Throbbing oil puzzle solved

Scientists in the US have solved an age old scientific problem that you may not have even realised existed - why is it that a mixture of oil, detergent and water throbs?

Professors Roman Stocker, left, and John Bush display a mixture of oil, detergent and water.

If you mix a small amount of mineral oil with an even smaller amount of detergent, squeeze a tiny drop of that mixture onto the water the oil appears to pump like a beating heart. 

It’s seems like a simple experiment, but explaining what makes the drop of oil throb has long mystified the scientific community. Now, in work that could have applications in fields from biology to environmental engineering, an MIT team has cracked the case.

The mechanism, they now know, is surface tension - or more precisely, evaporation-induced variations in surface tension. These changes in surface tension cause the drop to expand, then contract, and repeat the process every couple of seconds.

Professor John Bush of the MIT mathematics department said: “This is a bizarre and subtle mechanism. Everybody was flummoxed.”

Think of the oil detergent drop as a small lens with a rounded bottom. The detergent in the drop moves to the bottom surface of the lens, where it interacts with the water to decrease the surface tension where oil meets water. This change in tension increases the forces pulling on the outer edges of the drop, causing the drop to expand.

The centre of the drop is deeper than the edges, so more surfactant settles there, reducing the surface tension correspondingly. This causes the oil and surfactant near the outer edges of the drop to circulate. This circulation creates a shear (think of it as two velocities going in opposite directions), which generates very tiny waves rolling outward toward the edge. When these waves reach the edge, they cause small droplets to erupt and escape onto the water surface outside the drop. Those droplets of oil and surfactant disperse on the water and decrease the surface tension of the water surface, so the drop contracts.

As the surfactant evaporates, the surface tension of the water increases again, and the system is reset. Forces pull at the outer edges of the lens, and the cyclical process begins again.

So there you have it – next time you are in the pub and someone inevitably asks: “So you’re a science buff – just why does a mixture of oil, water and detergent throb?” you can amaze them with the answer.