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

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

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

Coupled ion interface dynamics and ion transfer across the interface of two immiscible liquids

When an ion moves across the interface of two immiscible electrolytes it moves together with the ion-induced protrusion of one solvent into the other. For an infinitely slow motion of an ion the height of the protrusion, h(eq), is a function of the position of the ion z. Due to a finite relaxation time the protrusion may not be able to spontaneously follow the motion of the ion, and this will cause slowing down of the ion transfer. The relaxation of the protrusion involves the movements of many solvent molecules and must be considered on the same footing as the motion along the coordinate of the ion. In this paper we develop a theory of such coupled motion which determines the kinetic laws of the ion transfer across the interface. When the equilibrium electrochemical potential for the ion has no barrier, the process of ion transport is purely diffusional and the effective diffusion coefficient may be evaluated as D-eff=k(B)T/{6eta[r(i)+(4/3)(h(max)/Lambda)L-2]}, where eta is the average viscosity of the liquids, r(i) is the Stokes radius of an ion, L and h(max) is the lateral size and the maximal height of the protrusion, and Lambda is the half width of the function h(eq)(z), which characterizes equilibrium ion-interface coupling. When there is a barrier, the theory recovers, depending on the height of the barrier, the mechanisms of ion transfer considered by Marcus or Gurevich-Kharkats-Schmickler. The effect of the nature of the ion and the solvents in contact is discussed. (C) 2002 American Institute of Physics

Ion penetration into an "unfriendly" medium and the double layer capacitance of the interface between two immiscible electrolytes

We develop a theory of the double layer at electrolyte I electrolyte interfaces with account for the finite thickness of the interfacial region. This includes the distribution of ions between the two phases and smooth variation of dielectric properties across the interface. The theory offers simple laws for the dependence of the double layer capacitance on the nature of ions, ionic concentrations and potential, which are in line with experimental observations. The theory shows which parameters reflect the nature of ions and the structure of the interface, and how these parameters can be extracted from the capacitance data. (C) 2001 Elsevier Science B.V. All rights reserved