Hydrogen and helium films as model systems of wetting

Abstract

Optical experiments on the wetting properties of liquid 4He and molecular hydrogen are reviewed. Hydrogen films on noble metal surfaces serve as model systems for studying triple point wetting, a continuous transition between wetting and non‐wetting. By means of optically excited surface plasmons, the adsorbed film thickness for temperatures around, and far below, the bulk melting temperature is measured, and the physical mechanisms responsible for the transition are elucidated. Possible applications for other experiments in pure and applied research are discussed. Thin films and droplets of liquid helium are studied on cesium surfaces, on which there is a first order wetting transition. Our studies concentrate on dynamical observations via surface plasmon microscopy, which provide insight into the morphology of liquid helium droplets spreading at different temperatures. Features corresponding to pinning forces, the prewetting line, and the Kosterlitz‐Thouless transition are clearly observed.

Adsorption Isotherms of Hydrogen: The Role of Thermal Fluctuations

“…This thickness at saturation increases with increasing temperature, and diverges for T → T 3 . This is triple point wetting [12].…”

“…While one would naively expect γ to be constant, we see that it is dramatically re-duced for small thickness. The jump at 0.7 nm may well indicate the boundary between the solidified part of the film close to the wall [12,19] and the liquid. At higher film thickness, a strong increase of the surface tension is seen, which approaches the bulk value above 25 Angstroms, corresponding to about 6.5 molecular layers (one layer is 3.8 Angstroms thick).…”

Untitled

“…Molecular hydrogen exhibits the effect of triple point wetting, which implies that although liquid films above the triple temperature T t can grow to arbitrary thickness, solid H 2 well below T t in thermal equilibrium forms only films of at most several monolayers. 1,2 This hampers the use of solid H 2 films for potential applications as well-defined matrix or substrate material. Experiments where homogeneous and smooth films of solid hydrogen isotopes would be quite useful cover such different areas as optical spectroscopy, laser fusion, studies of two-dimensional electron systems and the determination of the neutrino rest mass.…”

Prewetting transition on a weakly disordered substrate: Evidence for a creeping film dynamics

“…The transition reversibility has not always been tested. When it has been looked for, hysteresis has been found quite systematically [7][8][9][10][11][13][14][15][16][17]. This hysteresis has received diverging interpretations.…”

“…Hence a detailed view of the transition is not accessible. Conversely some experiments [14][15][16] have provided detailed images or movies of spreading films, but clearly out of equilibrium. In this letter, we report the first observation of the coverage map during a prewetting transition.…”