UC Irvine

We have used high-speed video and interferometry to investigate the impact, spreading, and eventual contraction of superfluid He drops on a sapphire substrate in a saturated atmosphere of helium vapor. We find the counter-intuitive result that the short-term kinetic spreading of superfluid drops (time t < 10 ms) is qualitatively similar to both normal helium and to 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 T lambda ~ 2.17K. The retracting 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 T lambda. 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.

Additionally, we present the results of our investigation of the Leidenfrost effect in liquid helium droplets impacting on a solid surface in an optical cryostat at temperatures between 3.5 K and 5.2 K at saturated vapor pressure. We use high-speed video to image the impacting drops and record the minimum temperature difference delta T necessary to levitate the drops, and also to observe the lifetime, changing radius, and termination of levitation. We observe that the Leidenfrost onset temperature Tl is a function of the ambient temperature and runs approximately parallel to the vapor pressure curve, with a lower delta T needed to levitate the drop at higher temperatures. We also observe that the delta T needed to levitate the drops is much smaller than has been predicted by previous authors examining film boiling in helium, requiring only 1-70 mK for levitation. We compare our results to previous models for Tl, and we calculate the a vapor film thickness of ~ 2-6 um, much thinner than for experiments for water at room temperature. We show that this levitation cannot be attributed to film squeeze-out or to the Marangoni effect, and we observe that helium drops levitate over both a warmer solid surface and a warmer thin layer of liquid helium.