Surface Energy and Contact Angle of the Superfluid ³He A-B Interface

Marek Bartkowiak^a, Shaun N. Fisher^a, Richard P. Haley^a, A. M. Guénault^a, George R. Pickett^a, Peter Skyba^a,b

^aDepartment of Physics, Lancaster University, Lancaster, LA1 4YB, United Kingdom

^bInstitute of Experimental Physics, Slovak Academy of Sciences, Watsonova 43, 04353 Ko20 sice, Slovakia

We have made two interface measurements below 300 mK, at zero pressure and in magnetic fields up to 400 mT. A variable magnetic field profile allows us to stabilize and precisely manipulate the position of the A-B interface. First, we can derive the difference in wall wetting energies from the behaviour of the phase boundary as it enters and exits a stack of glass capillary tubes. Secondly, we can measure the surface tension from the level of over- or under-magnetization needed to force the interface through an aperture. These are the first surface energy measurements in high magnetic fields in the zero-temperature limit. Our results are in surprising agreement with earlier measurements at high pressure close to T_c.

The Growth of the Non-Wetting Liquid ⁴He Film on Cs

Valentin Iov, Jürgen Klier, Paul Leiderer

University of Konstanz, Department of Physics, D-78457 Konstanz, Germany

It was theoretically predicted that the alkali metals Cs and Rb are the only surfaces non-wetted by superfluid ⁴He below a certain temperature T_w. This was experimentally proven for both systems. However, investigations have shown that the non-wetting thin-film state for the He-Cs system is extremely dilute for T << T_w but close to T_w it can be much thicker and of the order of monolayers. Using the photoelectron tunneling method we have sensitively measured the growth of the non-wetting thin-film state of ⁴He on a quench-condensed Cs surface. It turns out that far from co-existence there is little adsorption of helium. In contrast, close to co-existence a rapid growth up to two monolayers of helium is observed, but the surface is still non-wet under the usual convention. We interpret this behaviour on the basis of a 2-dimensional gas-liquid film and discuss the phase diagram in the non-wetting regime.

Apparent dewetting of helium-4 driven by a superfluid flow.

Mathieu Poujade, Claude Guthmann, Etienne Rolley

Laboratoire de Physique Statistique, E.N.S., 24 rue Lhomond, 75231 PARIS cedex 05.

Silicon is known for long to be totally wetted by superfluid helium-4 at all temperatures. Yet, on a silicon substrate covered with a thick superfluid film, we observed a meniscus displaying a finite contact angle of about 5 degrees at low temperature. The observed pseudo-contact angle goes down to zero when temperature increases to the superfluid transition. The same features were observed for single droplets on the same substrate. All those properties can be quantitatively explained by the existence of a superfluid flow leading to a pressure decrease in the film, as proposed by S. Herminghaus.