Wetting transition of 4He on alkali-metal surfaces from density functional calculations

We have studied the wetting properties of (4)He adsorbed on the surface of heavy alkali metals by using a nonlocal free-energy density functional which describes accurately the surface properties of liquid (4)He in the temperature range 0<T<3 K. Our results for (4)He on the Cs surface give both the temperature dependence of the contact angle and the wetting temperature in good agreement with the experimental findings. For the (4)He/Rb system we find that a wetting transition on the Rb surface occurs at T similar to1.4 K, whereas;the experiments show either wetting down to T=0 or a wetting transition at T similar to0.3,K; We suggest that this disagreement is due either to an inaccuracy of the fluid-substrate potential used in our calculations or the consequence of substrate roughness, which is known to affect the Rb surface, and whose effect would be to lower the wetting temperature. The sensitivity of the wetting transition to the He-surface potential is stressed for the He/Rb surface, which may justify the controversial experimental findings for this system.