Liquid⁴He on Cs: drying, depinning, and the non-wetting ‘thin-film’

Abstract

Liquid 4 He shows a dewetting phase on Cs and Rb at low temperatures. These are the only substrates which are not wetted by 4 He. On Cs the wetting temperature, T w , is about 2 K. Usually one observes a thick metastable helium film remaining on the Cs substrate when either the surface is cooled below T w (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 T w . This exceptional behaviour is analysed and the dependence of substrate roughness is discussed. Furthermore, we have studied the non-wetting 'thin-film' of liquid 4 He 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.

Wetting behavior of liquidHe4on rough Cs films: Pinning, memory effect, and micropuddles

“…At 1.55 Ͻ T Ͻ 1.65 K the same Cs spontaneously dries with the contact line receding with large scale jumps towards the wetted boundary. 31 In this case, the pinning force must decrease faster with temperature than F max ͑T͒ at this temperature, to account for this behavior. This is opposite to the behavior of the drops described above.…”

“…15 With medium pinning, the contact line of a helium film recedes slowly and in jumps, in some temperature range. 31 A small nonzero contact angle has been exhibited by a few quench-condensed Cs films, which are presumably smoother than those showing strong pinning. These Cs films were condensed onto a substrate at 4 K and then annealed at near 80 K for 40 min.…”

“…There are sometimes a small number of strong pinning sites, so that the receding contact line jumps from site to site and the contact line has a scalloped shape, as the line is concave between these sites ͑see Ref. 31 and Figs. 5-8͒.…”

“…31,34 The film thickness has been measured as a function of time, after the flooding, and it is found that after Ϸ5 s the film is too thin to be detected ͑Ͻ5 ML͒. It is assumed that the film disappears by evaporation 31 because the contact line appears stationary throughout the period that the film is getting thinner.…”

“…The mobility of the contact line can be very temperature dependent. On one Cs film 31 with an exceptionally low wetting temperature, T w = 1.68 K, if the surface is flooded at T Ͻ 1.55 K, the average helium film thickness decreases to Ͻ5 ML in a few seconds, without the contact line moving. Also in this temperature range, a memory of the contact line remained.…”