Slippery nanoworld

Quantum Matter and Optic

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

Thermodynamics of ferrofluids in applied magnetic fields

The thermodynamic properties of ferrofluids in applied magnetic fields are examined using theory and computer simulation. The dipolar hard sphere model is used. The second and third virial coefficients (B2 and B3) are evaluated as functions of the dipolar coupling constant λ, and the Langevin parameter α. The formula for B3 for a system in an applied field is different from that in the zero-field case, and a derivation is presented. The formulas are compared to results from Mayer-sampling calculations, and the trends with increasing λ and α are examined. Very good agreement between theory and computation is demonstrated for the realistic values λ≤2. The analytical formulas for the virial coefficients are incorporated in to various forms of virial expansion, designed to minimize the effects of truncation. The theoretical results for the equation of state are compared against results from Monte Carlo simulations. In all cases, the so-called logarithmic free energy theory is seen to be superior. In this theory, the virial expansion of the Helmholtz free energy is re-summed in to a logarithmic function. Its success is due to the approximate representation of high-order terms in the virial expansion, while retaining the exact low-concentration behavior. The theory also yields the magnetization, and a comparison with simulation results and a competing modified mean-field theory shows excellent agreement. Finally, the putative field-dependent critical parameters for the condensation transition are obtained and compared against existing simulation results for the Stockmayer fluid. Dipolar hard spheres do not undergo the transition, but the presence of isotropic attractions, as in the Stockmayer fluid, gives rise to condensation even in zero field. A comparison of the relative changes in critical parameters with increasing field strength shows excellent agreement between theory and simulation, showing that the theoretical treatment of the dipolar interactions is robust. © 2013 American Physical Society

Pair Correlations in a Bidisperse Ferrofluid in an External Magnetic Field:Theory and Computer Simulations

The pair distribution function g(r) for a ferrofluid modeled by a bidisperse system of dipolar hard spheres is calculated. The influence of an external uniform magnetic field and polydispersity on g(r) and the related structure factor is studied. The calculation is performed by diagrammatic expansion methods within the thermodynamic perturbation theory in terms of the particle number density and the interparticle dipole–dipole interaction strength. Analytical expressions are provided for the pair distribution function to within the first order in number density and the second order in dipole–dipole interaction strength. The constructed theory is compared with the results of computer (Monte Carlo) simulations to determine the range of its validity. The scattering structure factor is determined using the Fourier transform of the pair correlation func-tion g(r) – 1. The influence of the granulometric composition and magnetic field strength on the height and position of the first peak of the structure factor that is most amenable to an experimental study is analyzed. The data obtained can serve as a basis for interpreting the experimental small[1]angle neutron scattering results and determining the regularities in the behavior of the structure factor, its dependence on the fractional com-position of a ferrofluid, interparticle correlations, and external magnetic field. © Pleiades Publishing, Inc., 2014

Ripple formation induced in localized abrasion

The formation of nanometer-scale patterns while scratching a KBr(001) surface with a scanning force microscope in ultrahigh vacuum is reported. Wear of single atomic layers has been observed when the microscope tip is repeatedly scanned across a line. The initially flat surface is rearranged in a quasiperiodic pattern of mounds and pits. The distance between the pits is about 40 nm when normal forces of a few nanonewtons are applied, and it slowly increases with the load. If a square area is scanned, a pattern of ripples is formed. These features can be interpreted within an erosion process induced by a periodic increase of the strain produced by the scanning tip

Friction and wear on the atomic scale

Friction force microscopy experiments have been performed under ultrahigh vacuum conditions. Within a small force regime (below 1 nN), friction without wear is observed as a function of velocity on ionic crystals and metals. The results are discussed in the framework of a refined version of the Tomlinson model, which includes thermal activation. Higher normal forces lead to abrasive wear. The debris extracted from ionic crystals can be characterized with high lateral resolution. The dependence of wear rate on velocity and normal force is investigated

Transition from stick-slip to continuous sliding in atomic friction : entering a new regime of ultralow friction

A transition from stick-slip to continuous sliding is observed for atomically modulated friction by means of a friction force microscope. When the stick-slip instabilities cease to exist, a new regime of ultralow friction is encountered. The transition is described in the framework of the Tomlinson model using a parameter eta which relates the strength of the lateral atomic surface potential and the stiffness of the contact under study. Experimentally, this parameter can be tuned by varying the normal load on the contact. We compare our results to a recently discussed concept called superlubricity