MIT boffins unravel throbbing oil mystery

Scientists at the Massachusetts Institute of Technology (MIT) seem to have unravelled the mystery of oil droplets throbbing when mixed onto water.

Washington, July 18 : Scientists at the Massachusetts Institute of Technology (MIT) seem to have unravelled the mystery of oil droplets throbbing when mixed onto water.

In their study in the July 25 issue of the Journal of Fluid Mechanics, MIT Professors Roman Stocker of civil and environmental engineering and John Bush of mathematics explain what happens when an oil drop containing a water-insoluble surfactant (or material that reduces the surface tension of a liquid, allowing easier spreading) is placed on a water surface.

During the course of their experiments, the scientists found that evaporation induced variations in surface tension causes the drop to expand, then contract and repeat the process every couple of hours until it runs out of gas, which in this case, is surfactant.

"We're dealing with three interfaces: between the oil drop, the water in the Petri dish, and the air above it. A detergent is a surfactant, which reduces the surface tension of a liquid. The detergent molecules we added to the oil drop prefer to stay at the interface of the oil and water, rather than inside the oil drop," said Prof. Stocker.

"The surfactant in the drop moves to the bottom surface of the oil drop, 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," he said.

He said, as 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, which creates a shear and 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.

However, covering the experiment stops the process as it prevents evaporation of the surfactant, the researchers found.

If the surfactant can't evaporate, the oil drop remains stable. 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, they said.

ANI