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

Abstract

Friction between solids is responsible for many phenomena such as earthquakes, wear or crack propagation 1-4 . 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 FrenkelKontorova model, which considers a monolayer of interacting particles on a periodic substrate potential 5-8 . In addition to the well-known stick-slip motion, such models also predict the formation of kinks and antikinks 9-12 , 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.Friction is important in our daily life and it is not surprising that systematic investigations date back more than 300 years. According to Amontons and Coulomb, friction between solids is proportional to the normal force but independent of the contact area. This intriguing result was explained by realizing that macroscopic objects touch at asperities that are deformed by the normal force 13 . A different situation occurs when atomically flat surfaces slide against each other, as for example encountered in micro-or nanoelectromechanical systems. Then, extended contacts arise and the degree of commensurability between the surfaces determines the friction. For commensurate conditions, a dissipative stick-slip motion is typically observed 14 . In contrast, at incommensurate interfaces, atomic friction studies revealed a superlubrication regime, where the friction coefficient vanishes 15,16 . This behaviour can be explained by simple mechanical models such as the FrenkelKontorova model, a generalized Prandtl-Tomlinson scheme or the double-chain model [17][18][19][20] . In the Frenkel-Kontorova approach the interface between two solids is described by a monolayer of elastically interacting beads on a periodic substrate potential [5][6][7][8] . In addition to stick-slip motion, the Frenkel-Kontorova model also predicts the formation of topological solitons, so-called kinks and antikinks [9][10][11][12] . These excitations are believed to dominate the frictional properties at atomic length scales because they provide an efficient mechanism for mass transport; so far, such excitations have never been observed in sliding friction experiments 21 .Here, we report the observation of kinks and antikinks in a colloidal system that is driven across commensurate and incommensurate substrate potentials. We use highly charged polystyrene spheres with radius R = 1.95 μm, which are suspended in water. In the presence of gravitational and op...

Quantized transport for a skyrmion moving on a two-dimensional periodic substrate

“…There are other examples of particles moving over ordered substrates, such as vortices in type-II superconductors with periodic pinning arrays [41][42][43] or colloids placed on optically created periodic substrates 44,58 . In these systems the dynamics is overdamped; however, there can be directional locking effects in which the particles preferentially move along symmetry directions of the underlying substrate as the direction of drive is rotated with respect to the substrate lattice [45][46][47][48][49][50][51][52] .…”

Microscale Motion Control through Ferromagnetic Films

“…Phenomena such as superlubric transition, the effect of commensurability and contact geometry, and kink motion, can thus be investigated even in our proposed magnetic mesoscale system. Similar strategies have been recently proposed, making use of model systems such as ion traps [5,18] and colloidal suspensions [8,24,27], however magnetic domains can provide more freedom and flexibility, their properties being continuously tunable over many orders of magnitude. We will focus here only on FFs with perpendicular anisotropy, i.e., the easy axis of the magnetization is perpendicular to the film surface.…”

Current trends in the physics of nanoscale friction

“…Until recently, it has been demonstrated or implied in a relatively small number of cases [29,[42][43][44][45][46]. There are now more evidences of superlubric behavior in cluster nanomanipulation [32,33,47], sliding colloidal layers [48][49][50], and inertially driven rare-gas adsorbates [51,52] (see Fig. 2).…”

“…Thanks to a highly innovative experimental apparatus [48], a brand new light is cast on elemental tribological processes by exploiting the versatility of charged colloidal systems driven across interfering laser-generated potentials, whose geometry can be tuned at will. While AFM, SFA and QCM provide, a system response in terms of crucial, but averaged, physical quantities, colloidal friction provides an unprecedented real-time insight into the basic dynamical mechanisms at play, excitingly observing what each individual particle is directly doing at the sliding interface.…”

“…Specifically, by driving highly charged polystyrene spheres, naturally forming in water a 2D triangular crystal [142,143], across both a commensurate and incommensurate lasergenerated substrate geometry, the colloid approach [48] highlights the crucial tribological role played by localized superstructures (such as kinks and antikinks), which emerge as Figure 6. Zoom-in (front) of a MD simulated frictional interface between a colloid monolayer and an optical corrugated substrate potential.…”