Using superfluid mixtures to probe convective instabilities

Abstract

When the height d of a convecting layer of a dilute solution of 3He in superfluid 4He is much greater than a characteristic dissipation length λ 0, the superfluid mixture behaves as if it were a single-component classical fluid, but one with the extraordinarily broad and interesting Prandtl number range of 0.04 < Pr < 1.5. We have made the first direct measurements of λ 0 by observing its effect on the critical Rayleigh number R c, thus determining quantitatively how closely superfluid mixture convection matches standard Rayleigh-Bénard convection. Two-fluid effects can be only a few percent for 1.0 < T < 2.0 K. We also report a series of measurements near the crossing of the skewed-varicose and oscillatory instabilities showing the effects of instability competition.