Morgan - 25-1-2014 at 18:44
Somehow the burning polyethylene forms a resonance as it falls through the air. I tried melting some HDPE containers and the waxy drops really do
sound like zippers being pulled up and down as one viewer remarked. I wonder if you could increase the effect, make them more zippy?
Dripping Flaming Tyvek
http://www.youtube.com/watch?v=Q7b-yabEa7g
http://www.youtube.com/watch?v=RAuNsfhOybQ
Behold a high density polyethylene pipe burning with dramatic background music.
Burning Plastic Pipe
http://www.youtube.com/watch?v=XV5pBTylDak
[Edited on 26-1-2014 by Morgan]
bfesser - 25-1-2014 at 18:59
My initial thought was that the decomposing polymer is boiling off lower molecular weight components, and that the sound is caused by the bubbles of
this gas bursting at the surfaces of the drops and igniting in little explosions as they mix with oxygen in the air. But I suspect that it could be
much more complex, as the <a href="http://youtu.be/c3fJRRCAIdk" target="_blank">Barking Dog Demonstration</a> <img
src="../scipics/_yt.png" /> turned out to be.
[edit] Sorry, IrC. I tend to favor brevity and leave it to the reader to interpolate the details. I like your detailed
explanation.
[Edited on 26.1.14 by bfesser]
IrC - 25-1-2014 at 19:04
I'll bite. Just bear in mind I could be completely wrong, but I have noticed that sound from many different burning/dripping plastics. I do not think
it must be related to your particular example of polyethylene alone, rather any burning plastic with similar physical characteristics in a
molten/burning state. I think the sound is a flame front going out and reigniting rapidly many times as it falls through the air. The molten plastic
is cooling but not rapidly enough to get below the ignition point of vapors streaming from it as it falls. These vapors keep flashing into ignition. I
do not think I would call it a 'resonance', but possibly you could. I was thinking more in terms of a relaxation oscillator occurring at audible
frequencies. I suppose the frequency of extinguishing-reigniting would have a 'frequency' based upon temperature, viscosity, rate of air in fall
cooling and extinguishing the flame in front of the fall, where the specific heat is high, this trailing heat reigniting trailing vapors which then
flash all around the falling drops. Long sentence but it's really hard to say with few words a complex process I imagine going on. Cooling wind
putting out the forward portion (as in trying to blow a flame out) combined with enough heat to continuously produce new vapors trailing as well as
reigniting these vapors. Possibly I am getting redundant in this 'theory'.
Maybe I did a crappy job explaining my thought on this. Any ideas out here?
That figures, I took so long typing bfesser got in and beat me to the first reply.
[Edited on 1-26-2014 by IrC]
cyanureeves - 25-1-2014 at 19:12
red ants add a higher pitch sound when they get hit by those zoops. i think the flame extinguishes to almost nothing as gravity pulls down as it
falls but then reignites in a burst when oxygen now multiplied by speed/travel mixes with the flammable liquid.clorox bleach bottles are excellent for
this and as the plastic starts to burn you can see it turn clear and run like water just before it drips.it gets really loud when the bottle is about
half way burnt because for a couple of secs. it will be lit like a ring of fire until the top diameter half melts forming one big glob.
[Edited on 1-26-2014 by cyanureeves]
cyanureeves - 25-1-2014 at 19:15
et tu IrC ? ant killer you!
UnintentionalChaos - 25-1-2014 at 19:30
Just to throw a thought in there, there may be some parallels with a pulse jet in the repeated combustion/partial extinguishing cycle.
Morgan - 25-1-2014 at 20:15
Seems like a fun material to launch in a trebuchet if you could get it to stay lit and fragment or have a big melted blob meteor overhead instilling
fear with its zipping raining terror of waxy drops.
But more realistically, it would be interesting to see what level of oxygen in an air column the melted burning HDPE would work best in or if an
oxidizer or other additive could be blended into the plastic and then drop tested without destroying the effect, hopefully retaining and amplifying
the characteristic somewhat.
[Edited on 26-1-2014 by Morgan]
Pyro - 25-1-2014 at 20:40
Morgan, nowadays we have these things called modern weapons
http://en.wikipedia.org/wiki/Category:Weapons_post-1945
makes me think of this quote:
“Whoever said the pen is mightier than the sword obviously never encountered automatic weapons.” Gen. Douglas MacArthur
would the effect be a lot more extreme in an enriched O2 atmosphere?
violet sin - 25-1-2014 at 22:31
I sent off a request to Destin from "Smarter every day" channel on youtube just to see if he could use his slow motion camera and scientific thinking
to explain it. I have no idea when he may be able to get back to me, but it would definitely be cool to watch what is going on in super slow hi
definition.
in the past I for some reason thought it was a difference in viscosity between the center and more fluidized outer shell, kinda flipping on the way
down. but that was years ago. I think it was the strobe effect that made me think it tumbles. but the flame front idea seems to hold water.
Morgan - 26-1-2014 at 07:36
That's a good idea violet sin. The first thing I thought of was it would sure be nice if you could slow it down and study the flame front to see
what's going on.
Sometimes the first drops seem content to burn quietly as they fall, maybe flaming out prematurely and at other times the effect starts to kick in.
The temperature of the melt, the speed of the drops, and the size/surface area of the drops probably all figure in.
I wonder if you could Leidenfrost some polyethylene on a sawtooth maze, have the smoking melted drops skitter about and then light them on fire for an
added exotic "zippy" effect?
Leidenfrost maze for review
http://www.wimp.com/leidenfrosteffect/
[Edited on 26-1-2014 by Morgan]
Morgan - 26-1-2014 at 11:43
SmarterEveryDay did some good camera work with Prince Rupert's drops I recall.
I found a tidbit on high regression rate fuels but it that's about it. I remember mixing some prilled ammonium nitrate and aluminum powder and heating
it with a propane torch. It would melt into a liquid and then flash intermittently staccato style as I directed the torch on it, a "botryoidal"
bubbling mass that would go out if I stopped applying heat. It wasn't nearly the frequency of the polyethylene drops zipper effect however the
blinding on/off flashbulb effect reminded me of it.
Advanced Hybrid Rocket Fuels:
http://www.spg-corp.com/advanced-hybrid-rocket-fuels.html
[Edited on 26-1-2014 by Morgan]