Mailing Fish, or "Why you should care about Styrofoam."

I was approached with a seemingly simple question today:

How would I send frozen foods to my significant other in Afghanistan without them thawing on the way there?

My initial thoughts ran to refrigerated cargo services, or having a store in a location closer to Afghanistan ship it a shorter distance... but none of these solved the whole problem of getting it to an APO. So while I'm not sure my final solution will work for an ACTUAL package trying to make it through ACTUAL base security (I'll wait for one of my military readers to comment), I can virtually guarantee that food shipped in this method will remain frozen quite nigh on indefinitely. Or at least a week, whichever comes first.

First, some background - I LOVE the little cheap one dollar "we're going fishing" rounded-corner Styrofoam coolers. They're a dollar.

Doing a quick google search for a picture of such a cooler, I can't find one - only these:

Which might work, but I get mine at a local Roses department store. For a dollar. Go cheap or go home ;-)










Sure beats the hell out of these, though they might be a better choice when actually shipping something to an APO, since it's "real" packing material.

So there comes a point here where science applies, and I think I'll choose to make that point right about... now.

There's a very simple relationship between the temperature of a body, the temperature of its surroundings, and how quickly the two temperatures approach one another. If the body is cold, then the surroundings lose heat to warm it up, and vice versa.

Observe:


What we have here is an insulator and a conductor - since the heat of the room flows easily into the conductive plate, it passes immediately into the ice and melts it. The insulator obviously, and by definition, obstructs the flow of heat, and so the second ice cube is barely injured in the time it takes the first to completely melt.

This is direct conduction, but as you know, air is all around us - when air, or any fluid, takes heat away from an object, we call that process convection.



As you can see here, the candle flame causes a current of air to rise up as it is heated, carrying heat away from the flame. In our case of an ice cube or frozen food, the air current would be downward, but you can see the basic concept - air will be set in motion, bringing more warm, room-temperature air into contact with our food. Not a good prospect for keeping it frozen on a trip of several thousand miles!

So, our goals are clear - keep it out of contact with the surroundings by placing it in a non-conductive container, and seal the container so that convection doesn't carry heat away either. There is one more mechanism of heat loss (or in our case, gain) - radiation.

Here, we see the use of a thermal camera to visualize the radiated heat from objects in the form of infrared light. Notice the otherwise black trash bag is completely transparent at these wavelengths!



So since we want to block heat from entering our container, but plastics seem to be nearly invisible to infrared radiation, and we should avoid conductors, but metal (the aluminum mirror) seems to reflect infrared radiation well, what are we to do?

Aha! We combine the two! Yessir, your common everyday "space blanket" is made from aluminized mylar, developed initially as a thermal barrier material for the space program. Other metals have been used, like gold, but the principle is the same, with a thin layer of metal vapor-deposited on a nearly perfectly smooth film of plastic, resulting in a mirror that reflects upwards of 98% of the incident infrared radiation hitting it!

So basically, with a bit of insulation, a radiant barrier, and a good seal, you can keep something warm, or cold, for a very long time. This, by the way is exactly how a Thermos bottle works. Just a double walled container with vacuum pulled between the walls, and the insides are vapor coated with aluminum, just like the space blankets, so they reflect heat, and the vacuum is about the best insulator you can get. With a tight fitting top, there's no convection to move air about the contents, and you're only losing heat through the cap itself and the wee little bit of material that connects the inside and outside walls along the cap threads.

Another victory for science, and another hot meal, or cold pre-meal, anytime you want it.