Figure 8 shows the
results of a measurement of the wetting transition of the dilute liquid
between the concentrated liquid and the substrate. For this measurement,
a phase separated helium mixture was first cooled in steps from about 0.57
K to 0.47 K, and then warmed back to 0.57 K. At each point the temperature
was held constant and the frequency of the microbalance was monitored until
it stopped drifting and then the data was recorded. The settling time ranged
from about 20 minutes to over 2 hours at each temperature. The measured
hysteresis loop indicates a first order phase transition in the film. The
low temperature film phase has a higher 3He content and so can
be identified as a concentrated film. The high temperature phase which
has less 3He is a film with a thick layer of dilute liquid at
the substrate and a thick layer of concentrated liquid on top.
Since this wetting transition is first order, the two film states are possibly metastable so that the transition occurs at different temperatures upon cooling and warming. The bi-layer films on the strong binding quartz surfaces of the microbalance surrounding the cesium serve to nucleate qualitatively similar films on the cesium, so that the concentrated films are not metastable in this experiment. Consequently, the transition point measured while warming (the concentrated to bi-layer transition point) is identified as the wetting transition temperature, while the transition point measured while cooling is a point at which the metastable bi-layer film becomes unstable and so is a spinodal point.