Liquid 4He on Cs : drying, depinning, and the non-wetting ‘thin-film’

Liquid 4He shows a dewetting phase on Cs and Rb at low temperatures. These are the only substrates which are not wetted by 4He. On Cs the wetting temperature, Tw, is about 2 K. Usually one observes a thick metastable helium film remaining on the Cs substrate when either the surface is cooled below Tw (so starting from a wet film) or when bulk helium (in the form of a drop or an advancing film) is deposited on the Cs. The contact line of this film is generally very stable and so there is no dewetting. However, we have observed that spontaneous drying of helium on Cs can occur and that contact lines can recede for temperatures close to Tw. This exceptional behaviour is analysed and the dependence of substrate roughness is discussed. Furthermore, we have studied the non-wetting ‘thin-film’ of liquid 4He on Cs. Far away from coexistence this film grows as a two-dimensional (2D) gas until about one monolayer is obtained, then remains nearly constant for a certain range of chemical potential and, close to coexistence, rapidly forms a thicker 2D-liquid-like film as stable thermodynamical solution.