D. Ross

Adsorption of 3He on Cesium

Adsorption isotherms of3He on cesium substrates have been measured in the temperature range from 0.2 K to 1.5 K. At liquid-vapor coexistence3He wets cesium at all temperatures studied. Step-like features are found in the isotherms which are similar to the prewetting transitions of4He on Cs substrates, but the width of these steps is ∼20 times wider for3He than for4He. In the case of3He on Cs, the steps are located at a chemical potential about 0.6 K below liquid-vapor coexistence. If the low temperature behavior is interpreted to be first order prewetting, the prewetting critical point temperature is 0.6±0.1 K.

Contact Angle of Superfluid Helium Droplets on a Cesium Surface

We present pictures of superfluid drops on several cesium surfaces prepared in different ways. These pictures are used to determine the temperature dependence of the contact angle. The results show that the temperature dependence of the helium-cesium surface tensions depends on the wetting temperature. The relationship between these results and other recent measurements of the temperature dependence of the contact angle is discussed.

Direct Optical Imaging of Superfluid 4He Droplets on a Cesium Surface

The recent discovery that4He does not wet the surface of cesium makes possible the study of superfluid droplets. We have used a long focal length microscope to study superfluid4He droplets on a cesium surface. We find that4He droplets do not flow freely over the surfaces we prepare. Rather, they remain stationary even when the surface is inclined by as much as 10° to the horizontal. The contact angle, the angle between the surface and the liquid-vapor interface is also highly hysteretic. At 1.2 K, where the advancing angle is near 25°, the receding angle varies between 0° and 7°, depending on the cesium film.

Wetting transitions of binary liquid mixtures at a weakly attractive substrate

The adsorption of a binary liquid mixtures onto a weakly attractive substrate has been studied in the vicinity of bulk liquid phase separation. A wetting transition between two different kinds of wetting films has been found. Below the wetting transition temperature, homogeneous films of one liquid phase wet the substrate. Above the wetting temperature, the wetting films contain both liquid phases. The surface phase transition line associated with this wetting transition is found to extend to both sides of the bulk phase separation line. On one side it is a prewetting line, and on the other side it becomes a line of triple point induced dewetting transitions. The experimental system used to study this behavior was liquid helium mixtures on a cesium substrate.

The solid-liquid surface tension at a helium/cesium interface

The existence of a wetting transition for helium adsorbed on cesium provides an experimental opportunity to investigate the solid-liquid surface tension. We have extracted the temperature dependence of the solid-liquid surface tension at the4He/Cs interface from our earlier measurements of the wetting temperature of pure4He on thin layers of cesium overlaying gold. More recently we have measured the wetting temperature as a function of the3He concentration for3He-4He mixtures. An analysis of these experiments paralleling Andreevs investigation of the free surface reveals3He bound states at the helium/cesium interface.