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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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