Viscous Heating in Compressed Liquid Films

The greatest challenge in high-shear viscometry is the recognition of the difference between constitutive behavior and thermal feedback. The failure to realize this distinction was the cause of a serious mistake which has had a profound effect upon the EHL field for four decades. A traction curve is not a rheological flow curve. The classical approach to EHL has ignored the measurement of viscosity in viscometers at elevated pressures. As a consequence, the field has not benefitted from the understanding gained from directly observing the thermal softening effects of viscous heating. The Eyring sinh-law appears in high-shear viscometry when viscous heating causes thermal softening. The thermal limit of pressurized thin film Couette viscometers with high thermal conductivity cylinders is well-defined by the Nahme-Griffith number. The Spikes and Jie equation overcorrects at low values of the Nahme-Griffith number and undercorrects at high values. The viscous heating in a shear-thinning liquid may cause an apparent transition from power-law to sinh-law above some viscous power. Surprisingly, the sinh-law in this instance extrapolates back to the correct low-shear viscosity. The same response has been observed in one set of NEMD simulations.