109 research outputs found
Optimal Energy Dissipation in Sliding Friction Simulations
Non-equilibrium molecular dynamics simulations, of crucial importance in
sliding friction, are hampered by arbitrariness and uncertainties in the
removal of the frictionally generated Joule heat. Building upon general
pre-existing formulation, we implement a fully microscopic dissipation approach
which, based on a parameter-free, non-Markovian, stochastic dynamics, absorbs
Joule heat equivalently to a semi-infinite solid and harmonic substrate. As a
test case, we investigate the stick-slip friction of a slider over a
two-dimensional Lennard-Jones solid, comparing our virtually exact frictional
results with approximate ones from commonly adopted dissipation schemes.
Remarkably, the exact results can be closely reproduced by a standard Langevin
dissipation scheme, once its parameters are determined according to a general
and self-standing variational procedure
Experimental observation of the Aubry transition in two-dimensional colloidal monolayers
The possibility to achieve entirely frictionless, i.e. superlubric, sliding
between solids, holds enormous potential for the operation of mechanical
devices. At small length scales, where mechanical contacts are well-defined,
Aubry predicted a transition from a superlubric to a pinned state when the
mechanical load is increased. Evidence for this intriguing Aubry transition
(AT), which should occur in one dimension (1D) and at zero temperature, was
recently obtained in few-atom chains. Here, we experimentally and theoretically
demonstrate the occurrence of the AT in an extended two-dimensional (2D) system
at room temperature using a colloidal monolayer on an optical lattice. Unlike
the continuous nature of the AT in 1D, we observe a first-order transition in
2D leading to a coexistence regime of pinned and unpinned areas. Our data
demonstrate that the original concept of Aubry does not only survive in 2D but
is relevant for the design of nanoscopic machines and devices at ambient
temperature.Comment: 12 pages including 4 figures + 9 pages supplemental informatio
Controlling Microscopic Friction through Mechanical Oscillations
We study in detail the recent suggestions by Tshiprut et al. [Phys. Rev.
Lett. 95, 016101 (2005)] to tune tribological properties at the nanoscale by
subjecting a substrate to periodic mechanical oscillations. We show that both
in stick-slip and sliding regimes of motion friction can be tuned and reduced
by controlling the frequency and amplitude of the imposed substrate lateral
excitations. We demonstrate that the mechanisms of oscillation-induced
reduction of friction are different for stick-slip and sliding dynamics. In the
first regime the effect results from a giant enhancement of surface diffusion,
while in the second regime it is due to the interplay between washboard and
oscillation frequencies that leads to the occurrence of parametric resonances.
Moreover we show that for particular set of parameters it is possible to
sustain the motion with the only oscillations
Recent highlights in nanoscale and mesoscale friction
Friction is the oldest branch of non-equilibrium condensed matter physics and, at the same time, the least established at the fundamental level. A full understanding and control of friction is increasingly recognized to involve all relevant size and time scales. We review here some recent advances on the research focusing of nano- and mesoscale tribology phenomena. These advances are currently pursued in a multifaceted approach starting from the fundamental atomic-scale friction and mechanical control of specific single-asperity combinations, e.g., nanoclusters on layered materials, then scaling up to the meso/microscale of extended, occasionally lubricated, interfaces and driven trapped optical systems, and eventually up to the macroscale. Currently, this "hot" research field is leading to new technological advances in the area of engineering and materials science
Detachment Dynamics of Graphene Nanoribbons on Gold
Metal-surface physisorbed graphene nanoribbons (GNRs) constitute mobile nanocontacts whose interest is simultaneously mechanical, electronic, and tribological. Previous work showed that GNRs adsorbed on Au(111) generally slide smoothly and superlubrically owing to the incommensurability of their structures. We address here the nanomechanics of detachment, as realized when one end is picked up and lifted by an AFM cantilever. AFM nanomanipulations and molecular-dynamics (MD) simulations identify two successive regimes, characterized by (i) a progressively increasing local bending, accompanied by the smooth sliding of the adhered part, followed by (ii) a stick-slip dynamics involving sudden bending relaxation associated with intermittent jumps of the remaining adhered GNR segment and tail end. AFM measurements of the vertical force exhibit oscillations which, compared with MD simulations, can be associated with the successive detachment of individual GNR unit cells of length 0.42 nm. Extra modulations within one single period are caused by steplike advancements of the still-physisorbed part of the GNR. The sliding of the incommensurate moir\ue9 pattern that accompanies the GNR lifting generally yields an additional long-period oscillation: while almost undetectable when the GNR is aligned in the standard "R30" orientation on Au(111), we predict that such feature should become prominent in the alternative rotated "R0" orientation on the same surface, or on a different surface, such as perhaps Ag(111)
About females and males: continuity and discontinuity in flies
Through the decades of relentless and dedicated studies in Drosophila melanogaster, the pathway that governs sexual development has been elucidated in great detail and has become a paradigm in understanding fundamental cell-fate decisions. However, recent phylogenetic studies show that the molecular strategy used in Drosophila deviates in some important aspects from those found in other dipteran flies and suggest that the Drosophila pathway is likely to be a derivative of a simpler and more common principle. In this essay, I will discuss the evolutionary plasticity of the sex-determining pathway based on studies in the common housefly, Musca domestica. Diversification appears to primarily arise from subtle differences in the regulation of the key switch gene transformer at the top of the pathway. On the basis of these findings I propose a new idea on how the Drosophila pathway may have evolved from a more archetypal system such as in M. domestica. In essence, the arrival of an X counting mechanism mediated by Sex-lethal to compensate for X linked gene dose differences set the stage for an intimate coupling of the two pathways. Its precedent recruitment to the dosage compensation pathway allowed for an intervention in the regulation of transformer where it gradually and eventually' completely substituted for a need of transformer autoregulation
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