Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Laura_Kelly on 11/07/2009 10:30:02
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Laura_Kelly asked the Naked Scientists:
At school, we have seen Faraday Waves form on cornstarch mix placed in a speaker that vibrates at different frequencies.
How do the waves form? We can't find any suitable explanation for this phenomenon.
What do you think?
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A particularly good video of cornstarch on speaker ...
The corn flour is a (non-Newtonian) shear-thickening fluid ... http://en.wikipedia.org/wiki/Dilatant
Faraday waves in plain-old Newtonian fluid (like water) on a speaker are standing waves, which have regular symmetrical geometric patterns ... (unlike the chaotic cornflour above).
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The corn starch (or sand or whatever) will be moved about as the speaker vibrates. Some parts of the speaker will vibrate much less than others (may even be stationary) and the particles which land there will stay there, once they have landed. So you get a build up of particles in the so-called nodes ( where there is no d(e)isplacement) and very few particles in the antinodes because they are constntly being kicked into motion. So you can see the pattern of the standing waves on the speaker.
It's a bit like the effect on a windy day, when leaves and rubbish find themselves in sheltered spots and are constantly being removed from the middle of the road, where the wind is strongest.
A 'good' loudspeaker unit should not have resonances and should, ideally, move backwards and forwards like an ideal piston - that's why they are made with a conical shape which gives them strength even though they are very light.
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Thanks for that. What we are really looking for is an explanation of how the different patterns are formed. Especially since there are no specific frequencies for each pattern, showing it is a continuous as opposed to discrete phenomena, which does not fit with a theory of resonant frequencies. We have found some interesting patterns ranging from geometric patterns to flower shapes, to labrinth-like patterns, where there seems to be no set rule or placement of nodal and anti-nodal lines.
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The patterns are wavelength dependent, rather than frequency dependent. (The reason I make the distinction is that the speed of the vibrations over the surface will be hard to determine.) Standing waves (which is what you are seeing) are very complex - particularly because the effective edges of the surface are not clear - unlike the ends of a clamped, vibrating string, which is much easier to analyse because it is a relatively simple one dimensional problem. Rectangular plates are the next easiest to analyse but round and conical plates are much much harder to analyse.
You will see that the higher the pitch (shorter wavelength), the smaller are the patterns. More than that is hard to predict.
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More cornstarch waves ...