  One of the most fundamental questions in tribology concerns the area
dependence of friction at the nanoscale. Here, experiments are presented where
the frictional resistance of nanoparticles is measured by pushing them with the
tip of an atomic force microscope. We find two coexisting frictional states:
While some particles show finite friction increasing linearly with the
interface areas of up to 310,000nm^2, other particles assume a state of
frictionless sliding. The results further suggest a link between the degree of
surface contamination and the occurrence of this duality.
