SIMULATION OF LUBRICANT RHEOLOGY IN THIN-FILM LUBRICATION .2. SIMULATION OF COUETTE-FLOW

In the second part of this two-part study, molecular dynamics simulati ons are performed for a fluid of spherical molecule in Couette flow. T he simulation uses the same system as that in Part 1, but with the top wall translating in the x direction to generate a Couette flow. The s hear equivalent viscosity eta(es) of a fluid is found to increase as f ilm thickness decreases. The dependence is similar to that of the flow equivalent viscosity eta(ef) obtained in Part 1, but the shear viscos ity eta(cs) exhibits a smaller value and slower increase rate. A furth er comparison between eta(es) and eta(ef) shows that at the him thickn ess where the viscosity eta(ef) diverges, the corresponding shear visc osity keeps a relatively small value, which is attributed to the large r shear rate applied in simulation of Couette flow. In a region where shear rate is low, the shear viscosity remains almost constant until a critical shear rate gamma(c) is reached, then the 'shear-thinning' fo llows, i.e, the viscosity declines in a power-law, eta(es) similar to gamma(-2/3), and the mean shear stress approaches a constant value-the 'limiting shear stress'. If the film becomes molecularly thin, the lu bricant behaves like a viscoelastic material, indicated by the conside rable value of shear stress existing at the zero shear rate. The mean velocity profile of molecular flow shows a linear distribution, but wi th inflexions on the profile near the wall-fluid interface. When a ver y high shear rate is applied, however, the flow in thin films seems to be divided into two parts, half stays almost at rest and half is rapi dly sheared. Once the stress exceeds the limiting shear stress, a slip in velocity appears at the solid-fluid interface.