Impact and lifecycle of superfluid helium drops on a solid surface

Abstract

We have used high-speed video and interferometry to investigate the impact, spreading, and eventual contraction of superfluid He4 drops on a sapphire substrate in a saturated atmosphere of helium vapor. We find that the short-term kinetic spreading of superfluid drops (time t<10 ms) is qualitatively similar to both normal helium and conventional fluids at room temperature. In contrast, the contraction phase of the superfluid drops is highly unusual. Superfluid drops survive for only a few seconds on the substrate due to superflow out of the drop into the surrounding helium film. The drop lifetime is strongly dependent on temperature and diverges at the superfluid transition temperature Tλ∼2.17 K. The contracting drops undergo a geometry-dependent two-phase contraction, which includes a toroidal phase where the radius decreases linearly in time and subsequently a spherical cap phase where the radius decreases with the square root of time. The receding contact angle is temperature dependent and becomes small near Tλ. We also observe that the superfluid outflow causes surprising edge effects, including the emergence of satellite droplets on the perimeter of the expanding drop, as well as ragged and frayed drop edges at lower temperatures.