Want to try the experiment at home? We did. Scroll down for details of how to do it.

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ADELAIDE: Drop oil and detergent in a bowl of water and the droplets will beat like hearts. While this experiment can be cooked up at home, it has taken researchers 30 years to crack the science behind it.

These small droplets can rhythmically expand and contract for almost half an hour, a phenomenon that was first formally described in 1975.

It's a simple experiment, but explaining what makes the drop of oil throb – and then stop when deprived of fresh air – has long mystified the scientific community. Now, in work that could have applications in fields from biology to environmental engineering, a U.S. team has cracked the case.

"Bizarre and subtle mechanism"

“Everybody was flummoxed,” by this natural phenomenon, said mathematician John Bush of the Massachusetts Institute of Technology in Cambridge. “This is a bizarre and subtle mechanism.”

Bush and co-author Roman Stocker – who studied the throbbing droplets in their spare time between other funded work – have now published their findings in the Journal of Fluid Mechanics. The key to solving the problem was when Stocker, a professor of environmental engineering, turned a video microscope on the droplets.

The researchers mixed mineral oil with a small amount of 'surfactant', such as a water insoluble detergent (this allows oil to spread over the surface of the water more easily as it is attracted to both water and oil). The oil droplets float on water and the surfactant sits at the interface between the oil and the water.

The pair was trying to understand why the oil forms beating droplets rather than spreading out on the water. “It's an easy experiment to make,” said Bush. “But getting the theory for it was not straightforward.”

Push and pull

The mechanism, they now understand, is surface tension of the liquid, or more precisely, evaporation-induced variations in surface tension. These variations in surface tension cause the drop to expand, then contract, and the process to repeat every couple of seconds until it runs out of surfactant. Covering the experiment stops the process because it prevents evaporation of the surfactant.

The researchers found that the oil droplet has a rounded bottom so more of the surfactant sits at the bottom of the drop where it lowers the surface tension between the oil and the water. This change in tension increases the forces pulling on the outer edges of the drop, causing it to flatten out and expand.

The changes in surface tension also cause very small waves within the droplet. This allows tiny droplets to escape the larger droplet, onto the surface around it, which the researchers saw using the video microscope. The surrounding tiny droplets cause the surface tension on the larger droplet to decrease, and so the droplet contracts back in.

However, 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 droplet, and the cyclical process begins again. The droplets expand and contract about every two seconds.

The researchers found the droplets stop beating entirely if the dish is covered; proving that evaporation of the surfactant is an important part of the beating cycle.

Driven by curiosity

It took about three years of sporadic work without funding, and the help of two undergraduate students for Stocker and Bush to solve the conundrum. To what end, the researchers don't yet know.

“I am all for science which is driven by sheer curiosity,” commented Alexander Frenkel a mathematician at the University of Alabama in Tuscaloosa, whose fluid dynamics work helped Bush and Stocker crack the problem. “Such science has value independently of practical applications."

But there might already be practical applications added Stocker: “Oil contamination of water resources is a prominent problem in environmental engineering… Awareness of the fundamental mechanisms governing the interaction between the two phases is critical to devise sound engineering solutions for remediation.”

EDITOR'S NOTE

In the interests of scientific enquiry, and something to do on a Sunday morning after reading the newspaper, Cosmos Online has investigated how easy it is to replicate the experiment in an ordinary home kitchen with cooking oil. While the researchers used mineral oil, de-ionised water and a chemical surfactant called tergitol, it is quite possible to reproduce the experiment at home.

“Mineral oil can be purchased at any convenience store for a couple of dollars,” Stocker told us. “This said, it could also work with vegetable oil (I don't really see why not), but I cannot guarantee this, as I have not tried it.”

Well we did try it, and we can guarantee it. Instead of their Petri dish, we used a breakfast bowl filled with about a centimetre's depth of water, which we left on the sink to stop swirling. The researchers used a proportion of about 10 per cent surfactant in the oil. So in a small cup (a medicine cup with a 10 ml mark is ideal for the purpose) roughly one millilitre of dish detergent was put in the bottom, and filled it up to the 10 ml mark with canola oil.

Then a cake skewer was used to quickly whiz the oil and detergent together, before using the skewer to drip small droplets of the oil mix onto the water. The droplets beat quite obviously, expanding and contracting mysteriously on the surface of the water. We managed to keep the droplets beating for only a few minutes, but the researchers managed around 25 minutes with optimal conditions – so let us know if you can beat our effort!