Making a Freezer Mug

Most recent answer: 07/04/2013

Q:
I am trying to make a "freezer cup" or "freezer mug". I would like it to chill a drink more quickly and keep it cooler longer than the typical cup/mug. I've decided to have an interior of copper due to its high thermal conductivity an a exterior of PVC pipe. I'm wondering if there is a nontoxic and readily available liquid that would be better than water? I would want one with freezing point just above a common freezer, and a high heat capacity. It seems like salt water would be my best bet. Any other suggestions about the liquid and or the Copper or PVC? Thanks much, Jim
- Jim (age 16)
Sequim WA USA
A:

 A salt solution should work. So should an ethanol solution. Either way, remember that for these solutions there isn't a single sharp freezing point. The freezing point for the whole solution is the temperature where freezing starts or melting ends. At lower temperatures, the solution becomes more concentrated, lowering the freezing temperature, until the solution becomes as concentrated as it can get, "saturated". So for a solution of NaCl the lowest freezing temperature will be about -21°C even for a small amount of salt. 

On materials: 

Copper usually isn't used in direct contact with food because it can slightly dissolve and give off-flavors. Aluminum conducts heat almost as well and forms a hard oxide coating which protects it. Maybe you could start with a standard aluminum can.

You may need a small hole in the jacket to prevent large stresses when the water expands as it freezes. So salt water may work better than an ethanol solution, because the salt won't gradually evaporate, unlike ethanol. It'll be easy to top up the water occasionally.

How do you plan to seal the outer insulator to the inner conductor? Repeated thermal cycling may be hard on most glues. Some, such as epoxies, are toxic. You should check what materials are used in food-grade containers. Perhaps some sort of silicon rubber would work. I think this sealing problem will be your biggest difficulty. 

I wonder if it might work best to make both the inner and outer cans out of aluminum, and just use some spacers in between for stability. This would freeze a little more quckly for faster re-use. Then you could use a removable foam insulating sleeve when it's out of the feezer.

Mike W.

 


(published on 07/04/2013)

Follow-Up #1: freezer mug

Q:
Thanks for the GREAT suggestions! -- Esp. about the aluminum can -- you probably just saved me about $10 for a few inches of 3" copper pipe {and also some bad tasting drinks :) } Jim
- Jim (age 16)
Sequim, WA, USA
A:

Let us know how this works out. It just might catch on.

Mike W.


(published on 07/06/2013)

Follow-Up #2: cooling mugs

Q:
This is the coolest thing! Since we're just referring to a salt water solution between the aluminum, what do you think about just leaving it unsealed? I was thinking that I would try this with a skinny can and normal size can both with the tips cut off and then with foam to protect from getting cut. Then putting a salt water solution in between and just freeing it upright... I guess then I'd have to drink with a straw though to avoid spilling salt water while drinking. What do you think?
- Casey (age 27)
Los Angeles, ca
A:

I think you've put your finger on the biggest problem- spillage. Having recently lost a computer screen to a water spill, I'm sensitive to that issue. Some sort of silicone rubber seal should be easy to include. 

Mike W.


(published on 07/23/2016)