Molecular Tribology of Fluid Lubrication: Shear Thinning

Abstract

An investigation of the boundary layer viscosity of films of non-polar liquid lubricants, hexadecane and a silicone oil, octamethylcyclotetrasiloxane, confined between parallel atomically smooth single crystals of Muscovite mica, is reported. The effective viscosity (ηeff) showed a zone of linear response and a second zone of extreme shear thinning. In the zone of linear response, ηeff increased exponentially with increasing net normal pressure, increasing from 250 to 2200 poises as the pressure was raised from 0.03 to 0.15 MPa. However, very large shear thinning (decay by more than one order of magnitude) was observed as the effective shear rate was varied from 1 sec⁻¹ to 10⁴ sec⁻¹, and the transition to glassy solid response (critical shear stress behavior) was impeded by continuous shear. These experiments show that the rheology of a lubricant within a zone of contact can be strikingly different from that in the bulk. Possible engineering applications of these findings are discussed.