If you made lasagna recently and had a oily film on the casserole dish you probably can see that oil and water do not mix. From the time you were small people were telling you 'oil and water don't mix!' You learn in general chemistry later in life more about why they do not mix. Oil molecules are non-polar (the charge is spread evenly among the structure). On the other hand water molecules are polar (their oxygen and hydrogen have a specific charge this relates to how surface tension forces are made). Also oil particles are less dense because of their non-polarity and float on the surface of the water like in your casserole dish. Oil molecules 'float' on top of water. Right? Well sometimes they do not and water can float on oil.
Science's Jon Cartright gives a nice explanation the conditions necessary for water to 'float' on oil. This was published recently in the surface chemistry journal Langmuir by an Australian team:
So researchers dropped water onto a surface of oil. They calculated the forces acting on the water. They showed that from the surface tension the water droplet can 'hang' on the oil's surface. The surface of the oil droops- like when you stand on a trampoline- allowing air to extend beneath the surface's average level. The surface tension helps this air pocket to balance the weight of the water droplet, preventing it from sinking. You can try this at home.
When they tested this in real-world experimental trials, the researchers found that water droplets could accommodate up to 170 microliters of water before losing "bouyancy." This is not a lot of room but it could be enough space to make some interesting changes in the world. How? The researchers believe it's enough space to house oil-munching microbes. The microbe could be spread in droplets of water and aerosolized. The aerosilized microbe could be sprayed easily to help efforts like oil spills.
A tensiometer would be needed to also measure the surface tension of the microbes in the bulk spray solution to get that perfect droplet size. The perfect droplet size would help to optimize the solution and see if the microbe will float or sink.