Acoustic Levitation: Physics Law & Forces Explained

[mnb96]

Hello,
can anyone clarify what's the principle (or physics law) according to which small objects are kept suspended in air in acoustic levitation devices?

I understood that the device essentially creates an acoustic standing wave inside a container. This means that, in some specific regions of the container air remains "compressed" while in other regions it remains "rarefied". Such regions are located in correspondence of nodes/antinodes of the standing wave.

So far, so good.
Now what should I do if wanted to compute the force exerted upon a small object placed inside the container? Actually, what IS the force that is supposed to lift the object upwards? What is this force due to?

Thanks.

 

[mikeph]

The force would come from a pressure gradient over the surface of the volume (when integrated, a lift). What this balances against, I believe it's some kind of buoyancy force relating to the relative densities of the object and surrounding air. However, since you're getting a density gradient in the air due to the sound waves, it is probably quite a complicated problem!

 

[mnb96]

Hi!
thanks for your quick reply.
I am still trying to make sense of your explanation.
Could you please follow my reasoning and tell me if it is wrong?

1) Due to the standing wave the object will be immersed in a column of air having alternately low and high pressure areas (the nodes/antinodes of the standing wave).

2) Assuming the object is placed in such a way that its bottom lies on a high pressure area, while its top lies on a lower pressure area, it will experience a force directed upward (due to buoyancy).

3) Let's assume the sound intensity is high enough to let this (buoyancy) force overcome the gravity pull ==> the object starts to move upward.

4) After moving upward, the object will eventually enter a minimum pressure area (located at one node), and this area will be surrounded both above and below by higher pressure areas (each node lies between two antinodes) ==> the object will stabilize itself at a location close to the low pressure node (NOT exactly at the low pressure node, because of gravity).


Is this wrong or correct?
Thanks again.

 

[mikeph]

Disclaimer- I don't know the exact answer or method, I'm just trying to give you answers without giving you any wrong information.

I don't know enough about buoyancy to help you that much (didn't take that module!), but I think you're basically correct. If you know the shape and size of the object, and the amplitude and wavelength of the sound, you can (I think) calculate the lift due to the pressure difference at different heights in the standing wave.

Equate that to the object's weight and you can find the equation describing the conditions for keeping the object stationary.

This is assuming the object itself doesn't affect the standing wave.

 

[mnb96]

Ok thanks a lot.
I think now it is more clear.

In fact, my main concern was to clarify what was the nature of the upward-lifting force, and it was important to realize that it was buoyancy.
Some articles on science magazines I happened to read were occasionally mentioning "acoustic radiation pressure", which confused me a bit.
I still don't know what 'acoustic radiation pressure' is, but to me the mental picture involving buoyancy balancing gravity makes sense (hopefully it is correct).

 

[chrisbaird]

The principle is similar to http://www.youtube.com/watch?v=uFMdiw5dO7c": air pressure gradients exert forces.

 

[mnb96]

Thanks chrisbaird and mikeyw!

it seems that several articles (e.g. http://books.google.fi/books?id=66V...nepage&q=acoustic radiation pressure&f=false") report "acoustic radiation pressure" as the cause of levitation. I am not really convinced of what those articles say.

Could you elucidate the difference between the force due to bouyancy/aerodynamic lift and the force due to "acoustic radiation pressure"?