Understanding rain drops

Understanding Raindrops

The classical raindrop shape we see in illustrations everywhere is wrong! This may be the shape of a droplet the instant it lets go from a faucet, or some other surface, but it does not stay that shape for long (microseconds maybe). The reason is a phenomenon called surface-tension.

Surface-tension exists when two substances (particularly liquids or gases) are in contact with one another. The more different the two substances are from an electronic properties perspective, the larger will be the surface-tension between them. When two substances are very similar in their electronic properties, they will mix in each other or become miscible. In that case, the surface-tension is nearly zero. A good example of low surface-tension would be that between water and alcohol. Carefully pour a bit of alcohol into water, and no mater how carefully you pour, the two will combine into a single liquid, a solution of alcohol and water.

On the other hand, pour a little olive oil into water, and no matter how hard you shake it, it will eventually separate into two different liquids. When you do that, watch the shape that the oil droplets take as they rise through the water. If they are rising slowly, they will be almost perfect spheres. The smaller the droplets the closer to perfect spheres they will be. There is a large surface-tension between oil and water. If the droplets are large, or rising very rapidly, they will start to flatten out. This is very similar to raindrops.

Just like oil-in-water, very tiny raindrops will take on a spherical shape, and the smaller the droplet the closer to a perfect sphere it will remain. The reason is that a sphere is the shape with the smallest surface area for a given volume of material. Because air and water have very different properties, the molecules in the raindrop will prefer to be in contact with other water molecules rather than air molecules. Therefore, their attraction to each other will pull them into a shape that minimizes contact with air molecules. And the best shape for that is a sphere.

Now that is the ideal, and if you were aboard the International Space Station and carefully formed and released a drop of water into the air, it would eventually settle into a sphere (in the absence of air currents or other disturbances). However, on earth and in the atmosphere there are two forces that tend to distort the spheres.

The first of these forces is gravity. Gravity pulls the water through the air, because water