The wetting behavior of fluid on solid surfaces is of critical importance to many systems- fabrication of semiconductor devices, application of inks to paper, and even the survival of animals.   One possibility for the fluid is that it completely wets the surface, forming a thick uniform film.  However, if the surface tensions are altered, the fluid may not wet the surface and instead bead-up like water on a waxed surface. Wetting phenomena is an issue in all systems where a fluid phase is at an interface of another fluid and solid (or between two other fluids).  For most systems the substrate is sufficiently attractive that all fluids wet the interface.  However, recently several new systems have been explored that allow for wetting temperatures where the fluid goes from a wet to non-wet phase.
Most systems studied display a first order wetting transition, but it is believed that another variety of wetting was possible- continuous wetting.  For these systems the film thickness is continuous across the wetting temperature.  Continuous, or second order, wetting has not yet been seen in a physisorbed system.  It was recently predicted that xenon on alkali metals, plated with a monolayer of gold would be a possible candidate for continuous wetting.  With my experience of first order wetting and preparation of alkali metal surfaces, I believe that it would be possible to construct an experiment that would look for this new transition.
The experiment would require a cryostat capable of precise temperature regulation near the bulk triple point of xenon.  The design and construction of such an apparatus would give a student hands on experience with temperature control.  The elimination of radiant heat, equilibrium time constants, and handling of cryogenic liquids would be issues that need addressing.  Because surface contamination is of great concern, students would also be able to work with UHV techniques such as turbo pumps and leak detection.
Surface preparation would likely be done using evaporative techniques already used in thin film preparation.  Even outside the context of the wetting experiment, the preparation and analysis of thin films could be a valuable experience for a student.  Measurement of both the substrate and xenon film thickness could be done with quartz crystal microbalances.  These devices allow for resolutions of fractions of a monolayer, which would be necessary at all stages of the experiment.  Operation of the cryostat will require some computer programming and interface control for data collection.  This experience with LabVIEW or other similar language would give a student practical experience for running future experiments.
After completion of the xenon wetting experiment, the cryostat could be used for any number of low temperature experiments.  One perplexing puzzle in adsorbed films is the solid layer mobility.  For warmer temperatures, when the vapor pressure is significant, the major process for mass transport is through the vapor.  For situations when the vapor pressure is negligible, there are some questions as to the mechanism of transport.  In my helium experiments it is seen that at low temperatures, even before the adsorbed film goes superfluid, the mass transport is very rapid.  Experiments have also been performed on the kinetics of solid hydrogen mobility with little theoretical understanding, as the transport is not simple diffusion of a 2-D gas.  Further work on these and other systems could be performed with a cryostat that has optical access.  A simple experiment would burn a hole in the adsorbed film using a heater or laser.  Then using ellipsometry techniques a student would be able to accurately measure the film regrowth as a function of time.  Once optical capabilities have been added to the cryostat, other experiments on the adsorption and phase transitions of various two-dimensional films would be possible.
My interests in phase transitions and soft condensed matter have also grown beyond low temperature physics.  One recent project that I have been working on in my spare time is learning fluid dynamics.  In particular, I want to apply fluid dynamics to soap film made from standard dishwashing detergent.  Several groups are conducting research on turbulence in soap films suspended between two nylon wires.  This system is incredibly easy to assemble, at least on the elementary scale, and yet it provides one of the best apparatuses for testing fluid dynamics in two-dimensions.  I’ve been interested in relating the film thickness, in the laminar regime, to the various fluid parameters.  For a student who enjoys computer modeling this would be a nice project as they could perform calculations that can be easily compared with experiments.
Another system that is deceptively simple to assemble is granular materials.  The basic setup for the study of avalanches, pattern formation and convection only requires a few trips to the hardware and grocery stores, yet there is much to be learned from granular materials.  While the materials are simple macroscopic particles, which only interact with repulsive forces, the physics is still a mystery.  A sandpile at rest behaves like a solid, yet if the slope is increased so that is greater than the angle of repose the grains will begin to flow.  However this movement is not completely fluid-like, as the Navier-Stokes equations do not hold.  The discrete nature of the particles may imply a gas-like state but the thermal energy of the system is negligible when compared to the gravitational energy.  After all of the contradictions, one is left to conclude that new laws need to be found that can predict the behavior of granular materials.  The study of granular materials is a field that rapidly expanding in scope and popularity as evidenced by the recent articles in Physics Review Letters, Science, and Nature. 
Working on phase transitions and critical phenomena, especially at low temperatures, is something that I find very stimulating.  However, I think it is possible to use the experimental techniques I’ve learned to create other experiments.  Experiments such as soap films, granular materials, binary fluids and liquid crystals can offer students a different type of research experience.  Depending on the student’s interests and personality, working on a smaller, self-contained project may be more rewarding than working on one portion of a large experiment.  Offering the students different options of physics and technology could help to match the students with the project that is best for them.

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