Liquid motion lamps (which most people know as "lava lamps") have been around for decades. The theory behind a liquid motion lamp goes something like this:
In the lamp you have two liquids which are:
- Very close in density
- Insoluble in one another
Oil and water are insoluble in one another (that's where the expression "oil and water don't mix" comes from), but oil and water have very different densities (a volume of water weighs a lot more than the same volume of oil). They won't work, so you search to find two liquids that are very close in density and are insoluble. This site can help you in that search.
Now you apply heat to the bottom of the mixture. In a liquid motion lamp, the heat usually comes from a light bulb. The heavier liquid absorbs the heat, and as it heats up, it expands. As it expands it becomes less dense. Because the liquids have very similar densities, the formerly heavier liquid is suddenly lighter than the other liquid, so it rises. As it rises, it cools, making it denser and therefore heavier, so it sinks.
This all happens in slow motion because heat absorption and dissipation are fairly slow processes, and the density changes we are discussing here are very slight.
Here are some interesting links: