by Erin Dahlstrom
figures by Utsarga Adhikary
Popularized by films like Star Trek and Star Wars, the tractor beam is an iconic science fiction technology that allows the user to manipulate objects in space without making physical contact with the objects. While humans today may not be ready to use this technology in outer space, the ability to manipulate objects without touching them could potentially be very useful as a microsurgical technique and in the handling of dangerous chemicals. One exciting technology that has recently shown promise for the development of a real-life tractor beam is acoustic levitation. Acoustic levitation uses sound waves to generate forces that can levitate and move small objects. Previous forays into the use of this technology were limited by strictly prescribed setups and restricted movement capabilities, but in a recent breakthrough, scientists in Spain and the UK successfully used an array of speakers to create an acoustic field that can manipulate a polystyrene bead from up to 30-40 cm away. Using this new setup, these researchers can not only trap the bead at a specific point in 3D space, but also rotate it and move it around at a speed of up to 25 cm/s [1,2,3].
What is the physics behind the technology of acoustic levitation?
Let’s first go back to the basic nature of waves – more specifically, the nature of sound waves. Waves are disturbances that travel from one point in space to another through a medium or substance. For example, consider a slinky. If you pull on the first coil of a slinky, then release it, you see a pulse travel through the length of the slinky to the last coil, as shown in Figure 1. When you pull on the first coil, you transfer energy to the slinky, which the first coil then passes on to the next coil, and so on. After the first coil transfers this energy to the second coil, it moves back to its initial, or equilibrium, position in the slinky. Repetitive pulling on the first coil will create a disturbance that will travel through the slinky, called a wave. It’s important to note that what is actually traveling in the wave is energy, rather than the particles that make up the medium. Also, note that the slinky coils are moving back and forth in the same direction that the energy is moving; this is called a longitudinal wave. The other type of wave is a transverse wave, like the waves found on the surface of the ocean, where the medium moves perpendicular to the direction that the energy is traveling.
A sound wave is another example of a longitudinal wave, but instead of using slinky coils, the medium through which it transports energy is the air around us. The initial source of energy in a sound wave is a vibrating object; consider our vocal cords, which vibrate to produce our voices, or the vibrations you feel when you place your hand on a speaker as it plays music. As the speaker vibrates, it moves outward, compressing the particles of air in front of it, then moves inward, spreading out the particles in front of it. These regions of squashed particles and spread out particles are called compressions and rarefactions, similar to peaks and troughs (high and low points) on a transverse wave, as seen in Figure 2. The compressions are areas of high pressure (lots of particles in close quarters), while the rarefactions are areas of low pressure. As the speaker continues to vibrate at a regular frequency, more compressions and rarefactions are formed, creating a series of high- and low-pressure areas – a sound wave [7,8]!
How do sound waves produce forces?
Because sound waves are composed of areas of high and low pressure, they have the ability to produce forces due to differences in pressure. In a region of high pressure, lots of particles are close together and bumping into one another. If there is a region of low pressure nearby, which contains fewer particles that are spread far apart with lots of space between them, particles in a crowded space will move to the less crowded region. This movement of particles from a high-pressure region to a low-pressure region produces a force that can affect anything in its way. An everyday example of this is found in a canister of compressed air that you might use to blow dust away. Pressure is very high inside the sealed canister compared to outside the canister. When you press the trigger on the canister, you create an opening for the air particles to move from the inside of the can to the outside, which they do, and this generates enough force to move dust.
Acoustic levitation uses the forces that sound waves generate from differences in pressure to trap or move objects, such as small plastic beads. If we could create an area of low pressure surrounded completely by high pressure, a bead placed into that low-pressure zone would be trapped there in a high pressure “cage.” If the bead tried to move in any direction, the high pressure surrounding it would effectively push it back into the center of the acoustic trap. Then, by moving the location of the lower pressure zone, we could move the bead around. One can imagine more complex manipulations of the bead, as well, like rotation, produced by careful manipulation of the high- and low-pressure zones. What allows us to manipulate pressure patterns like this is a property of waves called interference. When two or more waves meet in a medium, they interact, causing the particles to respond in a way that corresponds to the net effect of the multiple waves . For example, if two identical waves traveling toward each other meet in a pool of water when they are both at their peak, they will create a wave twice as high as each of the original waves. If they meet when one is at its peak and the other at its trough, they will cancel each other out. The researchers who developed this new acoustic levitation technique used computational models of interference to design interactions of sound waves that could create a pattern of high and low pressures as described above. These pressure patterns produced acoustic levitation of a 3 mm polystyrene bead. By shifting the sound waves in real time to change the interference pattern, they could shift and rotate the target bead.
How can this technique be applied to solve existing problems?
While previous setups that used acoustic levitation required the target to be enclosed within the speakers generating the acoustics, the European scientists’ recent iteration of the technology only requires application of sound waves from one side to generate both high and low pressure patterns. This new versatility may find key applications in medicine, as the speaker array can be applied to the skin to generate manipulations inside the human body. This technology could be adapted for use in microsurgery, the removal of blockages like clots or kidney stones, or in drugs intended for limited delivery to a particular site in the body, such as a tumor [2,3]. Because the manipulation would be controlled by an acoustic setup located outside of the patient, there would be no need for incisions, making these procedures much less invasive than current surgical techniques. To make such medical applications possible, this system would need to be miniaturized one thousand fold to manipulate objects on the micron scale, rather than the millimeter scale. Bruce Drinkwater, a co-author of the paper, predicts that this miniaturization would just require the use of higher-frequency sound waves, a “relatively simple tweak” . While this particular advance in acoustic levitation technology is too new to have been tested in applications with biological samples, less advanced setups have been used to manipulate cells and the small worm C. elegans in 2D, implying that future medical applications certainly are feasible .
Another possible application that does not require miniaturization of the setup is non-contact manipulation of hazardous or easily contaminated materials for handling or transportation. Containerless transportation would eliminate human contact with these chemicals that is currently necessary and potentially dangerous. Prior work has tested the feasibility of using acoustic levitation to mix droplets of liquid without contact and to mix liquids with solids, like combining a small granule of instant coffee with a water droplet . While this is a simple example, it could be extended and applied to radioactive materials or DNA, which are easily contaminated by human contact.
Unfortunately, since sound waves require a medium to propagate, this technology cannot be applied in the vacuum of space. That means it won’t be much use for creating an actual tractor beam like the ones in Star Wars or Star Trek. Nevertheless, to demonstrate the capabilities of this new technology, the scientists who developed this technique created as close an approximation of this sci-fi dream as we are likely to see. As shown in this video, by mounting their speaker array upside down and covering it with a cardboard UFO, they used their “tractor beam” to suck a target particle up into the belly of a “spacecraft!” Tractor beams in space will continue to live only in sci-fi for now, but the advent of new technologies like acoustic levitation moves our universe ever closer to that of science fiction.
Erin Dahlstrom is a fourth year graduate student in the Physics Department at Harvard University. She works in the Levine Lab at the interface of physics and biology, studying the stress response of a small worm called C. elegans.
 Holographic acoustic elements for manipulation of levitated objects Marzo, et al., Nature Communications, 2015.
 The force awakens: tractor beam becomes a reality Ian Sample, Science editor, The Guardian.
 ’Tractor beam’ grabs beads with sound waves Jonathan Webb, Science reporter, BBC News.
 Acoustophoretic contactless transport and handling of matter in air Foresti et al., PNAS, 2013.
 Particle manipulation by a non-resonant acoustic levitator Andrade et al., Applied Physics Letters, 2015.
 What is a Wave? The Physics Classroom.
 How Sound Waves Work Media College.
 Sound is a Pressure Wave The Physics Classroom.
 Why does the shower curtain move toward the water? David Schmidt. Ask the Experts, Scientific American.
 How Acoustic Levitation Works Tracy V. Wilson. How Stuff Works.
 Real-Life ‘Tractor Beam’ Can Levitate Objects Using Sound Waves Tia Ghose, Senior Writer, livescience.
 On-chip manipulation of single microparticles, cells, and organisms using surface acoustic waves Ding, et al., PNAS, 2012.
 Acoustophoretic contactless transport and handling of matter in air Foresti, et al., PNAS, 2013.