Casimir forces, surface fluctuations, and thinning of superfluid films

Recent experiments on the wetting of $^{4}$He have shown that the film becomes thinner at the $\lambda$ transition, and in the superfluid phase. The difference in thickness above and below the transition has been attributed to a Casimir interaction which is a consequence of a broken continuous symmetry in the bulk superfluid. However, the observed thinning of the film is larger than can be accounted by this Casimir force. We show that surface fluctuations give rise to an additional force, similar in form but larger in magnitude, which may explain the observations.