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Falling Plastic Drops and Sound
Name: Eve W.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 1/13/2003
Question:
Last week I saw a demonstration where a newspaper
plastic bag was suspended and burned. Hot droplets would fall from the
suspended plastic bag. During this process there different noises made.
I was wondering what would cause this sound.
Replies:
Without actually being present, it is hard to say with certainty. Assuming
the sound was not just the usual sounds that accompany flames that are caused
by the rapid implosion of hot air pockets, one possibility comes to mind.
Newspaper plastic bags are low density polyethylene (it is the least
expensive). When the molten drop of polymer falls, it leaves a thread of
fluid polymer because the time required for the chains to untangle is long
compared to the rate at which the molten drop is falling. This thread cools.
The cooling comes from two effects. First, the thread is thin compared to
the "blob" as so heat is dissipated more rapidly. Second, the polymer chains
are being stretched and aligned along the axis of fall. This stretching
results in a decrease in entropy of this part of the thread, which in turn
cools the thread. Cooling causes the polymer thread to increase in viscosity
and stiffens the thread. However, if the inertia of the falling drop exceeds
the elongation strength of the polymer thread, it will break with a
"snapping" sound. A similar thing happens if you stretch a rubber band
beyond its elongation strength. An aside: Polymers that are lightly
cross linked, like a rubber band, it cools when you stretch it and warms when
you release the tension. This is the opposite of the behavior of most
materials. If you want to look into this effect in more detail, do a web
search on the terms "rubber elasticity."
Vince Calder
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Update: June 2012
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