14 research outputs found
Role of surface roughness in superlubricity
We study the sliding of elastic solids in adhesive contact with flat and
rough interfaces. We consider the dependence of the sliding friction on the
elastic modulus of the solids. For elastically hard solids with planar surfaces
with incommensurate surface structures we observe extremely low friction
(superlubricity), which very abruptly increases as the elastic modulus
decreases. We show that even a relatively small surface roughness may
completely kill the superlubricity state.Comment: 11 pages, 17 figures, format revte
Surface molecular dynamics simulation with two orthogonal surface steps: how to beat the particle conservation problem
Due to particle conservation, Canonical Molecular Dynamics (MD) simulations
fail in the description of surface phase transitions involving coverage or
lateral density changes. However, a step on the surface can act effectively as
a source or a sink of atoms, in the simulation as well as in real life. A
single surface step can be introduced by suitably modifying planar Periodic
Boundary Conditions (PBC), to accommodate the generally inequivalent stacking
of two adjacent layers. We discuss here how, through the introduction of two
orthogonal surface steps, particle number conservation may no longer represent
a fatal constraint for the study of these surface transitions. As an example,
we apply the method for estimating temperature-induced lateral density increase
of the reconstructed
Au (001) surface; the resulting anisotropic cell change is consistent with
experimental observations. Moreover, we implement this kind of scheme in
conjunction with the variable curvature MD method, recently introduced by our
group.Comment: 9 pages, 5 figures, accepted for publication in Surface Science
(ECOSS-19
Velocity plateaus and jumps in carbon nanotube sliding
The friction between concentric carbon nanotubes sliding one inside the other
has been widely studied and simulated, but not so far using external force as
the driving variable. Our molecular dynamics (MD) simulations show that as the
pulling force grows, the sliding velocity increases by jumps and plateaus
rather than continuously as expected. Dramatic friction peaks (similar to that
recently noted by Tangney {\it et al.} in Phys. Rev. Lett. 97 (2006) 195901)
which develop around some preferential sliding velocities, are at the origin of
this phenomenon. The (stable) rising edge of the peak produces a velocity
plateau; the (unstable) dropping edge produces a jump to the nearest stable
branch. The outcome is reminiscent of conduction in ionized gases, the plateau
correspon ding to a current stabilization against voltage variations, the jump
corresponding to a discharge or breakdown.Comment: 9 pages, 5 color figures, format latex Elsart. Surface Science, in
press, http://dx.doi.org/10.1016/j.susc.2007.05.03
Ballistic nanofriction
Sliding parts in nanosystems such as Nano ElectroMechanical Systems (NEMS)
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 novel "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
Equazioni differenziali stocastiche per superfici: analisi numerica del corrugamento in condizioni di crescita
Dottorato di ricerca in fisica. 12. ciclo. Supervisore Andrea C. Levi. Referee Enzo TosattiConsiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7, Rome; Biblioteca Nazionale Centrale - Piazza Cavalleggeri, 1, Florence / CNR - Consiglio Nazionale delle RichercheSIGLEITItal
Towards realistic simulations of polymer networks: tuning vulcanisation and mechanical properties
Simulations of coarse-grained network models have long been used to test theoretical predictions about rubber elasticity, while atomistic models are still largely unexplored. Here we devise a novel algorithm for the vulcanisation of united-atom poly(cis-1,4-butadiene), characterize the topology of the resulting networks and test their mechanical properties. We observe clear changes in the network structure when using slower vulcanisation, contrary to the traditional view that cross-linking simply freezes the melt configuration. Non-ideality of our networks reverberates on the distribution of strand length and on the strands deformation, which is highly non-affine, especially for short strands. Nevertheless, we do recover some of the trends observed on ideal bead-and-spring networks and controlled laboratory experiments, such as the linear relationships linking the degree of cross-linking and the density. We also compare different deformation methods and find step-equilibrium protocols to be more reliable. Regardless of the adopted method, it is advisable to precede the deformation by a pre-stretching cycle in order to release internal stresses accumulated during the vulcanisation
Viscoelasticity of Short Polymer Liquids from Atomistic Simulations
The viscosity â or more generally the viscoelasticity â of polymer liquids is a key property for the processing as well as the performance of these materials. Molecular theories and numerical methods can provide these quantities, but they all require certain input parameters that nowadays are typically obtained by experiment. In the long term, it would be desirable to obtain these parameters or the whole viscoelastic response by purely computational methods, enabling a full âin silicoâ design of new materials and processes. In this perspective, we present several test calculations of the viscosity of n-hexadecane, a short-chain analogue of polyethylene. Our calculations are based on both equilibrium and non-equilibrium molecular dynamics (MD) simulations, which are applied to models based on a united-atom force field, a conventional atomistic force field, and the AIREBO-M reactive force field. We compare both the computational cost of the different strategies and the reliability of the different models and we provide some general guidelines for their application to more complex systems
A Coarse-Grained Force Field for SilicaâPolybutadiene Interfaces and Nanocomposites
We present a coarse-grained force field for modelling silicaâpolybutadiene interfaces and nanocomposites. The polymer, poly(cis-1,4-butadiene), is treated with a previously published united-atom model. Silica is treated as a rigid body, using one Si-centered superatom for each SiO2 unit. The parameters for the cross-interaction between silica and the polymer are derived by Boltzmann inversion of the density oscillations at model interfaces, obtained from atomistic simulations of silica surfaces containing both Q4 (hydrophobic) and Q3 (silanol-containing, hydrophilic) silicon atoms. The performance of the model is tested in both equilibrium and non-equilibrium molecular dynamics simulations. We expect the present model to be useful for future large-scale simulations of rubberâsilica nanocomposites
Fracture in Silica/Butadiene Rubber: A Molecular Dynamics View of DesignâProperty Relationships
Despite intense investigation, the mechanisms governing the mechanical reinforcement of polymers by dispersed nanoparticles have only been partially clarified. This is especially true for the ultimate properties of the nanocomposites, which depend on their resistance to fracture at large deformations. In this work, we adopt molecular dynamics simulations to investigate the mechanical properties of silica/polybutadiene rubber, using a quasi-atomistic model that allows a meaningful description of bond breaking and fracture over relatively large length scales. The behavior of large nanocomposite models is explored systematically by tuning the cross-linking, grafting densities, and nanoparticle concentration. The simulated stressâstrain curves are interpreted by monitoring the breaking of chemical bonds and the formation of voids, up to complete rupture of the systems. We find that some chemical bonds, and particularly the SâS linkages at the rubberânanoparticle interface, start breaking well before the appearance of macroscopic features of fracture and yield