Frictional Duality Observed during Nanoparticle Sliding
  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. 
Dirk Dietzel, Claudia Ritter, Tristan Monninghoff, Harald Fuchs, Andre   Schirmeisen, and Udo D. Schwarz