In 1997, a team of aerospace engineering sophomores from Georgia Institute of Technology guided by Professor Komerath showed that the idea of levitating single droplets could be easily extended to form entire walls of complex shape, using audible-frequency, low-intensity sound.
The motivating purpose of their project was of course to do what is seen in the figure below left: fly around in microgravity.
The basic engineering question asked by their experiments was: would particles of arbitrary shape migrate towards a single point of minimum potential, or remain along entire surfaces of low potential? See below the results from the initial tests on the NASA KC-135. Stable walls did indeed form, single-particle thick. There was no particle spin to be seen, unlike what is observed in ultrasonic experiments where the particle size is larger than the wavelength of the excitation. The reason why the KC-135 Reduced-Gravity Flight Laboratory is called the "Vomit Comet" is seen in the picture of the "Mission Patch" of the program, below middle..
|Interior of the KC-135 cabin, looking forward. The LED at left middle shows the "g" level in centi-Gs. Oxygen bottles can be seen strapped to the bulkhead beneath it. The walls are padded - flyers routinely bounce off them. Photo of Tairon Cofer by Sam Wanis, team leader.||Mission Patch of the Reduced Gravity Student Flight Opportunities Program.||Styrofoam walls in the acoustic chamber provide first flight test proof that particles would self-align into single-particle-thick walls along nodal surfaces. No particle spin.|
For further information on the Acoustic Shaping project, please see http://www.ae.gatech.edu/research/windtunnel/aclev/aclhome.html