Properly seasoned, a droplet of oil can throb, expanding and contracting like a living thing. Now an engineer and a mathematician have put their finger on the source of that pulsing. Their analysis could yield insights into the oscillations of living things, too.
Oil and water may not mix, but they can get it together to perform a weird trick that has puzzled scientists for decades. Season a droplet of oil with a compound called a surfactant, which reduces the oil's surface tension, and plop it on water, and it will spread into a little circular slick, or "lens," that throbs as if alive, expanding and contracting like a living thing for as long as an hour. Until now, scientists have not been able to explain exactly what animates the oil, however.
To find out, engineer Roman Stocker and mathematician John W. M. Bush of the Massachusetts Institute of Technology in Cambridge used video microscopy. They mixed a drop of mineral oil with a little bit of liquid detergent, a surfactant, and placed the combination in a petri dish containing purified water. The researchers then scrutinized its motion in great detail as it expanded and contracted every couple of seconds. Through those observations and accompanying mathematical analysis, they say they've cracked the mystery and exposed what Bush calls the "bizarre and subtle mechanism" underlying the pulsing.
It works like this. Because one end of a detergent molecule is attracted to water and the other to oil, the molecules migrate to the bottom surface of the lens. There they reduce the oil's surface tension, and the reduction in tension allows the drop to spread flatter and wider. The detergent then circulates toward the edge of the oil, and a series of tiny eruptions, called spontaneous emulsification, releases minuscule droplets of oil and detergent from the main drop. The released detergent evaporates from the water, allowing the surface tension of the drop to increase, triggering contraction. Then the cycle is repeated, as the remaining detergent molecules again migrate toward the oil-water interface, the researchers report in the 25 July Journal of Fluid Mechanics. If a lid is placed on the petri dish to prevent evaporation of the detergent, the oscillations stop.
The authors explain that the pulsing is an example of the conversion of chemical to kinetic energy, a phenomenon governing many rhythmic movements in organisms, from walking to the beating of hearts. The researchers have produced "a nice and persuasive theory" to explain the seeming "magic" to an oscillating oil lens, says Richard Craster, a fluid dynamicist at Imperial College London. He says the experiment may offer scientists guidance in constructing simple physical models to help explain oscillations in living organisms.