6,179 research outputs found
Lubricated friction between incommensurate substrates
This paper is part of a study of the frictional dynamics of a confined solid
lubricant film - modelled as a one-dimensional chain of interacting particles
confined between two ideally incommensurate substrates, one of which is driven
relative to the other through an attached spring moving at constant velocity.
This model system is characterized by three inherent length scales; depending
on the precise choice of incommensurability among them it displays a strikingly
different tribological behavior. Contrary to two length-scale systems such as
the standard Frenkel-Kontorova (FK) model, for large chain stiffness one finds
that here the most favorable (lowest friction) sliding regime is achieved by
chain-substrate incommensurabilities belonging to the class of non-quadratic
irrational numbers (e.g., the spiral mean). The well-known golden mean
(quadratic) incommensurability which slides best in the standard FK model shows
instead higher kinetic-friction values. The underlying reason lies in the
pinning properties of the lattice of solitons formed by the chain with the
substrate having the closest periodicity, with the other slider.Comment: 14 pagine latex - elsart, including 4 figures, submitted to Tribology
Internationa
Numerical Methods for Multilattices
Among the efficient numerical methods based on atomistic models, the
quasicontinuum (QC) method has attracted growing interest in recent years. The
QC method was first developed for crystalline materials with Bravais lattice
and was later extended to multilattices (Tadmor et al, 1999). Another existing
numerical approach to modeling multilattices is homogenization. In the present
paper we review the existing numerical methods for multilattices and propose
another concurrent macro-to-micro method in the numerical homogenization
framework. We give a unified mathematical formulation of the new and the
existing methods and show their equivalence. We then consider extensions of the
proposed method to time-dependent problems and to random materials.Comment: 31 page
Melting-freezing cycles in a relatively sheared pair of crystalline monolayers
The nonequilibrium dynamical behaviour that arises when two ordered
two-dimensional monolayers of particles are sheared over each other is studied
in Brownian dynamics simulations. A curious sequence of nonequilibrium states
is observed as the driving rate is increased, the most striking of which is a
sliding state with irregular alternation between disordered and ordered states.
We comment on possible mechanisms underlying these cycles, and experiments that
could observe them.Comment: 7 pages, 8 figures, minor changes in text and figures, references
adde
Improved Algorithms for Simulating Crystalline Membranes
The physics of crystalline membranes, i.e. fixed-connectivity surfaces
embedded in three dimensions and with an extrinsic curvature term, is very rich
and of great theoretical interest. To understand their behavior, numerical
simulations are commonly used. Unfortunately, traditional Monte Carlo
algorithms suffer from very long auto-correlations and critical slowing down in
the more interesting phases of the model. In this paper we study the
performance of improved Monte Carlo algorithms for simulating crystalline
membrane, such as hybrid overrelaxation and unigrid methods, and compare their
performance to the more traditional Metropolis algorithm. We find that although
the overrelaxation algorithm does not reduce the critical slowing down, it
gives an overall gain of a factor 15 over the Metropolis algorithm. The unigrid
algorithm does, on the other hand, reduce the critical slowing down exponent to
z apprx. 1.7.Comment: 14 pages, 1 eps-figur
Topological mechanics of gyroscopic metamaterials
Topological mechanical metamaterials are artificial structures whose unusual
properties are protected very much like their electronic and optical
counterparts. Here, we present an experimental and theoretical study of an
active metamaterial -- comprised of coupled gyroscopes on a lattice -- that
breaks time-reversal symmetry. The vibrational spectrum of these novel
structures displays a sonic gap populated by topologically protected edge modes
which propagate in only one direction and are unaffected by disorder. We
present a mathematical model that explains how the edge mode chirality can be
switched via controlled distortions of the underlying lattice. This effect
allows the direction of the edge current to be determined on demand. We
envision applications of these edges modes to the design of loss-free, one-way,
acoustic waveguides and demonstrate this functionality in experiment
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