Vortices in metastable 4He films

Liquid helium wets almost all surfaces. This behaviour results from its small dielectric permittivity, ε=1.057, which leads to attractive forces between the helium atoms weaker than those between helium and other atoms. In 1991 Cheng et al., however, predicted that 4He does not wet certain alkali metals. Instead, for some of these metallic substrates, a first-order wetting transition was foreseen. In the alkali metals, weakly bound outer electrons cause a short-range repulsion for the helium atoms. The resulting temperature-dependent balance between repulsive short-range and attractive long-range forces is at the origin of the wetting phenomenon in liquid helium. (orig.)