  Using atomic force microscopy experiments and molecular dynamics simulations
of gold nanoislands on graphite, we investigate why ultra-small friction
commonly associated with structural lubricity can be observed even under
ambient conditions. Measurements conducted within a few days after sample
synthesis reveal previously undiscovered phenomena in structurally lubric
systems: rejuvenation, a drop in kinetic friction of an order of magnitude
shortly after the onset of sliding; aging, a significant increase in kinetic
friction forces after a rest period of 30 minutes or more; switches,
spontaneous jumps between distinct friction branches. These three effects are
drastically suppressed a few weeks later. Imaging of a contamination layer and
simulations provide a consistent picture of how single- and double-layer
contamination underneath the gold nanoislands as well as contamination
surrounding the nanoislands affect structural lubricity but not lead to its
breakdown.
