When a drop of liquid is placed on the surface of another substance, unless
it dissolves, it has two choices. It can spread to cover the surface, like
oil on water, or it can remain a drop, like water on wax. The first
behavior is called "wetting" and the second called "non-wetting". A deeper
analysis reveals that these choices are separated by a thermodynamic phase
transition. Just as temperature determines whether a collection of H2O
molecules will be ice or water, it determines whether a liquid drop will
wet or not. This so-called wetting transition has been a central topic in
the statistical mechanics of surfaces for years.
Intermolecular forces play a big part. If drop molecules are much more strongly attracted to substrate molecules than to each other, the drop wets. Until recently the He - He interatomic attraction was thought to be the weakest in nature. As an immediate consequence, helium should wet everything! Many years of empirical experience supported this conclusion. Thus the prediction a few years ago that helium was so weakly attracted to the surface of cesium that it would form drops on it was a major surprise.
The accompanying photographs, taken by Dr. David Ross while a post-doctoral researcher working with Professors Peter Taborek and James Rutledge, offers vivid evidence of the nonwetting of superfluid helium on cesium. They are notable because they are the first look at drops on a surface in the entire 87-year history of superfluid helium research. The photos, taken through a long focal length microscope, show a drop of superfluid helium at 1.1 K on a flat cesium surface. The picture is taken at a glancing angle just above the surface so that the top of the drop and its reflection on the shiny cesium metal are both visible. The sharp corners on the right and the left side mark the plane of the surface. The non-zero contact angle between the drop and the surface is a signature of nonwetting. The dark object intersecting the drop from the top of each photo is the tube used to place the drop on the surface. Notice that the liquid surface is tangent to the tube. This shows that helium wets the tube's surface and only gravity prevents the drop from immediately spreading over it and disappearing. In the top two pictures, liquid is being added to the drop through the tube and it is growing in size. In the bottom picture, liquid is being removed from the drop and it is shrinking. The difference in contact angles between growing and shrinking drops, and several other issues in surface and superfluid physics, are currently being investigated.