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Physics Question July 3, 2016

Posted by Sobek in News.

So here’s the Slow Mo Guys, smashing open spraypaint cans in super slow motion because they can (and because they’re really cool).

Obviously that’s just pure awesome.  But here’s my question.  If you go to 1:40, Dan shoots a red spraypaint can with a pellet gun, and you can see (very clearly, if you pause at the 1:43 mark) that the paint is not coming out uniformly, but rather in a wave pattern.  That’s my physics question – what causes the wave-like nature of the spray, rather than a more uniform effect?  My first thought, when I watched some of the other explosions, was that the paint is affected by Brownian motion, but that wouldn’t explain wave patterns.

The next thing I thought was this reminded me of an astrophysicist’s explanation for the arms of a spiral galaxy.  The arms are kept separate by pressure waves.  I assume there are pressure waves involved here, but I can’t think of what the source would be, either in the case of the paint or the galaxy.


1. Retired Geezer - July 4, 2016

Cohesion wants to keep the paint together while the propellant is pushing it out.
It holds together until the tension breaks and then… uh…
Lost my train of thought there.

I agree with what Mrs. Peel opines.

2. Sobek - July 4, 2016

Brother-in-law said it’s because of a vacuum. The rapidly-exiting paint creates a vacuum that over-powers the outward pressure of the paint and gulps in some air unti they equalize, and then more paint can spray out. Like dumping out a jug of milk. It’s pretty wild to think how strong of a force a vacuum is that it can over-power all that paint.

All that said, I also agree with Mrs. Peel and Geoff on the matter.

Next question: assuming it’s a vacuum phenomenon, could the amplitude of the wave be modulated by changing either the size of the hole, or the amount of pressure in the can, or both? Are there any other variables that could be modified?

3. geoff - July 4, 2016

I believe it’s the same phenomenon that occurs when liquid drops form out of your faucet: Fluid thread breakup.

4. Sobek - July 4, 2016

Geoff’s answer sounds basically like Geezer’s. [high five, Geezer] But fluid thread breakup, as I understand it, is a function of gravity and the surface tension of water, and it doesn’t look (in my unprofessional opinion) like the spray paint phenomenon is about gravity. The waves are present immediately after the paint starts to exit the can.

5. Retired Geezer - July 4, 2016

Interesting that until the advent of super slo-mo, you would not be able to observe any of those behaviors.

Also, the axe/sledge guy had the worst of that gig.

6. skinbad - July 5, 2016

He would never get a job pounding railroad spikes.

7. geoff - July 5, 2016

Good old Harold “Doc” Edgerton pioneered high-speed photography. He’s famous for the photos of the bullet through the playing card, the kid popping the balloon, and the milk drop.

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