Simulated Non-Newtonian Lubricant Behaviour under Extreme Conditions

Equilibrium and non-equilibrium molecular dynamics techniques are used to simulate a molecular liquid (n-hexadecane) at extreme physical conditions (high temperatures, pressures and shear rates), typical of those encountered in many elastohydrodynamic lubrication applications, and to study the effects of pressure and temperature on the calculated theological properties. Pressure and temperature effects are found to be more pronounced at low shear rates than at high shear rates, where the non-Newtonian behaviour is dominated by the strong external kinematic effects. The observed non-Newtonian behaviour of the pressure and temperature coefficients of viscosity and the variation of certain molecular geometric functions calculated to assist the microscopic understanding are combined to provide a possible explanation for the phenomenon of limiting shear stress under elastohydrodynamic lubrication conditions. Further implications for the behaviour of lubricant-like molecules in practical applications are also discussed.