Measurement of the viscosity of supercooled liquids at high shear rates with a Hopkinson torsion bar

Abstract

The Eyring theory of viscous flow suggests that lubricating oils should exhibit shear thinning when the shear stress exceeds about 5 MPa. The results of friction experiments in rolling-contact disc machines where very high pressures are generated in the lubricant film support this prediction, but are open to the criticism that the fluid is subjected to a high pressure for such a short time (ca. 10$^{-4}$ s) that an equilibrium state may not be reached. In the present investigation the appropriate condition of the lubricant is achieved, not by subjecting it to very high pressures but by maintaining it in the supercooled state. The lubricant is thus in a condition of equilibrium and the shear experiments are carried out at atmospheric pressure. The lubricant specimen is retained in a suitably adapted split Hopkinson torsion bar, and at the high rates of shear applied (ca. 10$^4$ s$^{-1}$) the shear stress at sufficiently low temperatures can exceed 5 MPa. By this technique the shear pulse is applied for a sufficiently short time (ca. 10$^{-3}$ s) to avoid viscous heating of the sample, which bedevils normal viscometry at high shear rates. Two fluids were tested: polyphenyl ether 5P4E and a mineral oil Shell HVI 650. Nonlinearity in the shear-stress-shear-strain-rate relation was found when the stress exceeded about 3 MPa. The elastic shear modulus G$\infty$ was also measured, yielding ca. 500 MPa for 5P4E and ca. 50 MPa for HVI 650. These values compare with ca. 1100 MPa and 300 MPa as found by the high-frequency oscillating shear technique at small strains.