically pinned state and quantized sliding is further investigated by varying sliding speed, temperature, load, and lubricant film thickness. The possibilities of experimental observation of quantized sliding are also discussed.
    Physical Review B 02/2014; 89(9). DOI:10.1103/PhysRevB.89.094301 · 3.74 Impact Factor
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    Rosalie Laure Woulaché · Andrea Vanossi · Nicola Manini
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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_{RP}(x,s), whose shape can be varied continuously as a function of s, recovering the sine-Gordon potential as a 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 affect significantly and not trivially the existence and robustness of the recently reported velocity quantization phenomena [A. Vanossi 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.
    Physical Review E 07/2013; 88(1-1):012810. DOI:10.1103/PhysRevE.88.012810 · 2.33 Impact Factor
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    ABSTRACT: 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.
    Review of Modern Physics 04/2013; 85(2). DOI:10.1103/RevModPhys.85.529 · 42.86 Impact Factor
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    Davide Mandelli · Andrea Vanossi · Erio Tosatti
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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.
    Physical review. B, Condensed matter 03/2013; 87(19). DOI:10.1103/PhysRevB.87.195418 · 3.66 Impact Factor
  • Davide Mandelli · Andrea Vanossi · Erio Tosatti
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    ABSTRACT: Trapped cold ions are known to form linear or planar zigzag chains, helices or clusters depending on trapping conditions. They may be forced to slide over a laser induced corrugated potential, a mimick of sliding friction [1,2]. We present MD simulations of an incommensurate 101 ions chain sliding subject to an external electric field. As expected with increasing corrugation, we observe the transition from a smooth-sliding, highly lubric regime to a strongly dissipative stick-slip regime. Owing to inhomogeneity the dynamics shows features reminiscent of macroscopic frictional behaviors [3]. While the chain extremities are pinned, the incommensurate central part is initially free to slide. The onset of global sliding is preceded by precursor events consisting of partial slips of chain portions further from the center. We also look for frictional anomalies expected for the chain sliding across the linear-zigzag structural phase transition. Although the chain is too short for a proper critical behavior, the sliding friction displays a frank rise near the transition, due to opening of a new dissipative channel via excitations of transverse modes.[4pt] [1] A. Benassi et al, Nature Comm. 2, 236;[0pt] [2] T. Pruttivarasin et al, New Jour. of Phys. 13, 075012;[0pt] [3] S.M. Rubinstein et al, Nature 4, 1005.
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    A. Vanossi · A. Benassi · N. Varini · E. Tosatti
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    ABSTRACT: The increase of sliding friction upon increasing load is a classic in the macroscopic world. Here we discuss the possibility that friction rise might sometimes turn into a drop when, at the mesoscale and nanoscale, a confined lubricant film separating crystalline sliders undergoes strong layering and solidification. Under pressure, transitions from N to N-1 layers may imply a change of lateral periodicity of the crystallized lubricant sufficient to alter the matching of crystal structures, influencing the ensuing friction jump. A pressure-induced friction drop may occur as the shear gradient maximum switches from the lubricant middle, marked by strong stick-slip with or without shear melting, to the crystalline slider-lubricant interface, characterized by smooth superlubric sliding. We present high pressure sliding simulations to display examples of frictional drops, suggesting their possible relevance to the local behavior in boundary lubrication.
    Physical review. B, Condensed matter 01/2013; 87(4). DOI:10.1103/PhysRevB.87.045412 · 3.66 Impact Factor
  • Andrea Vanossi · Nicola Manini · Erio Tosatti
    Proceedings of the National Academy of Sciences 12/2012; 109(50):20774-20774. DOI:10.1073/pnas.1219292109 · 9.81 Impact Factor
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    ABSTRACT: We study the slippage of a tribological system of particles confined between a horizontally driven top plate and a vertically oscillating bottom plate. As shown in a recent article (Capozza et al., Phys Rev Lett 103:085502, 2009), tiny vibrations, when applied in a suitable range of frequencies, may suppress the high dissipative stick–slip dynamics reducing drastically the lateral friction force. Here, we generalize and prove the robustness of the results against the effect of quenched disorder in the confining substrates and the presence of adhesive and cohesive forces at the interface. The observed phenomenology is shown to hold true by moving from the previously considered two dimensional modeling to a more realistic three dimensional geometry. A detailed analysis is devoted to the case of short vibration pulses. These findings are relevant for nanoscale mechanics and in the context of earthquake or avalanches triggering.
    Tribology Letters 10/2012; 48(1). DOI:10.1007/s11249-012-0002-0 · 2.15 Impact Factor
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    Andrea Vanossi · Nicola Manini · Erio Tosatti
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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. DOI:10.1073/pnas.1213930109 · 9.81 Impact Factor
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    A. Benassi · A. Vanossi · G. E. Santoro · E. Tosatti
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    ABSTRACT: 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.
    Tribology Letters 08/2012; 48(1). DOI:10.1007/s11249-012-9936-5 · 2.15 Impact Factor
  • Andrea Vanossi · Andrea Benassi · Nicola Varini · Erio Tosatti
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    ABSTRACT: The sliding friction during pressure squeezout of a boundary lubricated contact has been shown [1,2] to undergo upward jumps every time a lubricant atomic layer is expelled. Here we ask the question whether the jump could not be downward. Whereas most studies focus on the layered structure which the confined lubricant takes in the normal direction, the element we wish to consider is a possible change of parallel periodicity occurring at the squeezout transition. Such changes have been reported in simulations [3], but their effect has not been discussed so far. One possible effect could be a transition of the slider-lubricant interface commensurability, producing a switch of the frictional mechanism, from lubricant melting-freezing in a commensurate state, to superlubric in an incommensurate one -- in this case with a drop of friction for increasing load. We exemplify this effect by MD simulations, where we replace for convenience the open squeezout system with a closed system, where the lubricant is sealed between the sliders. As the number of layers drops under pressure, the planar lubricant structural lattice parameter also drops. This change reflects in a sliding friction jump, which is easily observed to be downwards. The potential observability of load-induced friction drops will be discussed. [4pt] [1] J.N. Israelachvili et al., Science 240, 189 (1988). [0pt] [2] J. Gao et al., J. Phys. Chem. B 102, 5033 (1998). [0pt] [3] U. Tartaglino et al., J. Chem. Phys. 125, 014704 (2006).

Publication Stats

531 Citations
388.28 Total Impact Points


  • 2009–2015
    • Scuola Internazionale Superiore di Studi Avanzati di Trieste
      Trst, Friuli Venezia Giulia, Italy
    • INO - Istituto Nazionale di Ottica
      Florens, Tuscany, Italy
    • University of Milan
      Milano, Lombardy, Italy
  • 2014
    • Empa - Swiss Federal Laboratories for Materials Science and Technology
      Duebendorf, Zurich, Switzerland
  • 2012
    • National Research Council
      Roma, Latium, Italy
  • 2002–2011
    • Università degli Studi di Modena e Reggio Emilia
      Modène, Emilia-Romagna, Italy
  • 2001–2003
    • Los Alamos National Laboratory
      • Center for Nonlinear Studies
      Los Alamos, CA, United States
  • 2000–2001
    • University of Bologna
      Bolonia, Emilia-Romagna, Italy