Paint that gets cooler rather than hotter in direct sunlight.
That sounds like it violates the laws of physics, but actually, it doesn't -- there's a trick to it. The trick is to make a material that emits its infrared radiation at a specific frequency that is not absorbed by the atmosphere. The atmosphere is transparent to visible light which is why sunlight comes straight down to us (at least on a clear day). But the atmosphere isn't transparent in the infrared. As such it traps a lot of infrared radiation. But there are certain wavelengths where it is transparent.
He doesn't say what wavelength is ideal for this but he does say the best material for this is barium sulfate. But the second best is calcium carbonate, which is what you can get from hardware or grocery stores.
But just buying a boatload of calcium carbonate -- or a material you can transform into calcium carbonate with a simple chemical reaction -- isn't enough. You need to make something called microspheres. So this video gets deep into microsphere synthesis. He illustrates why you want microspheres of certain sizes in order to maximizes packing density. You want the pigment jam packed with particles when viewed in the microscope so there isn't any place where light can slip through.
