Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

We all know that entropy is the measure of chaos in the system, and it's always increasing in the system.

Now comes my question - how does entropy work in smart materials? Here is a youtube video about smart material that I'm talking about, it remembers its shape - you can distort it, but when you heat it, it returns back to its original shape. So effectively it reverses from more chaotic state to a more uniform state:

Therefore my question - how come the system became less chaotic just by adding heat? It looks like entropy decreased, but the 2nd law of thermodynamics says that entropy can only increase.

share|cite|improve this question
The entropy is swamped by atomic disorder, so that the macroscale disorder is insignificant. Even if a material orders in a visible way, it is disordering in an invisible atomic way. The atomic entropy is so much larger, that it is misleading to characterize entropy in terms of disorder, because it leads to puzzles like this. – Ron Maimon Jul 17 '12 at 3:54

The law that entropy always stays constant or increases applies only to closed systems.

This material gets more ordered at the price of greater disorder in the closed system : energy generator + material.

It is the same with crystals, crystallizing out of a solution. They are highly ordered but the energy expelled to the total "solution +crystal" increases the entropy of the closed system.

And the paramount example are living things. They are highly ordered, but not a closed system. They continually absorb and expel energy to the surrounding environment increasing the entropy of a "living system" + "energy source" closed system.

share|cite|improve this answer

There are two parts to this.

  1. As @anna v said in her answer, the Second Law applies only to closed systems. If you want to try to apply it to this system, you need to take into account the entropy created by your heat source.

  2. As @Ron Maimon suggested in his comment, it's not obvious just by looking at the macroscopic shape of the material which scenario has a higher entropy. If you count the entropy of the material and its immediate surroundings, deforming the material can sometimes cause a lower entropy by allowing the immediate surroundings to rearrange themselves into a highly ordered state.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.