Impact and Spreading of Superfluid and Normal Fluid Helium Droplets*

We present the results of our investigation of superfluid and normal fluid helium droplets impacting on a solid dry surface in an optical cryostat at temperatures between 1.2 K and 5.1 K at saturated vapor pressure. We use high-speed video to image the impacting drops over a large range of Reynolds numbers and Weber numbers. We also use high-speed interferometry to measure the thickness and curvature of the droplets.

We find that the initial impact stages for both normal and superfluid helium droplets are similar to the results for conventional fluids. We observe that at longer spread times, the normal helium droplets do not completely wet the surface and maintain a small but finite contact angle indefinitely. This result is surprising because helium is expected to fully wet nearly all surfaces. The spreading dynamics of normal fluid and superfluid droplets are temperature-dependent, but the superfluid short-time spreading dynamics do not differ significantly from the normal fluid. We observe the Leidenfrost effect for normal fluid helium at high temperatures near the critical point.