Incomplete wetting ofHe4films on Ag and Au(111) surfaces

The wetting behavior of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ adsorbed on planar Ag and Au(111) surfaces has been studied by means of a quartz, high-frequency microbalance. Incomplete wetting was observed over the entire experimental temperature (T) range from below 1.4 K to 1 mK below the critical point. The limiting film thickness $d_0$ at coexistence with bulk liquid increases from about 10 layers at the λ point to more than 50 layers at the highest temperature. Over a considerable interval the T dependence of $d_0$ is consistent with the power law $d_0$∝($T_w$-T${)}^{\mathrm{-}1}$ proposed by Dietrich and Schick for critical wetting. Below the λ point the frequency isotherms develop pronounced dips at pressures well below saturated vapor pressures $P_0$. The dips are associated with superfluid onset, and the clear separation between the dips and the abrupt increases in frequency at $P_0$ show that superfluid onset and bulk fluid condensation are independent phenomena.