Nonwetting of Cesium by Neon near Its Critical Point

Abstract

We report quartz crystal microbalance measurements of the adsorption of neon on surfaces of cesium and rubidium at temperatures up to the critical point of neon. In the case of Ne͞Rb there is little adsorption until the temperature approaches 0.97 of T c , where a wetting transition occurs. In the case of Ne͞Cs no adsorption is seen all the way to T T c . Instead our data suggest the presence of a vapor film adjacent to the Cs surface when the sample cell is filled with liquid. This may indicate a crossover from wetting to drying.[S0031-9007 (97)02427-7] PACS numbers: 68.45.Gd, 68.15. + e, 64.60.FrIn 1977 Cahn [1] predicted that wetting transitions should be a general phenomenon in systems which are not wet at low temperature. In 1991 the first observation in a physisorption system of a wetting transition not tied to a bulk phase transition [2], and the associated prewetting transition [3,4], was made in the system 4 He on Cs, following the explicit prediction of Cheng et al. [5]. Taborek and Rutledge [4] determined the wetting temperature T w of 4 He on Cs to be 1.95 K. When the cesium layer deposited on the gold electrode of their quartz crystal microbalance was reduced to a few atomic layers, these authors found a significant reduction of T w [6]. The calculated potential well depth of such a composite substrate is intermediate between that of Cs and Au [7]. This result, and the finding that T w 0.31 K for 4 He͞Rb [8], confirms the expectation that a stronger substrate tends to move T w towards T 0 and a weaker substrate will push T w toward T c , the liquid-vapor critical point of the adsorbate. Cahn [1] showed that, in mean field treatment of a model with short-range interactions, wetting always occurs at some temperature below T c , or alternatively, if the substrate is sufficiently weak, a symmetrical "drying" transition occurs on the liquid side of the coexistence line, in which a vapor layer of diverging thickness is interposed between the substrate and the bulk liquid. More recent theoretical treatments which include long-range interactions have found that wetting and especially drying behavior near T c depends on a subtle interplay between short-range and longrange interactions [9][10][11].One way to quantify the strength of a substrate/ adsorbate pair is to compare the well depth D of the adsorbate molecule-substrate potential with the well depth of the molecule-molecule potential. For 4 He͞Cs the ratio D͞´is 0.40 [12], which may be compared to the value 8.4 for 4 He͞Au [13], for which wetting occurs from T 0. The systems Ne͞Rb and Ne͞Cs are estimated to have ratios D͞´of 0.30 and 0.25, respectively, making them the weakest substrate cases available [13]. Thus they are promising systems for investigation of wetting behavior near T c .In this Letter we report measurements of the adsorption of neon on cesium and rubidium, using the quartz crystal microbalance (QCM) technique [14]. The sensor is a beveled plano-convex AT-cut quartz crystal designed for and used in the third-overtone thickness shear mode...

Mobility Transition of Solid Rare Gases in Confined Environments

“…Nonwetting behavior has been studied both experimentally [19] and theoretically [20] and appears to depend on the interplay between short and long-range interactions. The behavior in our measurements is more reminiscent of a dewetting transition.…”

First-order wetting transitions of neon on solid CO2 from density functional calculations

“…7 Although wetting transitions are ubiquitous in nature whenever one has control over the surface-field strength ͑this may be achieved, for example, by preplating the substrate with a monolayer of a foreign species͒, in the case of one component films physisorbed on solid surfaces, genuine wetting transitions in the liquid phase have been experimentally observed only for quantum fluids on heavy alkali metals 8,9 and for Ne on Rb. 10 Recent theoretical calculations 11 suggest that among simple classical fluids ͑noble gases other than He͒, good candidates for showing wetting transitions above the triple-point might be Ar and Ne on the surface of light alkali metals. No experiment has yet confirmed this prediction.…”

Full correspondence between asymmetric filling of slits and first-order phase transition lines

“…These lines are plotted in Fig. 3(d 45 while the GCMC result reported in Table I of Ref. 27 suggests drying for Ne/Rb indicating that in such GCMC calculations the wetting properties are shifted toward higher T.…”