Explaining the Motion of Lotion in the Ocean

23 June 2005 (All day)

Path of least resistance. . A Teflon ball drags a bit during each wave's back-and-forth until it drifts to the wave crest (A).

Floating junk tends to clump. The tendency for dead leaves, oil slicks, and trash to cluster in the ocean is obvious to anyone who's ever strolled along a dock, but researchers have had a difficult time explaining the complicated patterns such objects make on the water. Now they have part of the answer: An object's movement in the ocean depends on how it displaces water.

Floating objects displace an amount of water equal to their own mass. This is known as Archimedes' Law. The law doesn't quite work for small, light objects, however. Small objects--tiny glass beads, for example--have a lot of surface area in comparison to their mass. If the surface likes water molecules, the bead will snuggle down into the water a bit further than its mass forces it to and consequently displace more water than Archimedes' Law predicts. If, on the other hand, its surface doesn't like water, the bead will sit higher and displace less.

All this surface action leads to some funny behavior in waves, researchers report today in Nature. When Gregory Falkovich, a physicist at the Weizmann Institute of Science in Rehovot, Israel, and colleagues took a small Teflon bead and floated it in a wave pool, the bead settled at the part of the wave that only moves up and down, not horizontally. The team suspects this happens because Teflon doesn't like water, and so the bead displaces less water than its mass. This makes it heavier than the water around it and the bead drags a little behind the surrounding water during each wave, falling further behind until it settles at a part of the wave where the water stays put (i.e. no horizontal motion).

When the physicists tried the same experiment with a bead of glass, which likes water, they found the opposite; the bead displaced more water than its mass, causing it to be lighter than the water and thus accelerate more than the water around it. Groups of glass beads clumped at the part of the wave that has lots of horizontal motion but never moves vertically. The Teflon and the glass beads behaved in similarly distinct ways when floated in more complex, oceanlike waves, and the researchers say the same forces work on trash in the ocean.

"This is an interesting, worthwhile piece of research," says Leo Kadanoff, a physicist at the University of Chicago. It's almost like a 19th century experiment, he adds, except nobody thought about it before. Falkovich says he hopes to expand on the bead study to develop a more sophisticated model for how litter and oil slicks clump on the ocean.

Related sites
The study
Gregory Falkovich's home page

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