Leidenfrost-Like Non-Coalescence of Liquid Helium Drops

We present the results of our investigation of Leidenfrost-like non-coalescent impact behavior of normal liquid helium drops in an optical cryostat at saturated vapor pressure at temperatures of 2.2-5.2 K. We observe non-coalescence when drops come into contact with both solid substrates and with bulk liquid. In contrast to other well-known types of non-coalescence, our drops exist in an almost perfectly isothermal environment and in the absence of a non-condensible gas such as air. Non-coalescence can occur throughout the entire temperature range of liquid helium and is particularly robust near the liquid-vapor critical point, where drop bouncing is observed at higher impact velocities. We examine the heat and mass balance in the drop and analyze the role of curvature-induced evaporation which generates a sub-millikelvin temperature difference between the drop and the substrate; this small temperature gradient between the drop and the surface produces and evaporation-driven gas film supporting the drop.