Temporary Reduction of Viscosity of Liquids at High Rates of Shear

Abstract

IN view of a recent statement¹ “that solutions of macromolecular substances may show an anomalous fall in the coefficient of viscosity at high rates of shear”, it is emphasized once again² that, at sufficiently high rates of shear, any liquid consisting of elongated molecules must be expected to exhibit a decrease of the coefficient of viscosity with increasing rate of shear. This means that there is only a quantitative, but no qualitative, difference between the dependence of viscosity on rate of shear of so-called Newtonian liquids and that of solutions of macro-molecular substances. Grunberg and Nissan³ estimated that at a rate of shear of about 10⁶ sec.⁻¹ pentane might begin to show an appreciable reduction of viscosity, and Bondi⁴ predicted that temporary reductions of viscosity would become apparent for shearing stresses of more than 10⁵ dynes/cm.² for straight hydrocarbon chains with molecular weights of 500 and more. In addition to our own previous results on castor oil and straight mineral oils⁵, experimental evidence is now presented to prove that a chemically defined liquid, such as tetradecane, suffers an appreciable fall of its coefficient of viscosity over a range of rates of shear extending to about 350,000 sec.⁻¹.