Magic Sand Saves Water in the Desert express
In the desert, the sun sucks water out of the ground. But a layer of water-repellent sand just 5–10 mm thick works like a lid: air passes through, but water is blocked. Evaporation drops by 63–83%.
Under such a lid, sand behavior flips upside down. Normally, fine sand loses moisture 37% faster than coarse sand because it heats up more. With the protective layer, it's the opposite: fine sand holds 40% more water, becoming the champion of retention.
The secret is that the lid forces moisture not to evaporate directly from the sun, but to slowly seep through a maze of sand grains. The thicker the layer, the longer the path and the lower the losses. Fine sand under the lid overheats less, so it preserves water better.
Water-repellent sand is made by coating ordinary sand grains with a thin layer of a paraffin-like substance. A drop of water on such sand doesn't spread but remains an almost perfect little sphere.