If you spend much time on the internet, you will see the same things pop up again and again. For me, it's these “cooling blankets” that people talk about on social media. I mean, it sounds great for summer—just like a blanket that warms you up but in reverse.
Sadly, these products don’t do what they claim. They might be breathable so they don’t make you as hot as an ordinary blanket would, but you’d still be cooler with no blanket at all. However, there is hope. Someone has created a real cooling blanket that's sort of awesome. Of course, there's a bunch of physics here, so let's get to it.
Temperature vs. Energy
Temperature is one of those words everyone uses and no one understands. In chemistry it’s the average kinetic energy, or vibrational motion, of the molecules in a substance. The greater the commotion, the higher the temperature.
But I like this more pragmatic definition: Temperature is the property two objects will have in common when they’re in contact for a long time. So, if you take a hot block of metal and set it against a cold block of metal, eventually they will have the same temperature. Heat flows from the warmer thing to the cooler thing until they equalize. (Note: It doesn’t work the other way around; you can’t transfer “coolness.”)
We also talk about objects having a certain amount of thermal energy, which you’d get by adding up the kinetic energy of all the particles inside it. It depends on three things: the mass of the object, its temperature, and the material it’s made of. So for instance, focusing on mass, big potatoes have more thermal energy than small potatoes at the same temperature.
Now, if you look at the type of material, every substance has a “specific heat capacity,” which is the amount of heat required to raise the temperature of that substance by one degree. Try this at home. Find two objects that have been sitting in your room for a while, so they’re both at room temperature. Here, I have a block of wood and a block of aluminum.
Touch both objects. They’re the same temperature, but the wood feels warmer, right? Why is that? It's not about temperature but thermal energy. When your hand touches an object, there is a heat conduction interaction. Energy is transferred from your warmer hand to the cooler object until the two are the same temperature. However, with the metal block it takes way more energy to reach the temperature of your hand. It feels cooler because it causes your hand to lose more energy.
You’ll notice the same thing when you go swimming. An air temperature of 75°F feels nice and comfortable, but wading into water of the same temperature feels really cold. That's because water has a much higher mass and specific heat capacity than air, which causes you to lose more thermal energy and feel colder.
All Blankets Cool
So, blankets, how do they work? A blanket is basically an insulator. That means it prevents energy transfer between objects at different temperatures. Wrapping yourself in a blanket on a cold day keeps you from losing body heat to the air around you, so you feel warmer. Similarly, if you put a blanket around a cold soda on a warm day, it will slow down the transfer of thermal energy from the air to the soda, keeping the soda cold longer.
But what if you feel hot and you put on a blanket? In that case, two things can happen at once. It can still act as a thermal insulator and slow down the transfer of energy between you and the air. Unless the ambient air is above 98.5°F, this is going to make you hotter, not cooler.
However, the blanket can also have a thermal interaction with your body. Suppose you have a 80°F blanket in contact with a 98°F person. This will raise the temperature of the blanket while reducing the thermal energy of your body. Yes, it will act as a cooling blanket—at least for a few minutes, until the temperatures are equalized.
So, what makes one blanket cool more effectively than another? First, it should have a high mass, so that it takes a lot of energy to warm up. Second, the blanket needs to make good contact with your skin to increase the thermal interaction. So, one of those light fluffy blankets won't cool you off that much. Other than that, it's just a normal blanket.
But I'm a sucker for trying these things, so I bought a cheap “cooling blanket” online. (I know, someone will say it doesn't work unless you get an expensive one.) For those who say their cooling blanket was out in the sun and they measured a 75°F temperature, I don't believe you. Check this out. I have three blankets on my sofa. One of them is the cooling blanket and the others are normal. In back is the same picture taken with an infrared camera so that different colors represent different temperatures.
Can you tell which one is the cooling blanket? Nope. You can't. There’s almost no difference in the temperatures. They are all cooling blankets. They are also all normal blankets.
A Real Cooling Blanket
But what if I told you it's possible to have a thermal interaction between two objects but one of them doesn’t change in temperature. Yes, this is a thing—it's called a phase transition. It happens whenever a material changes from solid to liquid or liquid to gas.
Here's an interesting experiment. Imagine I take a beaker with frozen water (aka ice) that's colder than the freezing point (maybe –10°C). Then I put the beaker on a hot plate and add energy to it, measuring the temperature of the water as I go. Here is what that would look like:
As you can see, the ice increases in temperature until it reaches the melting point, 0°C (32°F). At that point the temperature levels off, and it remains constant until all the ice melts—even though heat is still being added to the system. Why is that? It’s because the energy is being used to break the molecular bonds and turn the solid into a liquid. Once it’s entirely liquid, the temperature starts rising again—until it reaches the boiling point (100°C). Again, the temperature levels off until all the water turns to gas. (This is why it's nice to cook with boiling water—it stays the same temperature.)
You can see how this would make for a better cooling blanket. Because of these temperature plateaus during a phase change, you can keep transferring thermal energy from your body to the blanket without the blanket getting warmer and becoming ineffective.
Does this mean you could use an ice blanket, maybe with the water in a flexible lining, to cool yourself? Sure. But it would be excruciating and you might get frostbite. Also, once you melt the ice and heat up the water, you’d need to put your blanket back in the freezer before you could use it again.
But how about a material that has a melting point closer to the temperature of the human body? In this video from the YouTube channel NighthawkInLight, Ben Cusick makes just such a phase-change material from common salts (sodium sulfate and sodium chloride).
Of course I had to try making some of this stuff myself. I'm not a chemist, but I think it turned out pretty well.
It depends on your mixture and the type of salts used, but these kinds of materials have a melting point somewhere around 18°C (65°F). Why does that matter? Well, first, it's not so cold that it hurts. Second, you don’t need a freezer; a cool place like a basement floor will make it refreeze.
And best of all, the high melting point means it will melt slowly at room temperature, so the phase transition lasts a long time—hours instead of minutes—and it will cool you off during this whole time. Put this stuff in the lining of a blanket and voilà! Pretty cool, right?
