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...
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.PACS numbers: 82.70. Dd, 05.60.Cd, Particles which are driven across periodic substrate potential landscapes show a number of intriguing phenomena. Depending on the direction of the applied driving force F, the orientation of the particle's motion can substantially deviate from F but is locked-in to directions determined by the substrate's symmetry [1,2]. Examples of such kinetically locked-in states range from atom migration on crystalline surfaces [3], driven charge density waves [4] to flux flow in type-II superconductors [5][6][7]. Also, it has been demonstrated that directional locking can be employed for sorting colloidal particles according to their size, refractive index or chirality [8,9]. In contrast to the above examples which have been carried out with diluted systems, only little is known about directional locking in the presence of non-negligible particle interactions. Then, the competition between interparticle forces and those with the substrate leads to complex dynamical ordering phenomena [10]. Interestingly, dynamical ordering is not limited to periodic surfaces but is also found on vortex lattices driven across quasiperiodic and disordered pinning sites [11][12][13][14]. Recently, directional locking and dynamical ordering was even predicted for interacting colloidal systems on quasiperiodic substrate potentials [15]. However, both an experimental demonstration and a microscopic understanding of such ordering transitions on quasiperiodic substrates, is still missing.In this Letter we experimentally demonstrate dynamical ordering of a colloidal monolayer on a quasiperiodic optical interference pattern [16]. When the direction of the driving force is varied with respect to the substrate, we observe directionally locked states with smectic-like order in agreement with recent predictions [15]. We demonstrate, that this is due to the interaction of particle lanes formed by interstitial and non-interstitial particles. When the angle of F deviates from a substrate symmetry direction, the colloidal monolayer partitions into domains which are aligned along different symmetry directions of the substrate.Our experiments are performed in a thin sample cell with 200µm height which is filled with an aqueous suspension of charged polystyrene spheres with diameter σ = 1.95µm. Due to their surface charge they interact via a screened Coulomb potential φ(r) ∝ exp(−κr)/r where κ −1 is the Debye ...
We experimentally investigate the phase behavior of a dense two dimensional system of interacting colloidal particles subjected to a decagonal quasiperiodic potential landscape created by the interference of five laser beams. Upon increasing the intensity I 0 of the laser field, we observe the initial triangular crystal to change into a quasicrystal via a two step process. To characterize this transition, we apply an algorithm that describes the resulting structures in terms of a polygonal tiling comprised of triangular, square and pentagonal tiles. First, square tiles develop at the expense of triangular tiles and assemble into bands. Only at higher laser intensities, pentagonal tiles, which reflect the decagonal quasiperiodic ordering, occur. For certain particle densities, an Archimedean like tiling occurs where the bands of square extend across the entire system. We demonstrate how the alignment of these bands can be related to phasonic strain fields in the laser pattern.
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