Since the wetting transition is first order, there should be a prewetting line associated with it [1,2]. The measurements of this prewetting line are shown in Figure 9. Data was taken at several concentrations, five of which are shown: X3 = 0.905 (red), 0.873 (blue), 0.775 (green), 0.747 (magenta), and 0.726 (orange). Again, the frequency of the microbalance was allowed to settle at each point before the data was recorded. For clarity, the data are offset slightly form each other vertically and are plotted vs. T - Ts, where Ts is the bulk phase separation temperature for each mixture. Because the 3He used for these mixtures had been distilled from a more dilute mixture, the concentration was not well known directly and so was determined from the measurement of Ts and the data of Ref. [20].
As
the experiment was cooled, the frequency of the microbalance remained fairly
constant for each concentration until a temperature at which there was
a break in the slope of the curve and the frequency shift began to decrease
as a layer of superfluid formed at the cesium surface. For each concentration,
this transition temperature is marked with an arrow in Figure 9. For the
X3 = 0.905 measurement (shown in red), no transition was observed
because for this concentration Ts < Tw so that
the experimental trajectory does not cross the prewetting line. The transitions
became progressively less sharp as the 3He concentration was
reduced and Ts approached Ttc, the bulk tricritical
point temperature. For concentrations less than 0.726, the transition could
no longer be resolved.
The X3
= 0.873 measurement is far sharper than the others and is shown in greater
detail in Figure 10. This transition was hysteretic and therefore first
order. To verify the hysteresis, the loop was repeated twice (only one
loop is shown in the figure) and the transition temperatures were found
to be the same each time. The measurements at the higher concentrations
were also checked for hysteresis and none was found. Therefore, of the
measurements shown in Figure 9, only the X3 = 0.873 transition
is actually a first order prewetting transition. The others form a line
of superfluid onsets that extends upward from the surface critical point.
For some of the superfluid onsets, a peak in the dissipation characteristic
of a Kosterlitz-Thouless transition was visible in the data.