Abstract
Introduction
Experiment
Results
Discussion
References
Projects Page
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Introduction
 The manner in which a liquid film
grows on a surface as bulk coexistence is approached is determined by the
binding strength of the surface. On a strongly binding substrate the attractive
surface potential serves to stabilize the liquid phase so that wetting
films grow smoothly and continuously as bulk coexistence is approached.
In contrast, on a weakly binding substrate the free energy balance is more
subtle, and the growth may proceed through a first order film thickness
transition known as prewetting [1,2]. The locus of prewetting transitions
in the P - T (or m - T) plane is the prewetting
line. The prewetting line typically intersects the bulk coexistence line
at a first order wetting transition and extends upwards in temperature
where it ends at a prewetting critical point. The first example of a prewetting
phase diagram was found in studies of the adsorption of 4He
on cesium [3], and the phenomena has subsequently been observed in a number
of systems [4,5,6].
Since the
liquid-vapor surface tension of 3He is much smaller than that
of 4He, 3He is expected to wet cesium down to zero
temperature [7]. Despite the fact that there is no wetting transition in
the 3He/Cs system, theoretical predictions indicate that wetting
should occur via a prewetting transition [7]. In this case the prewetting
line would not intersect the bulk coexistence line but would instead terminate
at zero temperature at a chemical potential below coexistence. The theory
of Ref[7] also suggests that the Fermi statistics of the 3He
should play an important role in the prewetting behavior, and should result
in a series of smaller film thickness transitions in addition to a prewetting
transition.
Previous
work in this laboratory [8] studied the adsorption of 3He on
Cs at 1.2 K. One of the conclusions of the previous study was that if prewetting
occurs in this system, the prewetting critical point temperature lies well
below 1 K. The purpose of the work reported here is to extend these measurements
to as low a temperature as possible and to reexamine the question of prewetting.
We were able to obtain adsorption isotherm measurements of 3He
on Cs down to 0.2 K. Below this temperature the equilibrium time for the
experiment became prohibitively long. As expected 3He wets cesium
at all temperatures. Additionally, the 3He films thicken at
a broadened step in the isotherms located at a chemical potential about
0.6 K below saturation. As the temperature is lowered the steps become
steeper until 0.6 K where the steepness of steps becomes constant. This
general behavior of the steps is reminiscent of the 4He prewetting
line. However, the steps at their sharpest remain ~20 times wider for 3He
than for 4He prewetting steps on the same substrates and by
naive comparison do not look like first order transitions. In order to
decide whether first order prewetting does occur in the 3He/Cs
system, one must reconcile these two apparently conflicting observations.
Our attempts to do so are discussed below.
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