Most everyone knows that oil and water don't mix. Oil molecules are non-polar, meaning their charge is distributed more or less evenly throughout their structure. Water, on the other hand, is polar — different parts of each water molecule carry a weak positive or negative charge.
Generally speaking, polar molecules like water and non-polar molecules like oil don't attract, so they can't possibly mix. Add to this the fact that many oils are less dense than water, and you begin to understand why they "float."
Except when they don't.
Research published in a recent issue of the journal Langmuir outlines the conditions necessary for water to actually "float" on oil. Science's Jon Cartwright gives a tidy explanation:
Using a theoretical model, the scientists calculated the forces acting on water when it is dripped onto an oil surface [in this particular instance, the researchers performed the experiment with vegetable oil]. Taking into account surface tension, the property that allows some insects to walk on water, they showed that a water droplet can "hang" from the oil's surface. The oil surface droops like a rubber membrane, allowing the above air — which is much lighter than oil and water — to extend beneath the surface's average level. With help from the surface tension, this air pocket balances the weight of the water droplet, preventing it from sinking.
In real-world experimental trials, the researchers found that water droplets like the one featured here could accomodate up to 170 microliters of water before losing "bouyancy." That might not sound like a lot of room, but researchers think it's enough space to house oil-munching microbes.
If microbe-containing water could be aerosolized and sprayed over oil slicks, the researchers think it could go a long way in speeding recovery efforts after oil spills like 2010's Deepwater Horizon disaster. [Langmuir via Science]
Images and video via Langmuir