CONTACT-ANGLE OF LIQUID-HE-4 ON CS - EVIDENCE FOR RIPPLONS AT THE HE-CS INTERFACE

Two independent methods to measure the contact angle, Theta, of isotop ically pure He-4 on Cs are described, and applied to the same Cs sampl e. The two methods agree well. The contact angle as a function of temp erature is measured and clearly shows a first order surface phase tran sition with a wetting temperature T-w = 2K. From 12 Theta(T) the diffe rence in surface fi ee energy of the Cs-vapour interface and the Cs-li quid interface is calculated. Its large temperature dependence suggest s the existence of ripplons at the liquid He-Cs interface. This surpri sing possibility is discussed.