108 research outputs found
Manipulations of individual molecules by scanning probe microscopy
In this Letter we suggest a new method of manipulating individual molecules
with scanning probes using a "pick-up-and put-down" mode. We demonstrate that
the number of molecules picked up by the tip and deposited in a different
location can be controlled by adjusting the pulling velocity of the tip and the
distance of closest approach of the tip to the surface
Static and dynamic friction in sliding colloidal monolayers
In a pioneer experiment, Bohlein et al. realized the controlled sliding of
two-dimensional colloidal crystals over laser-generated periodic or
quasi-periodic potentials. Here we present realistic simulations and arguments
which besides reproducing the main experimentally observed features, give a
first theoretical demonstration of the potential impact of colloid sliding in
nanotribology. The free motion of solitons and antisolitons in the sliding of
hard incommensurate crystals is contrasted with the soliton-antisoliton pair
nucleation at the large static friction threshold Fs when the two lattices are
commensurate and pinned. The frictional work directly extracted from particles'
velocities can be analysed as a function of classic tribological parameters,
including speed, spacing and amplitude of the periodic potential (representing
respectively the mismatch of the sliding interface, and the corrugation, or
"load"). These and other features suggestive of further experiments and
insights promote colloid sliding to a novel friction study instrument.Comment: in print in the Proceedings of the National Academy of Sciences
U.S.A. This v2 is identical to v1, but includes ancillary material. A few
figures were undersampled due to size limits: those in v1 are far sharpe
Atomic scale engines: Cars and wheels
We introduce a new approach to build microscopic engines on the atomic scale
that move translationally or rotationally and can perform useful functions such
as pulling of a cargo. Characteristic of these engines is the possibility to
determine dynamically the directionality of the motion. The approach is based
on the transformation of the fed energy to directed motion through a dynamical
competition between the intrinsic lengths of the moving object and the
supporting carrier.Comment: 4 pages, 3 figures (2 in color), Phys. Rev. Lett. (in print
Molecular motor that never steps backwards
We investigate the dynamics of a classical particle in a one-dimensional
two-wave potential composed of two periodic potentials, that are
time-independent and of the same amplitude and periodicity. One of the periodic
potentials is externally driven and performs a translational motion with
respect to the other. It is shown that if one of the potentials is of the
ratchet type, translation of the potential in a given direction leads to motion
of the particle in the same direction, whereas translation in the opposite
direction leaves the particle localized at its original location. Moreover,
even if the translation is random, but still has a finite velocity, an
efficient directed transport of the particle occurs.Comment: 4 pages, 5 figures, Phys. Rev. Lett. (in print
Surface Roughness and Effective Stick-Slip Motion
The effect of random surface roughness on hydrodynamics of viscous
incompressible liquid is discussed. Roughness-driven contributions to
hydrodynamic flows, energy dissipation, and friction force are calculated in a
wide range of parameters. When the hydrodynamic decay length (the viscous wave
penetration depth) is larger than the size of random surface inhomogeneities,
it is possible to replace a random rough surface by effective stick-slip
boundary conditions on a flat surface with two constants: the stick-slip length
and the renormalization of viscosity near the boundary. The stick-slip length
and the renormalization coefficient are expressed explicitly via the
correlation function of random surface inhomogeneities. The effective
stick-slip length is always negative signifying the effective slow-down of the
hydrodynamic flows by the rough surface (stick rather than slip motion). A
simple hydrodynamic model is presented as an illustration of these general
hydrodynamic results. The effective boundary parameters are analyzed
numerically for Gaussian, power-law and exponentially decaying correlators with
various indices. The maximum on the frequency dependence of the dissipation
allows one to extract the correlation radius (characteristic size) of the
surface inhomogeneities directly from, for example, experiments with torsional
quartz oscillators.Comment: RevTeX4, 14 pages, 3 figure
Velocity tuning of friction with two trapped atoms
Our ability to control friction remains modest, as our understanding of the underlying microscopic processes is incomplete. Atomic force experiments have provided a wealth of results on the dependence of nanofriction on structure velocity and temperature but limitations in the dynamic range, time resolution, and control at the single-atom level have hampered a description from first principles. Here, using an ion-crystal system with single-atom, single-substrate-site spatial and single-slip temporal resolution we measure the friction force over nearly five orders of magnitude in velocity, and contiguously observe four distinct regimes, while controlling temperature and dissipation. We elucidate the interplay between thermal and structural lubricity for two coupled atoms, and provide a simple explanation in terms of the Peierls–Nabarro potential. This extensive control at the atomic scale enables fundamental studies of the interaction of many-atom surfaces, possibly into the quantum regime
Patterns and flow in frictional fluid dynamics
Pattern-forming processes in simple fluids and suspensions have been studied extensively, and the basic displacement structures, similar to viscous fingers and fractals in capillary dominated flows, have been identified. However, the fundamental displacement morphologies in frictional fluids and granular mixtures have not been mapped out. Here we consider Coulomb friction and compressibility in the fluid dynamics, and discover surprising responses including highly intermittent flow and a transition to quasi-continuodynamics. Moreover, by varying the injection rate over several orders of magnitude, we characterize new dynamic modes ranging from stick-slip bubbles at low rate to destabilized viscous fingers at high rate. We classify the fluid dynamics into frictional and viscous regimes, and present a unified description of emerging morphologies in granular mixtures in the form of extended phase diagrams
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