Equilibrium thickness of saturated superfluid ³He films on a copper substrate

Much theoretical interest focuses on the equilibrium thickness of liquid helium films adsorbed onto a substrate. To address this issue in liquid ³He, an experiment to measure the thickness d of a saturated superfluid ³He film, on a horizontal copper substrate which is a height h above the free surface of the liquid bath, was carried out. The free surface of the bulk superfluid ³He is slowly lowered, thus increasing h, by expanding a bellows (actuated with superfluid ⁴He) at the base of the cell. While doing this, continuous measurements of the thickness of the superfluid ³He film, from which we can obtain d(h), are made. These measurements then allow us to extract information on the attractive (van der Waals) potential between the superfluid ³He film and the copper substrate.