Anomalous Wetting of Helium Mixtures Near the Tri-Critical Point

Abstract

*Just below the tri-critical temperature, T t , we have observed that the interface between phase-separated 3 He- 4 He mixtures has a finite contact angle with a wall. Moreover, this contact angle increases with temperature, so that the 4 He-rich phase does not wet the wall below T t . This is the first experimental exception to the generally accepted phenomenon known as critical point wetting. We start with a brief description of the situation before our experimental description. Then we present our explanation: the long range critical Casimir force prevents complete wetting. Lastly, we present a critical discussion of our interpretation and pose a few open questions.