Chladni patterns

Chladni was a German physicist and a trained musician and is best known for inventing a technique to visualize the various modes of vibration of a rigid surface.

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Take a flat metal plate, fix it at any point (say at the center) and sprinkle some fine sand particles on it.

Using a violin bow, gently excite any edge of the plate to witness beautiful normal mode patterns (known as Chladni patterns/figures).

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Notice that by pinching the plate at different points one gets different patterns.

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                               Gif source video: Steve Mould

Inverse Chladni plates

There is much more than what meets the eye in this experiment as the patterns that emerge are dependent on the size of the particles that one chooses and the acceleration of the resonating plate.

Bigger particles ( > 0.15 micrometers ) – tend to accumulate at the nodes of the vibrating plate – Chladni figures.

Smaller particles ( e.g Lycopodium powder ) – tend to accumulate at the anti-nodes of the vibrating plate – Inverse Chladni figures.

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                   Source : “Air-induced inverse Chladni patterns

What causes them?

Although the traditional Chladni patterns are discussed extensively in the context of solid mechanics and normal modes of vibration in engineering.

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But that’s NOT where the story ends.

Michael Faraday argues in his remarkable paper that the ‘inverse Chladni patterns’ that one observes are not a run of mill mechanics problem with a secondary mode of vibration.

Instead, he demonstrates that the vibrating plate induces air currents on top of it, which drags the fine particles along to the antinode. This is completely counterintuitive since one doesn’t expect the air to play such a pivotal role in these patterns.

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A plot of the velocity profile above the Chladni plate.  Notice the air currents that are caused by the vibrating plate. This phenomenon is often referred to as “Rayleigh streaming”.

Thus far we have merely considered a plate vibrating in air.

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But what if we were to look at water atop a vibrating plate? For that, check out the accompanying FYFD post and stay tuned for more tomorrow.