Edge-dependent Static Friction of Adsorbed van der Waals Islands

Rare gas islands adsorbed through van der Waals forces on metal surfaces do not slide freely, but exhibit static friction in QCM experiments. Static friction appears, unexpectedly, even for incommensurate and defect-free crystal surfaces, where sliding should be frictionless. Via atomistic simulations of Kr islands on Au(111), we show that the island edges may be the ultimate culprits. Adsorbate sliding requires the flow of solitons - tiny density and corrugation modulations with the beat periodicity between the two periodicities. For an island, we find an edge-originated energy barrier that blocks the soliton flow, keeping the island pinned. As the static friction force is reached, the barrier vanishes at one point on the edge, and new solitons enter the island, which becomes depinned. Unsurprisingly, we find that low surface corrugation and high temperature facilitate this edge depinning. However, the island's thermal expansion is large and leads to changeable commensurability upon heating, which gives rise to the possibility of re-entrant static friction.