Question 1

What actually holds the bead up — is it just sound pressure pushing it?

Accepted Answer

No — simple radiation pressure alone can't produce a stable trap. What traps the bead is the acoustic radiation force, a small nonlinear (second-order) effect described by the Gor'kov potential, which depends on the spatial gradient of both the time-averaged pressure and velocity fields, not on the instantaneous push of the wave.

Question 2

Why do beads settle at pressure nodes rather than pressure antinodes?

Accepted Answer

For a particle denser and less compressible than the surrounding air — true for polystyrene beads and water droplets — the Gor'kov potential has its minimum energy at the pressure node (which is also the velocity antinode), so that location is where the bead is stable. Very light, highly compressible objects like bubbles in a liquid can behave the opposite way.

Question 3

Why is the node spacing always exactly half the wavelength?

Accepted Answer

Two coherent waves travelling in opposite directions interfere to form a standing wave whose pressure amplitude is zero every half wavelength (λ/2) — that spacing falls directly out of the interference condition and doesn't depend on transducer separation, only on frequency.

Question 4

Why does raising the frequency change how many beads I can trap?

Accepted Answer

Higher frequency means shorter wavelength, so node planes pack closer together and more of them fit in the same transducer gap — more node planes means more independent stable trapping sites between the arrays.

Question 5

Why does bead size matter so much for how strongly it's trapped?

Accepted Answer

The Gor'kov radiation force scales with the particle's volume, i.e. with the cube of its radius, while gravity (its weight) also scales with volume — but the acoustic term additionally carries an extra factor from the size-to-wavelength ratio, so trap stiffness falls off fast for beads that aren't small compared to the wavelength.

Question 6

What is this used for in the real world?

Accepted Answer

Acoustic levitation enables containerless processing of materials that would be contaminated by touching a container, contactless manipulation of cells and droplets in biomedical research, and — via open-source rigs like TinyLev — hobbyist experiments floating small water drops, foam beads and insects.

Question 7

Why does the field in the animation pulse so slowly compared to real ultrasound?

Accepted Answer

Real transducers oscillate 20,000-60,000 times per second — far too fast for any screen to render frame by frame. The pulsing here is deliberately slowed down purely to make the standing-wave shape visible; the actual trapping force depends only on the time-averaged field, which doesn't care how fast we animate it.

Question 8

Why don't the beads also drift sideways out of the trap?

Accepted Answer

Real levitators (including TinyLev) use curved or phased arrays that focus the beam and create a true 3-D potential well. This simulation uses flat opposing arrays, so lateral confinement is approximated with a simplified restoring term rather than a fully derived 3-D Gor'kov field — noted here for transparency.

🔊 声悬浮:驻波与辐射力

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