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Assume you are chewing some gum and pull it out of your mouth like so: enter image description here If you release the gum quickly it will spring back to your mouth as if it is elastic, but if you leave it for a few seconds then release it will just fall down like a piece of string. What is happening in those few seconds to get rid of its elasticity?

DannyH
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Gum acts springy on short timescales and like a very viscous liquid on longer timescales. That is, its stress-strain behavior is time-dependent. Here is why:

The gum consists of long molecules with kinks and bends in them, oriented in random directions and tangled up with one another. when you quickly pull on the gum, the bent molecules unbend like springs and also get snagged against their neighbors, urging them to unbend too- and they offer resistance, pulling back like springs. but with just a little time, they begin to slowly untangle and slip past one another, relieving the spring stresses, and then they flow like goop. this is called viscoelastic behavior and is common in rubber-like materials.

niels nielsen
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  • So what is happening on a molecular level in that time? – DannyH Jan 17 '18 at 21:30
  • the gum consists of long molecules with kinks and bends in them, oriented in random directions and tangled up with one another. when you quickly pull on the gum, the bent molecules unbend like springs and also get snagged against their neighbors, urging them to unbend too- and they offer resistance, pulling back like springs. but with just a little time, they begin to slowly untangle and slip past one another, relieving the spring stresses, and then they flow like goop. this is called viscoelastic behavior and is common in rubber-like materials. – niels nielsen Jan 17 '18 at 21:41
  • Why not put this explanation into the answer body? – Ruslan Jan 17 '18 at 21:45
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When you take the gum out of your mouth it cools.

MaxW
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