What you Need
What to do
Half fill one jar with water and then add a little oil
Almost completely fill the second jar with water and then top it right to the top with oil.
Give both jars one good shake. Do you notice any difference?
What may happen
You should find that the jar with air inside mixes far better, and far quicker that the one without.
Why does it happen?
To mix liquids together you need to cause lots of turbulence, with large chages in speed and direction of the fluid flow, breaking up the large lumps of liquid.
To achieve this you want to cause large high speed flows of liquid, so if you shake the liquid more violently it will mix better.
However because the fluids are floating on one another the important thing is the relative density, because a denser liquid will have more inertia than a less dense one, so they will move relative to one another. The density of oil is very close to that of water, so with just oil and water the relative movements will be much less than oil and air.
The other effect is viscosity, this is a measure of how thick a fluid is, so thicker fluids like oil or water will absorb energy slowing down the flows, and reducing mixing, so again air will assist mixing, as it has a lower viscosity.
Part of the show Why do Men's Bits Shrink in the Cold? from the 28th Nov 2010