Overview of active research fields
Modeling and molecular dynamics simulations of atomistic processes
Advances in the theoretical understanding of interatomic interactions in material science and computer-based modeling of complex systems have led to molecular dynamics simulations for exploring atomic-scale behaviors. Numerical simulations have revealed themselves to be extremely useful in investigating processes at the microscopic scale and, in the years to come, are expected to play an expanding role in science. Their distinctiveness consists of allowing scientists to follow and analyze the dynamics of all atoms in controlled computational “experiments” where the system geometry, driving conditions, interparticle interactions, and so on, can be varied at will to explore their effect on the physical system response. In the context of nanotribology, for example, this approach has revealed, no doubt, a great deal of information about the atomistic origins of static and kinetic friction, hysteresis, stick–slip dynamics, boundary lubrication, and the interplay between molecular geometry and tribological properties.
Nanomanipulation, dynamics and diffusion of nano-objects on surfaces
The high complexity of dealing with systems with many degrees of freedom under a strict size confinement arises especially in sliding friction phenomena, where the key mechanisms take place at a buried interface. In the last decades, growth of new materials with tailored features down to the nanoscale and developments in nanotechnology have extended the experimental study of friction, permitting the analysis on well-characterized surfaces at the smaller scales. Controlled manipulation of individual molecules and clusters can be used to build new molecular suprastructures, to explore the influence of the environment on a molecule, or to perform “engineering” operations at the ultimate limits of fabrication for new high-tech devices. By presenting case studies of deposited metallic clusters, physisorbed rare-gas islands and molecular motors, we investigate through modeling and MD simulations the frictional properties of nano-objects functioning at surfaces.
Theory and simulations of generalized Frenkel-Kontorova type models
Universal models which can be applied to describe a variety of different phenomena are very rare in physics and, therefore, they are of key importance. Over the years a relatively simple model, known these days as the Frenkel-Kontorova (FK) model, has become one of the fundamental and universal tools of low-dimensional nonlinear physics. In its simplest version, it describes a 1D chain of interacting particles subjected to a substrate on-site potential. This model, together with its several extensions, has been successfully applied to describe a broad spectrum of effects of distinct physical nature: the theory of dislocations in metals, domain walls in ferroelectrics, the problem of crowdion in a metal, superionic conductors, sub-monolayer films of atoms on crystal surfaces, the surface reconstruction phenomena, the DNA dynamics and denaturation, the theory of Josephson junctions, and, last but not least, tribology.
Trapped Optical Systems
From colloidal mesoscale suspensions down to the nanoworld of cold ions, the use of artificial tribology emulators has recently taken us to the ultimate limit of atomic friction. The field of nanotribology can now benefit from the experimental opportunities offered by handling nanoparticles with artificial optical potentials, opening the possibility to change parameters almost freely and to visualize directly the elemental mechanisms of sliding friction. Supporting, or even anticipating, the experimental findings, theory and numerical simulations expose how properties such as substrate geometry and corrugation, temperature, driving velocity and lattice mismatch may influence the tribological response, from pinned configurations and intermittent stick-slip dynamics to frictionless regimes of motion.
Nanomechanics and tribology of graphitic materials
The recent advances in the preparation techniques of graphene and its semiconductor or insulator counterparts (e.g. h-Si, h-BN, respectively) have made possible to control with very high accuracy the growth of sheets, nanotubes, and other variants, thus allowing large surfaces with few or no defects. This achievement is fundamental to permit an ideal comparison of experimental data with theoretical simulations, and to have a clear understanding (modeling) of the physics that governs these nanostructures at a fundamental level. Another advantage of low defects concentration is the reduced frictional force between sliding incommensurate sheets. This condition can be exploited in applications like nanobearings and nanomotors, that will drive the future robotics. Simulations can assist the engineering of these nano-objects at the atomic level, and can drive the developement of prototypes by assisting the experiments.
Confined systems under shear, lubricated friction and ionic liquids
Confined lubricants under shear display intriguing nano- and mesoscale tribological phenomena. The intervening lubricant film between two sliding solid surfaces generally changes from liquid (with hydrodynamic lubrication) to nearly solid when the film is only a few monolayers thick. Both experiments and simulations find that in this regime, the film develops a solidlike layered structure, supporting static friction, and a strong stick-slip dissipative behavior. Here, the sliding frictional properties turn out to be significantly affected by the interface incommensurability or the presence of nanoscale superficial patterning of the moving confining walls. In this framework, recent measurements suggest also the possibility to exploit ionic liquids as smart lubricants for nanocontacts, tuning their tribological and rheological properties by charging the sliding interfaces.
New approaches to control and modify tribological properties and dissipative mechanisms at the nanoscale
Exploring novel routes to achieve friction control is a fundamental goal currently pursued in nanoscience and nanotechnology. The traditional lubrication control of frictional forces in macroscopic mechanical contacts is impractical at the nanoscale, where mating surfaces are likelier to succumb to capillary forces. Therefore, novel methods for control and manipulation of friction in nano and intermediate mesoscale sliding interfaces (e.g., tuned mechanically-induced oscillations, mismatched contact geometries, applied external fields, substrate phase transitions) are constantly investigated via modeling and simulations.
Energy dissipation in molecular dynamics simulations of sliding friction
MD Simulations are making a large and increasing contribution to such growing fields as rheology, and tribology, which sit at the lively interface between basic condensed matter physics and technology, especially active in the nanoscience area. How to make the forced dynamics of a small system (the simulation cell), equivalent to the real infinite one, is a time-honored problem in non-equilibrium physics, one that has repeatedly been addressed in the literature, but, somewhat surprisingly, with no real attempt at realizing this program in real simulations. We implement a fully microscopic dissipation scheme which, based on a parameter-free, non-markovian, stochastic dynamics, absorbs Joule heat precisely as a semi-infinite solid substrate would. The problem is not just one of principle, for in many cases the resulting dynamical steady state (and its physical features) actually depends upon the choice of the ad-hoc introduced dissipative parameters.
Advances in several research fields have led to a progressive change of paradigm during the last decades, as the concept of order without periodicity has emerged to properly describe an increasing number of complex systems. Shortly after discovery of quasicrystals, it was realized that their well-ordered, yet non-periodic structure is dictated by a rule other than periodicity and closely related to the description of incommensurate structures (e.g., 1D Fibonacci sequence or 2D Penrose tiling). Wearless atomic friction force experiments have recently demonstrated that tribological response in quasicrystals could be related to the exotic atomic structure of the bulk material. By numerical simulations, we address the origin of the experimentally observed friction anisotropy on a twofold decagonal quasicrystal surface. We investigate 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.
Pattern formation and energy localization in complex multi-scale systems
Pattern formation in physics, chemistry and biology are but some of the fields in which these exciting “geometrical” entities can be observed, showing us how structures arising in so many disparate areas can be similar and interrelated. Methods are needed to characterize the differences and the similarities in patterns that develop in different systems, as well as in patterns formed in one system for different experimental conditions. Indeed, the ubiquity of the phenomenon of pattern formation leads to the search for theoretical formulations that unify the discussion of the diverse systems. From the viewpoint of fundamental science, patterns are of interest because their formation takes advantage of the spontaneous self-organized processes occurring in nature, a tendency often related to strong nonlinear effects and energy localization. Generally, the pattern-forming ability marks the point at which a nonequilibrium system is driven across a particular threshold as a result of competing forces, symmetry breaking, nucleation of dissipative structures, instabilities, scale invariance, bifurcations, and higher-order phase transitions.