Non-universal Casimir effect in saturated superfluid 4He films at Tλ

Measurements of Casimir effects in 4He films in the vicinity of the bulk superfluid transition temperature Tλ have been carried out, where changes in the film thickness and the superfluid density are both monitored as a function of temperature. The Kosterlitz-Thouless superfluid onset temperature in the film is found to occur just as the Casimir dip in the film thickness from critical fluctuations becomes evident. Additionally, a new film-thickening effect is observed precisely at Tλ when the temperature is swept extremely slowly. We propose that this is a non-universal Casimir effect arising from the viscous suppression of second sound modes in the film