Two liquids

There are two liquids, with different colours in the container. They cannot mix and their densities differ considerably. The coloured liquid is denser than the transparent one. Even if the liquids are stirred violently they do not intermingle. As a result, small balls form, but after a while they blend with the corresponding liquid.

The slow sloshing of the blue liquid is caused by the presence of the transparent liquid above it; the disturbance of the bottom liquid churns the top one. The surface of the top liquid, with air above it, sloshes in a regular manner, while the bottom liquid does not take part in this movement.

The wavy movement at the border of two liquids can explain the problems that Norwegian fishermen experience in spring. According to local folk tales, some elves awake in spring and hamper rowing so that boats do not move as fast as they usually do. In reality, a layer of fresh water of about fifteen centimetres forms on the surface of the sea as snow melts. Rowing in such a layer of light water sloshes the heavier salty water below and the boat does not move forward as fast as it is supposed to.