148 research outputs found
Sliding Nanofriction in Low Dimensional Model-Systems
Thanks to novel experimental techniques, physicists are now able to characterize the dynamics of interacting surfaces down to molecular length scales. This possibility has brought fresh interest in the field of friction and has opened a new branch of research, nanotribology, whose aim is the study of tribological properties of sliding systems in terms of fundamental atomistic dissipative mechanisms. Far from being a mere academic problem, understanding and controlling friction at the nano-scales can have a great impact on many technological applications, from energy conversion and saving, to transportation and micro-machining. To reach this goal it is essential to develop theoretical tools able to tackle this problem. In absence of a general theory of friction, molecular dynamics (MD) simulations represent, at the moment, one of the most powerful approaches able to explain and predict the behaviour of nanoscale interfaces. Within the context of nanotribology, this thesis deals with some fundamental aspects of friction between dry crystalline surfaces. Anticipating experiments, it reports results obtained by means of realistic MD simulations of artificial model-systems currently under experimental investigation for application in nanotribology.
Qualitatively, the sliding properties of crystalline interfaces can be interpreted based on the mutual interaction between the potential energy landscapes generated by the two touching lattices. These may be described as periodic, two dimensional sequences of wells and hills, corresponding to repulsive and attractive regions of the surfaces. Commensurate geometries correspond to atomically locked configurations, where the atoms of each interacting plane adapt into the wells of the potential landscape generated by the other one. When driven out of equilibrium by an applied shear force, this kind of atomic locking, or interdigitation, generally determines mechanical instabilities, which in turn lead to violent depinning events and high dissipation. Atomic locking may still occur, but may also be avoided in incommensurate interfaces, in which case friction shows more smooth and gentle sliding regimes. The ordered atomic arrangement of crystalline surfaces therefore offers a peculiar way to reduce friction, that is by controlling the geometry of the interface, e.g., by rotating relative to each other two originally aligned and commensurate surfaces.
The above picture applies to clean, chemically inert, and atomically flat crystal surfaces. To describe in a quantitative way real systems, one has to take into account many other effects whose interplay determine the overall tribological response. They include elasticity of the surfaces, plastic deformations, and the presence of steps, defects and impurities, not counting chemical interactions and other processes involving the direct excitations of electronic degrees of freedom. From the experimental point of view, the fine features of the interface are hardly accessible because buried. Usually only macroscopic average values of some arising physical quantities are measured, which hinders the possibility to keep track of each distinct mechanism at play. From the theoretical point of view, developing a general theory different from brute-force with quantitative predictive power is also difficult, and phenomenological models are usually adopted which apply only to a reduced number of cases. However, there exist a class of real systems where these complications are absent or mitigated, allowing for detailed experimental and theoretical investigations. On one hand, one (1D) and two dimensional (2D) artificial crystals of charged particles trapped inside optical lattices offer the possibility to study the dynamics of ideal crystalline interfaces with all interface parameters under control, including commensurability and substrate interaction strength. On the other hand, surface science and ultra high vacuum techniques supply clean and atomically flat substrates, suitable for the study of the sliding properties of two dimensional monolayers of adsorbate atoms. Both these systems are very well characterized, and allow for accurate realistic MD simulations where geometrical effects, and interface elastic and plastic deformations effects are investigated in great details.
In view of future experiments, this thesis reports results of MD investigations of some fundamental tribological aspects in low dimensional incommensurate interfaces formed by: (i) 1D cold-ion chains trapped in optical lattices, (ii) 2D charged colloids monolayers interacting with laser-induced periodic potentials, (iii) 2D islands of rare gas atoms physisorbed on clean metallic substrates. These simulations show that incommensurate linear chains of trapped cold ions display a rich dynamics when forced to slide over a periodic corrugated potential. That suggests that they can be adopted to investigate in detail the external-load dependent transition between the intermittent stick-slip motion and the smooth sliding regime, as well as the precursor dynamics preceding the onset of motion. Both of them are shown to display paradigmatic behaviours observed in sliding contacts at any length scales. Incommensurate two dimensional interfaces realized by colloidal crystals in periodic fields have been simulated to study the pinning transition from the locked (finite static friction) state, to the ``superlubric'' -- zero static friction -- free-sliding state, which is predicted to occur as a function of decreasing substrate potential strength. In a range of parameters compatible with recent experiments, that is a first order (structural) phase transition of the colloid monolayer, showing analogies with the superlubric to pinned Aubry transition" extensively studied in the one dimensional discrete Frenkel-Kontorova model of dry friction. Moreover, realistic simulations show that relative misfit rotations between the colloidal slider and substrate may significantly affect the dissipation under steady sliding even in genuinely incommensurate geometries, by changing the degree to which the soft deformable slider interdigitates within the hard, non deformable optical substrate. This is a result of more general validity, since a similar mechanism must be at play in any incommensurate 2D crystalline interface. Finally, in order to understand the persistent static friction force observed in quartz crystal microbalance experiments on highly clean surfaces, extensive numerical simulations of substrate-incommensurate model rare-gas islands have been performed, which, in absence of any other sources of pinning, describe how the island edges alone may play the ultimate role in determining the overall barrier preventing the onset of global sliding
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
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 of Physisorbed Nanotubes: Rolling or Sliding?
The structure and motion of carbon and h-BN nanotubes (NTs) deposited on
graphene is inquired theoretically by simulations based on state-of-the-art
interatomic force fields. Results show that any typical cylinder-over-surface
approximation is essentially inaccurate. NTs tend to flatten at the interface
with the substrate and upon driving they can either roll or slide depending on
their size and on their relative orientation with the substrate. In the
epitaxially aligned orientation we find that rolling is always the main
mechanism of motion, producing a kinetic friction linearly growing with the
number of walls, in turn causing an unprecedented supra-linear scaling with the
contact area. A 30 degrees misalignment raises superlubric effects, making
sliding favorable against rolling. The resulting rolling-to-sliding transition
in misaligned NTs is explained in terms of the faceting appearing in large
multi-wall tubes, which is responsible for the increased rotational stiffness.
Modifying the geometrical conditions provides an additional means of
drastically tailoring the frictional properties in this unique tribological
system
CNN-based fast source device identification
Source identification is an important topic in image forensics, since it
allows to trace back the origin of an image. This represents a precious
information to claim intellectual property but also to reveal the authors of
illicit materials. In this paper we address the problem of device
identification based on sensor noise and propose a fast and accurate solution
using convolutional neural networks (CNNs). Specifically, we propose a
2-channel-based CNN that learns a way of comparing camera fingerprint and image
noise at patch level. The proposed solution turns out to be much faster than
the conventional approach and to ensure an increased accuracy. This makes the
approach particularly suitable in scenarios where large databases of images are
analyzed, like over social networks. In this vein, since images uploaded on
social media usually undergo at least two compression stages, we include
investigations on double JPEG compressed images, always reporting higher
accuracy than standard approaches
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