Acoustic levitation: Next best thing to anti-gravity

A standing wave of high-frequency sound suspends a particle between two parts of the levitator.
Credit: Andrade/Univ. Sao Paolo

The Consumer Electronics Show (CES) started this week and has been dominating the news with revelations of gadgets that  are exactly as irrelevant to you as they were last year and the year before and every year back to the dawn of time. (Though that means the dawn of the age of of digital watches, in this case, not actual time.)

Somewhere in the rush of stories about USB-enabled oven mitts and Internet of Things SmartTissues is one truly original, brilliantly conceived, cunningly designed, wickedly effective bit of technology out there somewhere, just like every year, but you won't know for sure what it is until three years from now, when it will have been replaced with something better.

So forget CES gadget staples like the smart ring that lets you change channels by giving the finger; forget the smart coffee grinder/brewer that must pour out magic because you can push the "brew" button from ten feet away rather than by poking it with your finger or whacking it with a ring. And especially forget all the Internet of Things things that promise to make your house smart but can't tell you if a smart bulb is burned out, if the smart stove is still on, if the smart vacuum is chasing the dog or if the smart front door is standing open because the fracking brilliant door lock is lonely and heard there might be burglars in the area.

If you want to pay attention to something cool and gadgety, with Science, check out this bit of magic from researchers in South America who made a magic wand out of sound waves.

A group of researchers from Brazil and Uruguay build an acoustic levitator that can lift and move small objects without touching them and without having to leave them inside part of the levitator or rigidly locking a floating object into position a precise distance from the device.

This levitator comes with a small cylindrical emitter that produces the sound and a  reflector whose business end is a small concave dish that reflects the high-frequency sound waves back toward the emitter. On their return trip toward the emitter, the original sound waves run into new ones coming from the emitter. When they collide, the two waves of sound conflict with and obstruct each other to the point that they form a standing wave -- an apparently stationary set of waves that provide a consistent amount of pressure in one specific direction, with pressure points on each with enough energy to suspend an object at that point as if it were sitting on a shelf.

"Just turn the levitator on and it is ready," according to lead author Marco A. B. Andrade of the Institute of Physics at the University of Sao Paolo, who was quoted in the announcement describing the acoustic levitator that was published in the journal Applied Physics Letters.

The objects can't be very big, at least not yet. The objects in the test were polystyrene blobs 3mm across that don't weigh enough to feel their weight on your finger, but they should get bigger over time as researchers figure out how to strengthen and balance the acoustic wave more consistently.

In May a group of British researchers from an organization called the Engineering and Physical Sciences Research Council, which is affiliated with four universities, developed an acoustic tweezers they could use to pick strings of cartilage cells from the surface of a Petri dish and implant them precisely into place within a wound. They developed the technique to help with knee surgeries during which surgeons could use the acoustic tweezers to mold a string of cartilage cells into precisely the right shape to replace a missing tendon or ligament.

"Ultrasonic tweezers can provide what is, in effect, a zero-gravity environment perfect for optimizing cell growth," researcher Martyn Hill of the University of Southampton in Southampton, U.K. said in a statement. "As well as levitating cells, the tweezers can make sure that the cell agglomerates maintain a flat shape ideal for nutrient absorption. They can even gently massage the agglomerates in a way that encourages cartilage tissue formation." Acoustic levitation has been known for almost a century, but the first demonstration of a technique that could raise and hold and object in place was first demonstrated successfully in July, 2013 by researchers at the Swiss technical university Eidgenössische Technische Hochschule Zürich.

In January, 2014 a group of researchers from the University of Tokyo published a paper demonstrating a technique to control particles in three dimensions using acoustic waves that lift tiny objects and hold them in a pattern extending into three dimensions, and raise, lower or rotate the whole flight in unison.

The breakthrough for Andrade's team was the ability to suspend particles while the emitter and reflector moved, leaving an inconsistent distance between them.

The next step, Andrade says, is weight and practical application.

"Modern factories have hundreds of robots to move parts from one place to another," according to Andrade's statement. "Why not try to do the same without touching the parts to be transported?"

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