Orientational and directional locking of colloidal clusters driven across periodic surfaces

Abstract

12,13. This allows for an unprecedented control of cluster steering relevant for nanomanipulations on surfaces. An important step towards the bottom-up assembly of nanoscopic functional components from atomic building blocks is the controlled translation and positioning of atoms and molecular clusters on surfaces by external forces 14. At macroscopic scales, objects typically follow the direction of an external force. This is no longer true for microscopic components on atomically corrugated substrates, where particle trajectories can lock to substrate lattice directions since they provide low-energy corridors within the potential-energy landscape. Such directional locking has been previously observed for nanocrystals migrating over atomic surfaces 1,2 and microparticles on arrays of obstacles 3-5 or optical traps 6,7 but also in flux flow of type-II superconductors 8. In addition to their fundamental understanding, deviations between the direction of particle motion and the applied force have important consequences for the manipulation of atoms on surfaces and must therefore be considered in bottom-up assembly strategies. Contrasting with isolated or weakly interacting particles 8-11 , however, little is known about the driven motion of spatially extended crystalline clusters across periodic surfaces 15. Here we experimentally and numerically study the translational and the orientational motion of micrometresized colloidal clusters with up to N = 400 particles that are driven across a patterned surface. Although the governing forces and their range are very different in colloidal systems, tribological experiments suggest a close resemblance with observations in atomic systems 16. Such system-independent features are in perfect agreement with the Frenkel-Kontorova model 17 that ignores all details regarding the relevant forces and considers just a monolayer of interacting particles on a corrugated surface. Compared to atomic clusters, where well-defined driving forces are difficult to realize by means of

Structural lubricity in soft and hard matter systems

“…In addition to individual or weakly interacting particles 122 , 128 , 129 , 139 , directional locking is also observed in nanoscale manipulation and numerical simulations of Au-clusters on layered materials 140 , 141 as well as in mesoscopic experiments with rigid clusters comprised of several tens of colloids 142 , 143 (Fig. 7 ).…”

“…In addition to the direction of motion the orientation of the cluster becomes locked during the sliding process. Particles are colored corresponding to their displacement relative to the nearest substrate potential well 142 . b AFM-manipulated crystalline gold nanoparticles on molybdenum disulfide also displaying a pronounced deviation between the direction of translation (blue arrow) versus external forcing (red arrow) 141 , 10.1103/PhysRevB.98.165417.…”

Collective Directional Locking of Colloidal Monolayers on a Periodic Substrate

“…In previous experimental realizations directional locking has received much attention at the level of single particles, and it manifested with the presence of a Devil's staircase structure in the particle migration angle [21,23]. More recently, orientational and directional locking were observed for stiff clusters of microscopic particles driven by gravity across a periodic array of holes, and the resulting complex dynamics followed from competing symmetries between the two crystalline surfaces [24].…”

A moiré foray

“…This effect -known as directional locking -has been observed in many different contexts, from the pinning of electrons 1 or vortices in high-temperature superconductors 2 , to colloids in optical lattices 3 and flowing microfluidic streams 4 . Now, writing in Nature Physics, Xin Cao and colleagues have observed locking effects in clusters of colloids, driven by gravity across a periodic array of microscopic holes 5 . The work shows that studies on friction, traditionally relegated to the macro-or nanoscale world, can also be approached with microscopic particles.…”

“…At first glance, these colloidal systems may simply be considered toy models with which one can visualize ideal cases of perfectly symmetric sliding surfaces. But the studies 5,6 also open new research directions beyond confirming atomistic models on friction, because different physical interactions come into play at colloidal length scales. For example, hydrodynamic interactions can become important, and fluid flow between two sliding surfaces may lead to reduced contact forces.…”