When is a lubricant effective?

A plate shearing an interstitial granular medium and driven by an always ongoing spring feels less fricion when only few layers are present. Adding more layers results in an increase of friction,
nevertheless the plate slides more often. The ease of slipping is not determined by friction alone?


M A Annunziata, A Baldassarri, F Dalton, A Petri and G Pontuale, Increasing ‘ease of sliding’ also increases friction: when is a lubricant effective? J. Phys.: Condens. Matter 28 134001