Scientist Explains How to Levitate Objects With Sound
Released on 01/23/2020
[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.
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