Using a quartz microbalance, the adsorption of helium mixture films onto a cesium substrate was measured as a function of temperature and concentration. The measurements spanned temperatures from 0.15 K to above the tricritical (consolute) point and 3He concentrations from 0 to 1. The procedure for taking data was to fill the experiment cell with a sufficiently large quantity of helium mixture (of known concentration) so that bulk liquid was condensed in the bottom of the cell at a temperature well above the range of interest (typically between 1 K and 3 K) and then cool into the temperature range of the measurement. For each data point taken, the experiment cell was held at constant temperature until the frequencies of both (cesium and gold plated) microbalances stopped drifting, and then the data was recorded. The microbalances are relatively insensitive to the superfluid component of adsorbed films, so the frequency shift of the microbalance is essentially a measurement of the 3He content of the adsorbed films for this experiment.
We will refer to the two liquid phases of the helium mixtures by their 3He content, so that the 4He rich superfluid will be referred to as the dilute liquid, and the 3He rich normal liquid will be referred to as the concentrated liquid.
The results are summarized in the phase diagram of Figure 7, and are presented in greater detail in three parts dependent on the concentration of the mixture used:
Wetting transition of the dilute liquid between the concentrated liquid and the substrate. This result was obtained with a mixture at nearly the tricritical concentration so that both dilute and concentrated liquid phases (as well as the vapor) are present in the experiment cell.
Prewetting line. This set of data was obtained with mixtures of greater than the tricritical concentration.
Dewetting line. These results were obtained using mixtures of lower than the tricritical concentration.
