Helium prewetting transition on a disordered cesium metal surface

The prewetting transition of liquid helium on a thick cesium layer surface has been investigated using an optical method to image the coverage of the substrate. Starting below the transition temperature with a saturated film pinned on the lower half of the surface, the temperature is raised. On this surface, probably a highly disordered one, the film edge is found not to m ove. Instead the coverage difference between the two sides of the edge disa ppears gradually from 1.85 to 2 K, suggesting that a non-saturated helium f ilm, presumably lacunar, grows on the initially dry area. The advancing con tact angles have been measured on the same area for various temperatures, s uggesting a wetting temperature around 1.88 K, consistent with current valu es. (C) 2000 Elsevier Science B.V. All rights reserved.