Friction is a fundamental phenomenon that affects many different areas of human life and technology. In this Colloquium the microscopic origins of friction are reviewed in light of recent ...developments that are beginning to unveil the processes at the interface of two sliding materials. Large-scale simulations and theoretical modeling help reveal the complex nature of this basic phenomenon which is still the subject of much controversy. 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.
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 T=0, 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 two-dimensional (2D) case, simulations show a sharp Aubry transition between an unpinned and a pinned phase as a function of corrugation. Unlike one dimension, the 2D transition is now of first order, and, importantly, remains well defined at T>0. It is heavily structural, with a local rotation of moiré pattern domains from the nonzero initial Novaco-McTague equilibrium angle to nearly zero. In the temperature (T)-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 T=Tc, marked by a susceptibility peak. The expected static sliding friction upswing between the unpinned and the pinned phase decreases and disappears upon heating from T=0 to T=Tc. The experimental pursuit of this novel scenario is proposed.
The ultra-low kinetic friction Fk of 2D structurally superlubric interfaces, connected with the fast motion of the incommensurate moiré pattern, is often invoked for its linear increase with velocity ...v0 and area AS, but never seriously addressed and calculated so far. Here we do that, exemplifying with a twisted graphene layer sliding on top of bulk graphite – a demonstration case that could easily be generalized to other systems. Neglecting quantum effects and assuming a classical Langevin dynamics, we derive friction expressions valid in two temperature regimes. At low temperatures the nonzero sliding friction velocity derivative dFk/dv0 is shown by Adelman–Doll–Kantorovich type approximations to be equivalent to that of a bilayer whose substrate is affected by an analytically derived effective damping parameter, replacing the semi-infinite substrate. At high temperatures, friction grows proportional to temperature as analytically required by fluctuation–dissipation. The theory is validated by non-equilibrium molecular dynamics simulations with different contact areas, velocities, twist angles and temperatures. Using 6°-twisted graphene on Bernal graphite as a prototype we find a shear stress of measurable magnitude, from 25kPa at low temperature to 260kPa at room temperature, yet only at high sliding velocities such as 100m/s. However, it will linearly drop many orders of magnitude below measurable values at common experimental velocities such as 1μm/s, a factor 10−8 lower. The low but not ultra-low “engineering superlubric” friction measured in existing experiments should therefore be attributed to defects and/or edges, whose contribution surpasses by far the negligible moiré contribution.
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 sub(lub)/v sub(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 with 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.
We compare the electronic structure of the unreconstructed Au(1
1
1) surface calculated within density functional theory by means of plane waves and fully-relativistic ultrasoft pseudo-potentials ...(US-PPs) on one hand, and scalar relativistic US-PPs on the other hand, where spin-orbit is not included. Several surface states are identified and discussed, in particular their character and spin polarization caused by the spin-orbit interaction. Besides the well known splitting of the
L-gap shallow surface state near the Fermi energy, the spin-orbit interaction generates, modifies, and splits many other deep surface states in the range 1–10
eV below
E
F. The existence and spin-orbit splitting of these deeper states should be experimentally detectable by angle-resolved photoelectron spectroscopy.
We reconsider the applicability of classical nucleation theory (CNT) to the calculation of the free energy of solid cluster formation in a liquid and its use to the evaluation of interface free ...energies from nucleation barriers. Using two different freezing transitions (hard spheres and NaCl) as test cases, we first observe that the interface-free-energy estimates based on CNT are generally in error. As successive refinements of nucleation-barrier theory, we consider corrections due to a nonsharp solid-liquid interface and to a nonspherical cluster shape. Extensive calculations for the Ising model show that corrections due to a nonsharp and thermally fluctuating interface account for the barrier shape with excellent accuracy. The experimental solid nucleation rates that are measured in colloids are better accounted for by these non-CNT terms, whose effect appears to be crucial in the interpretation of data and in the extraction of the interface tension from them.
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