Scientist Explains How to Levitate Objects With Sound

[Narrator] When you hear the word levitation,

you probably picture the hoverboards

from Back to the Future or magic tricks like this.

But gravity-defying technology

isn't just the stuff of science fiction.

It's very real.

Acoustic levitation uses sound waves

to counteract gravity.

[Narrator] Acoustic levitation is unique because,

unlike magnetic levitation, for example,

it can effectively suspend both liquids and solids.

But there's a small catch.

The largest object we've levitated

has just been a three millimeter bead.

[Narrator] But even at that scale,

there are some exciting applications,

like analyzing chemical reactions in suspension,

the creation of better drugs,

and even improved robotic arms

that can manipulate tiny, delicate objects.

And you can track an object to counteract gravity

by creating a space where there's no force.

[Narrator] That's Chris Benmore,

a physicist with Argonne National Laboratory,

who uses this gravity-defying technology.

We spoke to him to find out how acoustic levitation works

and what exactly it's used for.

Walk us through what acoustic levitation is

and how it works.

Acoustic levitation uses sound waves

to generate a force to counteract gravity.

It was developed primarily by NASA

in the '60s and the '70s to do ground-based experiments

on looking at the effects of anti-gravity on earth.

And can you walk us through the different components

of the device that you have there

and how the sound waves actually come together

to produce the levitation?

These transducers basically drive these horns,

the silver part.

So this horn will vibrate at 22,000 times a second

up and down to generate a sound wave.

And we have a match transducer down here and horn,

and that will generate another sound wave.

And when these two waves interact,

you'll get what's called a standing wave.

So they'll cancel in places

and they're reinforcing others

to create nodes and anti-nodes.

And those particular places where they cancel,

you can put an object in and you can levitate.

All right, well, let's see this in action,

if you wouldn't mind giving us a demo.

Right now, I've created a standing wave.

These horns, which I'm not gonna touch,

are vibrating at 22,000 times a second,

creating a standing wave.

And so I can put an object in that little cavity

where the two standing waves cancel.

In fact, there are several cavities

where I can put objects.

And so if I just have this brass rod here,

you can see I can go through them.

If I come in from the side with my hand

and see I get some reflections, I will disturb it more,

so I will interfere.

Is there anything particularly special about

the sound waves themselves, or is it more the way

they're interacting that is really central

for producing the effect?

It is the way they're interacting

and the particular frequency.

So both of these devices operate at 22 kilohertz.

And so that's just on the edge of human hearing.

So you might hear it come in and out.

Okay, especially if I turn it up to the higher power.

[Narrator] Although it's loud, the sound waves

are at such a high frequency

it's almost imperceptible for humans.

You might just be able to make out a high pitched pulse.

At that frequency,

there is a spacing between the nodes of six millimeters.

Okay, so this standing wave that's created

will create pockets.

And this six millimeter spacing

limits you to how much you can put in there.

So you can put something in an object,

so maybe half of that size.

So something like three millimeters.

So when you add a little object, how do you know exactly

where to place it to get it in that right spot?

You can, of course, actually do the math

and calculate it between,

this is actually a very precise distance between the two.

When you actually spray a mist of water,

you'll get a vortex, and the droplets will be drawn

to the most stable places within this region.

And when you place them, they almost look like

they kind of snap into position.

Is that the case?

That's exactly the case, yeah.

So the standing wave is fixed by the geometry.

And so they're every six millimeters, okay?

So if I try and put it close there,

it will naturally lock into position.

And can we elaborate a bit on the limitations for size?

Why limit to smaller objects?

Why can't you, for instance, levitate me,

if I would want such a thing?

This is actually generating an awful lot of sound.

Even though it's a pretty small device,

it has about the same level of sound as a rock concert.

So you could build bigger transducers

and levitate larger objects,

but it would be deafening, for one,

and also very destructive for another.

You imagine 10 times a rock concert

to levitate an object that's maybe a centimeter in size.

So you can imagine, if you wanna levitate you,

you would've to build something enormous.

So given that you're working with focused sound waves,

would it be possible for actually an outside actor

if they wanted to mess with your experiment,

to throw their own sound waves at the device

to disrupt the object that you're manipulating?

They certainly could, particularly on this device,

because this is just a single axis levitator,

so it really only counteracts gravity.

It's pretty unstable in the horizontal direction.

So quite often, what people can do

is have another levitator,

say at 90 degrees to that to stabilize it.

I think one of the other interesting things

about the kinds of objects that you can put

in these levitator is you can do solids,

and as you had mentioned, liquids.

Why would that make this particularly

useful as a technology?

For us, it's an ideal device

for holding a droplet in space

with no other interactions around it.

So you can just study that droplet.

[Narrator] Benmore and his team are currently using

this device to analyze pharmaceutical drugs

with the help of an extremely powerful X-ray.

We have the most intense X-ray source

in the western hemisphere here at Argonne.

And so what we're able to do is look at the atomic structure

where all the atoms are arranged.

And so we identify the molecular shape

and how the molecules interact.

So what it allows us to do

is kind of trap that drug in that manufacturing process

and give an idea of what conditions you need

to actually make a more effective pharmaceutical.

At the present moment,

you are levitating fairly small objects.

Can you walk us through what it will take to actually

scale that up to levitate bigger and bigger things?

What people are trying to do now

and have successfully done to a certain extent

is to make arrays of these.

So if you have a whole array of these,

say you have five in a row in one direction

and five in another, so you have 25,

you can levitate a larger object

just by levitating in certain places.

Rather than having a bigger transducer,

you just have more of them.

And in fact, what they've able to do as well

is to actually move objects around,

'cause you can change the amplitude using software

to vary the power in one transducer compared to another.

So you'll be able to move the object laterally

as well as vertically.

[Narrator] For example, take a look at this robotic arm

that uses acoustic levitation

to move objects without ever touching them,

reducing the risk of damage or contamination.

In the future, this could give robots a more delicate touch.

So this isn't the only form of levitation out there.

Can you walk us through perhaps

some other methods of actually levitating objects.

and maybe why this is a more effective

method in certain ways?

So there are many types of levitation,

from magnetic, electromagnetic, to aerodynamic levitation.

And in fact, one we use quite a lot

is aerodynamic levitation.

And it's really quite an easy one.

If you've ever had a ping pong ball on a hair dryer,

you can imagine how that one works.

That is very effective.

[Narrator] Magnetic levitation or maglev

can suspend something as massive as a train

by using opposing magnetic forces.

But acoustic levitation is unique

because it's ideal for handling tiny fragile objects

and non-conductive substances like liquids.

This is soft enough that it'll keep the droplet together.

And of course, a lot of these droplets aren't magnetic,

so we can't use magnetic levitation.

And looking forward in the future,

might we be able to speculate

on other applications for this technology?

One application that's going on right now

is a combining of acoustic levitator

with an aerodynamic levitator.

So you can get the benefits of both.

And I have to ask, could you potentially make

something like a hoverboard out of this technology,

or are you inherently limited to the lab

because you have to have these two

devices interacting with each other?

You can actually make these device is very small now,

and you can have many of them.

So maybe not more powerful,

but you could have a lot of them.

I don't think it's enough to levitate a hoverboard,

let alone a person, but you can certainly

think about levitating heavier objects.

Thank you for blowing our minds today.

Well, thank you.