Melting and solidification of helium in restricted geometries

The processes of melting and solidification are determined to a large extent by microscopic conditions at the liquid-solid interface. Theoretical calculations and some direct optical observations suggest that the interface for helium is several atomic layers thick. In our own experiments on the solidification of 3He at low temperatures (<= 100 mK) within a restricted geometry (Vycor porous glass with pore diameter approximately 70A), we have made use of the adiabatic solidification process for cooling 3He itself (Pomeranchuk effect) to make measurements of entropy and volume changes as a function of pressure and temperature within 2/3 bar of the melting curve. Measurements of magnetisation have also been made. The observations show that the restricted geometry provided by the high surface-area solid glass substrate can have a profound effect: in particular, it induces a high density (rho>solid), high entropy phase which, although disordered, is probably mobile. Such a situation may also be representative of the interface between bulk liquid and solid helium, and we compare our measurements with others which give indirect evidence of the nature of this interface