210 research outputs found
Molecular motor with a build-in escapement device
We study dynamics of a classical particle in a one-dimensional potential,
which is composed of two periodic components, that are time-independent, have
equal amplitudes and periodicities. One of them is externally driven by a
random force and thus performs a diffusive-type motion with respect to the
other. We demonstrate that here, under certain conditions, the particle may
move unidirectionally with a constant velocity, despite the fact that the
random force averages out to zero. We show that the physical mechanism
underlying such a phenomenon resembles the work of an escapement-type device in
watches; upon reaching certain level, random fluctuations exercise a locking
function creating the points of irreversibility in particle's trajectories such
that the particle gets uncompensated displacements. Repeated (randomly) in each
cycle, this process ultimately results in a random ballistic-type motion. In
the overdamped limit, we work out simple analytical estimates for the
particle's terminal velocity. Our analytical results are in a very good
agreement with the Monte Carlo data.Comment: 7 pages, 4 figure
Saltatory drift in a randomly driven two-wave potential
Dynamics of a classical particle in a one-dimensional, randomly driven
potential is analysed both analytically and numerically. The potential
considered here is composed of two identical spatially-periodic saw-tooth-like
components, one of which is externally driven by a random force. We show that
under certain conditions the particle may travel against the averaged external
force performing a saltatory unidirectional drift with a constant velocity.
Such a behavior persists also in situations when the external force averages
out to zero. We demonstrate that the physics behind this phenomenon stems from
a particular behavior of fluctuations in random force: upon reaching a certain
level, random fluctuations exercise a locking function creating points of
irreversibility which the particle can not overpass. Repeated (randomly) in
each cycle, this results in a saltatory unidirectional drift. This mechanism
resembles the work of an escapement-type device in watches. Considering the
overdamped limit, we propose simple analytical estimates for the particle's
terminal velocity.Comment: 14 pages, 6 figures; appearing in Journal of Physics: Condensed
Matter, special issue on Molecular Motors and Frictio
Dynamical heat channels
We consider heat conduction in a 1D dynamical channel. The channel consists
of a group of noninteracting particles, which move between two heat baths
according to some dynamical process. We show that the essential thermodynamic
properties of the heat channel can be evaluated from the diffusion properties
of the underlying particles. Emphasis is put on the conduction under anomalous
diffusion conditions. \\{\bf PACS number}: 05.40.+j, 05.45.ac, 05.60.cdComment: 4 figure
Electrotunable lubrication with ionic liquids: the effects of cation chain length and substrate polarity.
Electrotunable lubrication with ionic liquids (ILs) provides dynamic control of friction with the prospect to achieve superlubrication. We investigate the dependence of the frictional and structural forces with 1-n,2-methyl-imidazolium tetrafluoroborate [C n MIM]+[BF4]- (n = 2, 4, 6) ILs as a lubricant on the molecular structure of the liquid, normal load, and polarity of the electrodes. Using non-equilibrium molecular dynamics simulations and coarse-grained force-fields, we show that the friction force depends significantly on the chain length of the cation. ILs containing cations with shorter aliphatic chains show lower friction forces, ∼40% for n = 2 as compared to the n = 6 case, and more resistance to squeeze-out by external loads. The normal load defines the dynamic regime of friction, and it determines maxima in the friction force at specific surface charges. At relatively low normal loads, ∼10 MPa, the velocity profile in the confined region resembles a Couette type flow, whereas at high loads, >200 MPa, the motion of the ions is highly correlated and the velocity profile resembles a "plug" flow. Different dynamic regimes result in distinctive slippage planes, located either at the IL-electrode interface or in the interior of the film, which ultimately lead, at high loads, to the observation of maxima in the friction force at specific surface charge densities. Instead, at low loads the maxima are not observed, and the friction is found to monotonously increase with the surface charge. Friction with [C n MIM]+[BF4]- as a lubricant is reduced when the liquid is confined between positively charged electrodes. This is due to better lubricating properties and enhanced resistance to squeeze out when the anion [BF4]- is in direct contact with the electrode
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
Theory and simulations of ionic liquids in nanoconfinement.
Room-temperature ionic liquids (RTILs) have exciting properties such as nonvolatility, large electrochemical windows, and remarkable variety, drawing much interest in energy storage, gating, electrocatalysis, tunable lubrication, and other applications. Confined RTILs appear in various situations, for instance, in pores of nanostructured electrodes of supercapacitors and batteries, as such electrodes increase the contact area with RTILs and enhance the total capacitance and stored energy, between crossed cylinders in surface force balance experiments, between a tip and a sample in atomic force microscopy, and between sliding surfaces in tribology experiments, where RTILs act as lubricants. The properties and functioning of RTILs in confinement, especially nanoconfinement, result in fascinating structural and dynamic phenomena, including layering, overscreening and crowding, nanoscale capillary freezing, quantized and electrotunable friction, and superionic state. This review offers a comprehensive analysis of the fundamental physical phenomena controlling the properties of such systems and the current state-of-the-art theoretical and simulation approaches developed for their description. We discuss these approaches sequentially by increasing atomistic complexity, paying particular attention to new physical phenomena emerging in nanoscale confinement. This review covers theoretical models, most of which are based on mapping the problems on pertinent statistical mechanics models with exact analytical solutions, allowing systematic analysis and new physical insights to develop more easily. We also describe a classical density functional theory, which offers a reliable and computationally inexpensive tool to account for some microscopic details and correlations that simplified models often fail to consider. Molecular simulations play a vital role in studying confined ionic liquids, enabling deep microscopic insights otherwise unavailable to researchers. We describe the basics of various simulation approaches and discuss their challenges and applicability to specific problems, focusing on RTIL structure in cylindrical and slit confinement and how it relates to friction and capacitive and dynamic properties of confined ions
Stabilizing stick-slip friction
Even the most regular stick-slip frictional sliding is always stochastic,
with irregularity in both the intervals between slip events and the sizes of
the associated stress drops. Applying small-amplitude oscillations to the shear
force, we show, experimentally and theoretically, that the stick-slip periods
synchronize. We further show that this phase-locking is related to the
inhibition of slow rupture modes which forces a transition to fast rupture,
providing a possible mechanism for observed remote triggering of earthquakes.
Such manipulation of collective modes may be generally relevant to extended
nonlinear systems driven near to criticality.Comment: 4 pages, 4 figure
Experimental Validation of Contact Dynamics for In-Hand Manipulation
This paper evaluates state-of-the-art contact models at predicting the
motions and forces involved in simple in-hand robotic manipulations. In
particular it focuses on three primitive actions --linear sliding, pivoting,
and rolling-- that involve contacts between a gripper, a rigid object, and
their environment. The evaluation is done through thousands of controlled
experiments designed to capture the motion of object and gripper, and all
contact forces and torques at 250Hz. We demonstrate that a contact modeling
approach based on Coulomb's friction law and maximum energy principle is
effective at reasoning about interaction to first order, but limited for making
accurate predictions. We attribute the major limitations to 1) the
non-uniqueness of force resolution inherent to grasps with multiple hard
contacts of complex geometries, 2) unmodeled dynamics due to contact
compliance, and 3) unmodeled geometries dueto manufacturing defects.Comment: International Symposium on Experimental Robotics, ISER 2016, Tokyo,
Japa
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