M. S. Pettersen

Prediction of reentrant wetting of3He-4He mixtures on cesium

We examine the effect of3He impurities on the wetting behavior of4He on cesium, predicting a phase diagram which includes reentrant wetting transitions. This phase diagram is shown to be very sensitive to effects such as a theoretically predicted bound state of3He at the liquid-cesium interface, and the contact angle may be sensitive to interesting temperature dependences of the helium-cesium surface tension resulting from surface rotons or Rayleigh waves.

Nucleation of bubbles in liquid helium

We give a brief survey of experiments that have been performed to study the nucleation of bubbles in liquid helium at negative pressures.

Thermally activated wetting dynamics in the presence of surface roughness.

From simple models of thermally activated contact line dynamics far below the depinning transition, one expects the velocity to depend exponentially on the applied force and the activation area to be the size of the defects on the surface. We study contact line motion on evaporated gold films and find that the dynamics are activated, but the activation area is not straightforwardly linked to the surface roughness. Surprisingly, the activation area can be significantly smaller than any features on the surface. Furthermore, it depends strongly on the liquid. We show that this indicates that the line is close to the depinning threshold at experimentally accessible velocities. A model based on independent defects is developed and used to show deviations from the purely exponential law. The dynamics are written entirely in terms of properties of the surface and partially wetting liquid. In addition, we are able to show that the region of validity of models of thermal activation on mesoscopically rough surfaces typically corresponds to velocities of less than 1 mm/s.

The multilayer melting transition in methane adsorbed on graphite

High resolution heat capacity measurements of multilayer methane adsorbed on graphite are presented and analyzed. The evidence indicates the presence of two wetting transitions: a first-order dewetting transition at T_w = 90.48 K, and a continuous wetting transition at the triple point, T_t = 90.66 K. This behavior is to be expected in connection with the melting transition in any system where both solid and liquid wet the surface. Heat capacity measurements can provide a valuable diagnostic tool for the wetting behavior of films too thick to be investigated by other means. In the thin film limit, we find that the latent heat of melting vanishes at about 4 layers.

Experimental observations of cavitation in superfluid helium-4

Abstract We describe the results of an experiment in which a focused ultrasonic wave is used to induce cavitation in superfluid helium.

NMR study of thick films of methane on graphite

We report the results of an NMR survey of the system methane adsorbed on graphite near the triple point temperature, in the thick-film regime (10–50 layers) where the methane behavior is substantially bulk-like. We study the interaction of the film with the substrate and find that T_1 is affected by a dipolar coupling with paramagnetic spin centers in the substrate. Since the systematics of these effects are now understood, they can be used as diagnostics in their own right. We are able to conclude that solid methane wets the graphite basal plane substrate to a thickness of at least 50 layers in the temperature range 70 K < T < 105 K. On the Grafoil substrate we employ, such thick films ought to be unstable with respect to capillary condensation, but capillary condensation is evidently not nucleated in our experiments.

Observation of a first-order triple-point dewetting transition in methane adsorbed on graphite

We report the results of high-resolution heat capacity measurements of multilayer methane adsorbed on graphite near the bulk triple point. These measurements indicate that the films studied, from 5 to 18 layers thick, undergo two wetting transitions in the process of melting. The first is a first-order dewetting transition at T_w ∼ 90.5 K, and the second is a continuous wetting transition at the triple point, T_t = 90.66 K. The melting transition in the film begins at T_w, and moves to lower temperatu for thinner films.

The Hydraulic Jump in Liquid Helium

We present the results of some experiments on the circular hydraulic jump in normal and superfluid liquid helium. The radius of the jump and the depth of the liquid outside the jump are measured through optical means. Although the scale of the apparatus is rather small, the location of the jump is found to be consistent with the assumption that the jump can be treated as a shock, if the surface tension is taken into account. The radius of the jump does not change when going down in temperature through the lambda point; we think that the flow is supercritical. A remarkable feature of the experiment is the observation of stationary ripples within the jump when the liquid is superfluid.

Surfactants for Exploring the Liquid–Solid Surface Free Energy

Helium-3 and helium-4 can be used as surfactants to modify the observed wetting transitions of hydrogen and deuterium on rubidium. The effect of these surfactants on the wetting temperature is calculated. Prewetting induced by4He in H2on Rb is studied, and found to be re-entrant. Experiments can use the effect to measure the difference in temperature and concentration dependence of a free solid surface and a liquid/solid interface. This may help to resolve some unresolved questions pertaining to the wetting behavior of helium isotopic mixtures on cesium.

Temperature Dependence of the Surface Tension between Liquid Helium-4 and Cesium

Pricaupenko and Treiner1predicted a small but non-negligible temperature dependence of the surface tension between liquid helium and cesium. However, when the effects of bulk phonons in both liquid and solid are taken into account, the temperature dependence is found to be significantly weaker. The effect of surface adsorption, surface stress and surface elasticity are also considered, and found to be negligible. Experimental results have so far been contradictory.