Effect of Isotopic Mass on Viscosity of Molten Lithium

The viscosity of the separated isotopes of molten lithium was measured by observing the damped oscillations of a torsion pendulum whose bob contained a spherical cavity filled with the material under test. For both Li⁶ and Li⁷, the viscosity varies approximately as exp(B/T) over the temperature range 180 to 300^∘{}C; B has the value 458^∘{}K for Li⁶ and 631^∘{}K for Li⁷. The viscosity of Li⁶ is 4.18±{}0.05 mP (millipoise) at 180.4^∘{}C, its melting point; of Li⁷, 6.00±{}0.05 mP at 180.7^∘{}C, its melting point. A linear interpolation for Liⁿᵃᵗ (7.4% Li⁶, 92.6% Li⁷) gives 5.86 mP at 180.5^∘{}C, only slightly lower than the value of 6.02 mP reported earlier by Andrade and Dobbs. The ratio of the viscosity of Li⁷ to that of Li⁶ is 1.44 at the melting point, in contrast with the value of 1.08, the square root of the mass ratio, as predicted on simple theoretical arguments. Actually, a thorough dimensional analysis shows that the viscosity varies as (mkT)1/2 times a complementary dimensionless function containing---among other arguments--- the mass as a consequence of quantum effects. The function evidently depends strongly on these effects, as indicated not only by the present results on molten Li⁶ and Li⁷, but also by the results of others on other isotopically-substituted liquids.