The growth of the non-wetting liquid on Cs

Abstract

It was theoretically predicted that the heavy alkali metals provide the only surfaces non-wetted by superfluid %e below a certain temperature Tw. This was experimentalIy proven both for Cs and for Rb. However, investigations have shown that the non-wetting thin-film state for the He-Cs system is extremely dilute for T Q T,, yet close to T, it can be much thicker and of the order of monolayers. Using the photoelectron tunneling method we have sensitively measured the growth of the non-wetting thin-film skate of 4He on a quench-condensed Cs surface. Tt turns out that far from coexistence there is littIe adsorption of helium. In contrast, close to GO-existence a rapid growth up to two to three monolayers of helium is observed, but the surface is still non-wet under the usual convention. These systems are excellent for studying the phenomena of non-wetting and pre-wetting, difficult to do with classical fluids. However, it turned out that the wetting behaviour, determined by measured contact angles [4,8,9] is strongly influenced 'by the method used to prepare the cesiated substrate: quench-condensed Cs, as it IS obtained by evaporation at low temperature, (expected to cause a rough surface) shows weaker nonwetting than Cs grown from the liquid state (which allows for a smooth surface to develop). This difference *Corresponding author.E-mail addresc: iov.valentin@hmi-konstanz.de (V. lov).in wetting behaviour is mainly attributd to the microscopic roughness of the Cs [lo].We have studied the non-wetting thin fiIm state on a quench-condensed (and hence rough) Cs surface using the photoelectron tunneling method [I 11. Close to liquid-vapour coexistence there is a sharp drop in photocurrent which indicates a transition from a monolayer to a multilayer film. Even in this regime the Cs surface shows non-wetting behaviour, according to the usual definition. From the temperature dependence of this transition a phase diagram for the monolayer and multilayer regime, respectively, can be constructed.

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

“…If the Cs surface is less rough the transition may happen suddenly once the surface density of the 'thin-film' state has reached a critical value. A transition in 2D was indicated by other experiments [27,28], and is discussed in more detail in the next section. Again these observation underline the role of the surface roughness and the need for simultaneous wetting studies and surface characterization.…”

“…The helium thickness in that region does not depend on temperature. We assume that at liquid-vapour coexistence a 2D-liquid has formed as the thermodynamic stable non-wetting film on the Cs surface [27,28].…”

Helium on planar and nanostructured alkali-metal surfaces

“…5 we plot the density profiles of the coexisting thin and thick films at T = 2.3 K. From these profiles we extract widths at half height of nearly 4 and 50 Å for the low and high coverages, respectively, which for the corresponding saturation density represent a thin film slightly above one monolayer, and a thick one with around 14 helium layers. On the experimental side, photoelectron tunneling measurements 37 indicate that below T w and far from bulk coexistence the helium system is dilute with a coverage of less than one layer. Ellipsometric measurements of film thickness of helium on evaporated Cs with submonolayer resolution 38 the thin film is around two layers wide and the prewetting jump could be estimated as 12-15 layers.…”