A Possible Molecular Dynamic Structure Contribution to Elastohydrodynamic Lubrication

Abstract

A possible third contribution to the elastohydrodynamic lubrication effect is proposed based upon differential diffusion of higher and lower molecular weight hydrocarbons in the tribological contact. The effects of molecular weight and structure on the physical properties of hydrocarbons show that lower molecular weight, linear hydrocarbons have lower viscosities, and therefore higher self diffusion coefficients, than higher molecular weight, extensively branched structured, hydrocarbons with higher viscosities and lower self diffusion coefficients. In an equilibrium, isotropic, situation separation of lower and higher molecular weight hydrocarbons does not occur. In the elastohydrodynamic contact, under certain rates of relative movement of the metal boundaries, a non-equilibrium, anisotropic, situation will allow the lower molecular weight hydrocarbons to preferentially diffuse forwards in the direction of movement, giving a transient enhancement of higher molecular weight hydrocarbons remaining in the contact. This will give a higher viscosity and higher viscosity-pressure coefficients in the contact generating a fluid pressure above that predicted from the bulk equilibrium physical properties of the mixed fluid. Implications for base oil formulation are discussed.