Phase Transitions of Weakly Physisorbed Films

9705270 Cole This Focused Research Group grant supports a collaborative theoretical-experimental study of phase transitions of weakly physisorbed films. The investigations focus on films of the noble gas helium, including mixtures of the isotopes of masses four and three, adsorbed on the substrates which offer the weakest possible binding. Such substrates include alkali metals and molecular hydrogen. This choice is made to more clearly display novel behaviors in the areas of superfluid onset and phase separation in submonolayer two dimensional films; in the varieties of wetting behavior; and in the adsorption of very thick films. In all cases the research is collaborative, involving close coupling between experiment, theory, and simulation. The experimental methods include dielectric constant, volumetric, capillary rise, torsional oscillator, heat capacity and quartz microbalance measurements. The theory will deal with film dynamics, statistical mechanics, van der Waals forces, and calculation of potentials. Grand canonical simulations will be directed primarily toward the wetting problem. %%% This Focused Research Group grant supports a collaborative study of the adherence of thin films of gas molecules to several types of substrates, with an interest in the ability of the molecules to wet the surfaces. The gases under study are relatively inert noble gases, specifically helium atoms of masses three and four atomic mass units, and the studies are carried out at temperatures near the absolute zero, where the helium atoms can appear in several different types of superfluid state. A superfluid state is one in which the motion of the helium is not at all impeded by its interaction with the surrounding container or substrate, in the case mentioned above. If, for example, the liquid superfluid is put into rotation (like swirling water in a glass) the swirl ing motion will persist forever, and this effect is the basis for a gyroscope which can detect extremely slow rotations. The proposed research will clarify the effects of the substrate on the temperature at which the superfluid behavior first occurs, and will probe these behaviors down to layers less than one atomic layer thick, on average. The results may lead to new insights useful in the variety of situations, such as chemical engineering, polymer science, catalysis, and others where wetting phenomena are important. The results will also contribute to general knowledge of situations where monolayer and submonolayer coverages of surfaces by atoms and molecules occur. The nation's science and engineering research base will benefit strongly from the training of graduate students in this program, in addition to the discoveries which may be made on monolayer wetting and phase transitions. ***