We all know what gravity feels like here on earth. In the Space Shuttle or the International Space Station, astronauts feel only a fraction of this gravity. It’s not quite zero-gravity, it’s very very small, so it’s called micro-gravity. A large proportion of the experiments performed in orbit are designed to exploit this micro-gravity condition. In micro-gravity, completely new materials can be formed, well-known materials behave differently and living things respond in strange ways. Many of these experiments are very delicate and require solids, liquids and living organisms to be carefully held in place. This is where magnetic, electric and acoustic levitation instruments are used. Acoustic levitation is particularly useful as it works with any material and can be achieved with a relatively simple apparatus. Of course, these are sound waves they need a gas to travel through and so these experiments are confined to on-board the spacecraft. The basic acoustic levitator, uses an ultrasonic loudspeaker and a reflector. Here we note that it should really be called ultrasonic levitation but NASA called it acoustic levitation and the name stuck. If the distance between the loudspeaker and the reflector is set to be an integer number of half wavelengths then a standing wave is formed. This results in a sequence of alternating nodes and antinodes and objects can be levitated in the nodes. The used of other loudspeakers enables rotation of the objects. This works on earth where the acoustic radiation force has to counter the earth’s gravity but it works much better in orbit where micro-gravity conditions exist.
The first acoustic levitation experiments in space date back to 1985 when the scientist-astronaut (called a Payload Specialist by NASA) Taylor Wang undertook pioneering experiments on-board the Space Shuttle Challenger (mission STS-51B). Famously, his instrument failed during launch and mission control forbade any repair as this was not part of the mission. Desperate not to miss this once in a lifetime opportunity to undertake these experiments Taylor told mission control “If you guys don’t give me a chance to repair my instrument, I’m not going back.” This was quite unprecedented and in credit to all, mission control backed down and Taylor fixed his instrument. So what was the experiment? Well, the idea was to study droplet formation, motion and fragmentation in a micro-gravity environment. These experiments were performed to provide experimental validation of theories of planet formation where a range of forces act to shape a mostly liquid object. The length scales are different, but the key theories are the same. In particular, when rotated, droplets form into a two-lobed shaped and then fragment which was thought to be part of the evolution of planets and stars. Anyhow, Taylor’s experiment did indeed validate these fundamental theories so the mission was a success. This was then the birth of acoustic levitation in space.
The next pioneer was the Payload Specialist astronaut Eugene Trinh who developed and used the United States Microgravity Laboratory-1 (USML-1) which was an instrument that flew on the space shuttle Columbia in 1992 (mission STS-50). The acoustic levitation part of this instrument was called the Droplet Physics Module and its purpose was to study the fundamental physics of droplet motion under zero-gravity. The scientific questions were to understand what role viscosity and non-linearity play in the motion. With gravity eliminated very high quality measurements were possible which fed into theoretical fluid mechanics developments in the coming decade. This work then formed the basis for future space shuttle levitation experiments which continued until then end of the space shuttle era (c. 2011)
After NASA’s pioneering work, acoustic levitation has become a standard tool of micro-gravity research in space. It is now routinely used alongside other levitation technologies to position solids and liquids without any contact with a container. Will we need acoustic levitation on future space missions? To perform careful scientific experiments the answer is a definite yes. There may be other uses though. Di Chen and Junru Wu from the University of Vermont have shown that acoustic radiation forces can be used to clean dust from surfaces such solar panels. So acoustic levitation could be one of the ways we cope with the dust storms on Mars.