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

Experimental observation of directional locking and dynamical ordering of colloidal monolayers driven across quasiperiodic substrates

We experimentally investigate the structural behavior of an interacting colloidal monolayer being driven across a decagonal quasiperiodic potential landscape created by an optical interference pattern. When the direction of the driving force is varied, we observe the monolayer to be directionally locked on angles corresponding to the symmetry axes of the underlying potential. At such locking steps we observe a dynamically ordered smectic phase in agreement with recent simulations. We demonstrate, that such dynamical ordering is due to the interaction of particle lanes formed by interstitial and non-interstitial particles.Comment: accepted at Phys. Rev. Let

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

Colloidal monolayers driven across light-induced substrates

Reibung lässt sich auf makroskopischer Längenskala durch das Amontonsche Gesetz beschreiben, welches besagt, dass Reibungs- und Normalkraft zueinander direkt proportional sind. Dieser einfache Zusammenhang beruht auf dem Scheren unzähliger Mikrokontakte, ein Mechanismus, der erst in den 1950er Jahren theoretisch verstanden und erst nach der Jahrtausendwende experimentell aufgelöst wurde. Um grundlegende Erkenntnisse über Reibung zu gewinnen, müssen allerdings die Mechanismen verstanden werden, die zum Brechen eines einzelnen Mikrokontakts führen, also Prozesse, die auf Längenskalen von Mikro- bis Nanometern ablaufen. Dies führte zur Entwicklung des Forschungsfelds der Nanotribologie, welches Reibung, Schmierung und Verschleiß auf der Nanoskala behandelt. Ein wichtiges theoretisches Werkzeug der Nanotribologie sind simplifizierte tribologische Modelle, wie das Tomlinson oder das Frenkel-Kontorova (FK) Modell. Das Tomlinson Modell beschreibt punktförmige Kontakte, für realistischere, d.h. ausgedehnte Kontaktgeometrien wird von theoretischer Seite das getriebene Frenkel-Kontorova Modell verwendet. Während die Vorhersagen des Tomlinson Modells durch Messungen mit dem Rasterkraftmikroskop bestätigt wurden, existiert bisher kein experimentelles System, um das FK Modell detailliert zu studieren. Von besonderem Interesse sind hierbei sog. topologische Solitonen, die im Rahmen des Frenkel-Kontorova Modells vorhergesagt werden und welche einen effizienten Mechanismus für den Massentransport auf kleinen Längenskalen darstellen. Die gezielte Erzeugung und Manipulation topologischer Solitonen bietet eine Perspektive, Reibung auch auf der Nanoskala zu reduzieren. In dieser Arbeit wird die erste experimentelle Realisierung eines zweidimensionalen getriebenen Frenkel-Kontorova Modells verwirklicht. Hierfür dienen kolloidale Monolagen miteinander wechselwirkender Partikel, welche über stationäre lichtinduzierte Substratpotentiale getrieben werden. Der Aufbau erweist sich dabei als ideales Modellsystem, da nahezu alle relevanten Parameter in situ variiert werden können. Die geladenen Partikel wechselwirken repulsiv mittels eines Yukawa Potentials, dessen Reichweite über die Ionenkonzentration der Lösung kontrolliert wird. Die Interferenz mehrerer Laserstrahlen erlaubt es, ausgedehnte Lichtfelder mit mehreren zehntausend Minima zu erzeugen, welche als Substratpotential für die kolloidale Monolage dienen. Durch Änderung von Anzahl und Anordnung der Strahlen können sowohl periodische, als auch quasiperiodische Substratpotentiale generiert werden, deren Längenskalen durch Änderung des Einfallswinkels der Strahlen variiert werden können.On a macroscopic length scale friction can be described by Amontons' law, which states that the friction and the normal force are directly proportional. This simple relation is based on the shearing of countless micro-contacts, a mechanism which was understood theoretically only in the 1950s. To gain a fundamental understanding of the mechanisms of friction, however, the processes leading to the breaking of a single micro-contact must be understood. This led to the emergence of the field of nanotribology. An important theoretical tool of nanotribological research are so called simplified tribological models, like the Tomlinson or the Frenkel-Kontorova (FK) model. The Tomlinson model describes point contacts, for more realistic, extended sliding geometries the driven Frenkel-Kontorova model is used. While the predictions of the Tomlinson model were confirmed by atomic force microscopy, yet no experimental system exists to study the FK model in detail. Of particular interest are so called topological solitons, which are predicted in the framework of the Frenkel-Kontorova model and which provide an efficient mechanism for mass transport on small length scales. The systematic creation and manipulation of topological solitons offers a novel perspective to reduce friction on the nanoscale. In this thesis the first experimental version of a two-dimensional driven Frenkel-Kontorova model is realized. For this purpose, interacting colloidal monolayers are driven across a stationary light induced substrate potential. The setup proves to be an ideal model system, because almost all relevant parameters can be varied in situ. The charged particles interact via a repulsive Yukawa potential whose range is controlled by the ion concentration of the solution. The interference of several laser beams allows to generate expanded light fields with tens of thousands of minima which act as a substrate potential for the colloidal monolayer. By changing the number and the arrangement of the beams, both periodic and quasi-periodic light fields can be generated. The length scale of the substrate can be varied by changing the angle of incidence of the beams

Observation of kinks and antikinks in colloidal monolayers driven across ordered surfaces

Friction between solids is responsible for many phenomena such as earthquakes, wear or crack propagation. Unlike macroscopic objects, which only touch locally owing to their surface roughness, spatially extended contacts form between atomically flat surfaces. They are described by the Frenkel-Kontorova model, which considers a monolayer of interacting particles on a periodic substrate potential. In addition to the well-known stick-slip motion, such models also predict the formation of kinks and antikinks, which greatly reduce the friction between the monolayer and the substrate. Here, we report the direct observation of kinks and antikinks in a two-dimensional colloidal crystal that is driven across different types of ordered substrate. We show that the frictional properties only depend on the number and density of such excitations, which propagate through the monolayer along the direction of the applied force. In addition, we also observe kinks on quasicrystalline surfaces, which demonstrates that they are not limited to periodic substrates but occur under more general conditions.publishe

Light-induced phase transitions of colloidal monolayers with crystalline order

We experimentally study the phase behavior of a charge-stabilized two-dimensional colloidal crystal which is subjected to a one-dimensional periodic light field. Such light fields are created by a scanned optical line tweezer which allows the variation of the periodicity without optical realignments. In order to realize a wide range of line spacings relative to the lattice constant, we use a suspension of silica particles in bromobenzene. This colloidal system has a Debye screening length of about 4.6 μm which results in the formation of crystals with lattice constants up to 20 μm. Because the refractive index of bromobenzene is larger than that of the colloids, optical gradient forces lead to the attraction of particles at regions where the intensity is smallest. Depending on the depth and periodicity of the optical potential, we observe the light-induced assembly of colloids into triangular, rhombic and square phases.publishe

Phase behavior of colloidal monolayers in quasiperiodic light fields

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Periodic average structures of colloidal quasicrystals

We experimentally study the phase behaviour of a charge-stabilized two-dimensional colloidal monolayer which is subjected to a one-dimensional quasiperiodic substrate potential. Upon increasing the laser intensity, we observe a transition from a periodic to a quasiperiodic state. It proceeds via the formation of an intermediate periodic average structure (PAS) which is related to the quasiperiodic lattice by a bounded 1-1 mapping. Because PAS can transform to crystals and quasicrystals by minute particle displacements, they provide a mechanism to allow for interesting insights into the relationship between periodic and quasiperiodic order.publishe