[stevegong]

My dad bought this thing in Beijing, I'm not sure how to describe it.

It's a medical device you use to warm up cold joints and stuff during winter...

It's basically a plastic vinyl sack with clear red liquid inside. In the sack there is also this round coin like flat piece of metal with a cross in the middle.

So basically you snap this piece of metal and suddenly the entire red liquid crystallises, and becomes absolutely boiling hot. It becomes as hard as a stone. Then it cools down gradually and becomes liquid again.

You snap it and the same process happens again, and apparently you can do this a thousand times or so.


So what is the chemical reaction going on and how does it do it? Why is it so repeatable? I can't understand how the metal thing can possibly release chemicals that would so suddenly make the entire liquid crystallise.


I've provided an image of when it's in the crystal state.

It's really pretty amazing...


http://www.people.virginia.edu/~rg8s/Misc/odd.jpg

[Kickaha]

Google is your friend.

http://ibs.howstuffworks.com/ibs/des/question290.htm

[stevegong]

Awesome, are these things really common? How did you know what to search for if you didn't know it's name already?

Thanks Man!

[Eugene]

Originally Posted by stevegong Awesome, are these things really common? How did you know what to search for if you didn't know it's name already? Thanks Man!

Yes, super-saturated liquid based heating pads are very common. I could walk into any Any Mountain or other outdoorsy shop and get one. Kickaha already knew the answer. He was just telling YOU to use Google.

[FFL]

Originally Posted by stevegong It becomes as hard as a stone. Then it cools down gradually and becomes liquid again.

Well, not exactly... you have to heat it back up to melt the solid back into a liquid. Conservation of Energy and all that...

"In this house, we obey the laws of Thermodynamics!"
-Homer Simpson

The ones I've seen have had clear liquid - the red stuff is a new one on me.

[stevegong]

yeah, you're right. it's cool but still hard.

So I'm supposed to boil it? Won't that melt the plastic?

[Eugene]

Originally Posted by FFL The ones I've seen have had clear liquid - the red stuff is a new one on me.

It's probably just dye.

Originally Posted by stevegong yeah, you're right. it's cool but still hard. So I'm supposed to boil it? Won't that melt the plastic?

Probably not, but if you're worried about that, you probably don't have to put it in a pot of boiling water. Just boil the water first and then dump it in. Or you could suspend somehow...use your creativity here.

[Kickaha]

Originally Posted by stevegong Awesome, are these things really common? How did you know what to search for if you didn't know it's name already?

Um 'metal disk heating pad' was the first run, found a site that sold them, ran a couple more terms in there, and voila. I think it was the third link.

Efficient googling is an artform.

[alcimedes]

i have a pile of those at home. if you're very careful, you can actually mircowave them, even though there's metal in them. that's by far the fastest way to get them back to liquid form.

for the most effecient heating, you should massage the crystals as they initially heat up. by keeping them from hardening quickly they give off a lot more heat.

[stevegong]

what does being careful consist of when microwaving them?

how long do you micro them?

sounds pretty scary after that time I put a cd in the microwave.

[alcimedes]

just watch it as you do it, and microwave it in 15 second intervals. of course, depending on how powerful your microwave is, you may need to change tha time somewhat. i found usually a minute to a minute and a half was long enough to return it to its full liquid state.

[Kickaha]

Originally Posted by stevegong what does being careful consist of when microwaving them?

Not sticking your head in there too. Cats are okay though.

how long do you micro them?

Until they turn purple and dance the robot.

sounds pretty scary after that time I put a cd in the microwave.

Feh. Try a CD *player*. Now that's fun.

[billybobsky]

that is exactly it -- the microwaves can eject electrons from the metal surface (although if positioned correctly in the bag) there shouldn't be a problem... you can just dump the bag in a bowl of water in the microwave and have the same effect (water will act as a microwave insulator)...

as for the "snapping the metal causes the molecules to solidify," that isn't exactly correct. There is a kinetic barrier to crystalizing, hence the ability to supercool things when there isn't a surface that would tend to catalyze crystal formation; and all the snapping does is produce enough local heat to allow a few molecules to crystalize and the cooperative crystalization of the rest of the mass occurs releasing heat...

[Ebby]

Mine that I bought from REI says to wrap it in a small cloth and boil it for about 10 minutes. You have to make sure to melt ALL the crystals, or else it slowly re-crystalizes again.

[FFL]

Originally Posted by Ebby Mine that I bought from REI says to wrap it in a small cloth and boil it for about 10 minutes. You have to make sure to melt ALL the crystals, or else it slowly re-crystalizes again.

Yep, that's the best method. You don't have to go to a full boil, either - a medium simmer works great. You just have to get it over the 135-degree F. melting point. Mash on 'em with a wooden spoon or other safe utensil to mix up the liquid and unmelted solid on the inside every minute or so and they'll melt (or "recharge") much faster.

I've had lots of practice at using and recharging these things. A partner and I sold several hundred thousand dollars worth of these things in shopping malls throughout Florida, about 10 years ago. Kind of like selling ice to Eskimos, except in reverse...