Sound Waves Levitate, Move Particles
People have been able to levitate small objects using sound for years. But applications for the technique are severely limited because scientists hadn't figured out how to control and manipulate the floating objects. Until now.
The idea of acoustic levitation is certainly not new. Several levitation methods exist today, including using electrostatic forces, magnetism and light. But each of these techniques has it own drawback. For example, light can only manipulate matter that's very tiny (up to about 50 microns, the size of a human liver cell), whereas magnetic levitation requires the material to have special properties.
Sound waves, on the other hand, can float larger objects, independent of material properties. The basic setup requires an acoustic emitter and a reflector placed some distance away. When the sound is turned on, it bounces off the reflector and back towards the emitter.
The two opposing waves (emitted and reflected) interfere with one another, creating a standing wave. Certain parts of the wave, called nodes, don't move. And when the standing wave is parallel with gravity, some portions of it have a constant downward pressure and other portions have a constant upward pressure -- the nodes, however, have very little (virtually zero) pressure.
"A sample can be pushed into this region of zero acoustic pressure and it will levitate due to net force on it," says Daniele Foresti, an engineer at ETH Zurich, a university in Switzerland. "But how do we try to move this object? Can we think about a system or new concept to move around a standing wave, taking the sample with it?"
As Foresti and his colleagues discovered, the answer to this problem is fairly simple. The researchers built an array made up of individual emitters, called Langevin piezoelectric transducers or LPTs, each separated by a small gap. The surface of each LPT was only 15 mm by 15 mm (0.6 inches by 0.6 inches). Next, they placed a single, flat reflector above the LPT array. With this set up, they essentially produced a long standing wave created from multiple sources.
"The idea then is to move a sample from one acoustic levitator to another by slowly shifting the LPTs' power," Foresti tells io9. If you slowly turn down the driving voltage of one LTP (which has the object) while slowly turning up the voltage of an adjacent LTP, you can horizontally move the sample and even pass it between levitators. "If you can do this using two levitators, you can think about having a line of them and produce planar motion."
To levitate small objects like water droplets, you need fairly high acoustic power, at around 160 or 165 dB (the sound level of a rocket launch), Foresti says. Sometimes scientists will fashion their acoustic levitators in special sound chambers to protect their hearing; Foresti and his team instead used frequencies above 20 kHz, the upper limit of human hearing.
The researchers demonstrated their system's applicability in several experiments. In one test, they brought two water droplets together, combined them and then separated them again. They also illustrated what can happen when a solid particle comes in contact with a water droplet: in one case, an instant coffee particle dissolved in the water particle; in another experiment, a particle absorbed the droplet.
Additionally, the team used their "acoustophoretic method" for DNA transfection, where genetic material is introduced into cells -- the transfection efficiency was comparable to standard methods, but required 50 to 75 percent less reagent.
Foresti says there's no real limitation for the length or size of the object you can manipulate with sound, equipment permitting. In one experiment, the team used a square array to transport and rotate a toothpick, which spans about five LTPs. "Density is the main issue," he says, adding that the more dense an object is, the more power you need to levitate it.
The technique has a number of potential applications, most notably in biopharmaceuticals. For example, acoustic levitation could be used to evaporate solutions without having them crystallize -- the body takes up amorphous drugs more efficiently than crystalline drugs, but solutions are more likely to crystallize if they evaporate in a container. With the acoustophoretic method, scientists could also conduct hazardous chemical reactions without running the risk of contamination from containers.
"We have simply a new tool that I believe can be used in many different fields," Foresti says. "My hope is that people can use it to discover new things."
The researchers detailed their work in the journal PNAS.
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