Contrary to popular belief, new research from Australia has found that water can float on oil in certain situations with potential applications for oil spill cleanups.
At school you were probably taught that less dense liquids float above more dense ones. This means crude oil should float on the sea surface. However, chemical engineers at Curtin University in Perth have discovered that this is not always the case.
The team tested three interfaces—water/oil, water/air, and oil/air—using a numerical model based on the Young-Laplace equation, which describes the difference in capillary pressure across the boundary between two static fluids.
They added small drops of water to an oil bath and found that water’s ability to float on the surface varies according to the droplet size as well as the type of oil and its surface tension—the cohesive force between liquid molecules that allows water to form beads.
The researchers found that the surface tension of commercial vegetable oil is sufficient to support the weight of a water droplet, whereas this was not the case for pure mineral oils. Furthermore, vegetable oil can only support water drops up to about 1/100th of a cubic inch.
“The stability of the floating droplet depends on the combination of three interface tensions [oil, water, and air], oil density, and water droplet volume,” wrote the authors in the study abstract.
“For practical purposes, however, the equilibrium contact angle has to be greater than 5° so the water droplet can effectively float.”
These findings could be useful in cleaning up oil spills by suspending biodegrading microbes in tiny water droplets that would mix better with the spill.
“This result can lead to a new and advanced mechanism in processing oil/water mixtures, such as biodegrading process of unwanted oils, including vegetable oils, sand oil tailings and oil spillages,” they wrote.
The study was published in American Chemical Society’s journal Langmuir.