A few weeks ago, Mariam and I were at a birthday party and somehow or another, the subject of this experiment came up. One of the other parents asked for the necessary materials (which are few and usually already on hand) and we turned the experiment into an impromptu party trick right then and there. It was a huge hit, of course, because this is one of those incredibly simple experiments that yields delightfully surprising results.
What you will need:
A shallow dish or plate
What you will do:
Start by pouring a small amount of milk into the plate so that the entire surface is covered with about a quarter to a half inch of milk. Dot a few drops of food coloring around the plate, noticing that the drops sit on the surface of the milk, mostly without moving.
Next, randomly add a few small drops of the dish soap to the plate. Just for fun, Mariam and I made a little video of what happens when you do this (because seeing it actually happen is much cooler than looking at a picture of after it's already happened...). You can check it out by clicking below.
So, why does this happen the way that it does? It's all about surface tension. The food coloring is less dense than the milk, so it sits on the surface and is mostly immobile when you add it. However, the dish soap immediately begins breaking down the fat molecules in the milk, changing its density and reducing the surface tension. This allows the food coloring to start moving through the milk, making the swirling effect.
I would call it good, clean fun except for the fact that it involves food coloring, which is pretty much always trouble. Nevertheless, enjoy!