Custom-made rip currents have real potential.
Water doesn't get nearly enough credit for being weird and genuinely mysterious. Sure, it enables life and occupies the coveted position of "universal solvent," but just how it moves is a towering wonder of physics. Its dynamic peculiarities enable what researchers at the Australian National University are calling the water "tractor beam," a method of creating very powerful flow patterns with relatively simple wave generators.
The ANU research, published in today's edition of Nature Physics, describes the creation of patterns of liquid waves that force a floating object to move in the opposite direction of those waves. This in itself isn't a radical phenomenon and it occurs commonly in the form of rip currents—the fast, narrow streams of water that return the water delivered by breaking waves back to the open sea. Water moves in with the waves, but then has no clear exit as all of the wave energy is directed in one direction (sideways against the shore, say); in essence, the accumulated water, held in a kind of limbo along the beach, is then forced to burrow its way back out against the waves via powerful rip currents.
The current research describes a method by which these currents can be both created and directed using finely-tuned wave patterns. "We found that above a certain height, these complex three-dimensional waves generate flow patterns on the surface of the water," said MIchael Shats, the research team's leader, in a ANU statement. "The tractor beam is just one of the patterns, they can be inward flows, outward flows or vortices."
Strangely enough, noted ANU investigator Horst Punzmann, is that this effect has until now gone mostly undescribed, at least mathematically. "It's one of the great unresolved problems, yet anyone in the bathtub can reproduce it," he said.
Using a ping pong ball and a wave tank, the researchers were able to very finely tune wave frequencies and sizes, with the resulting "beam" being able to pull the ball in pretty much whatever direction they wanted. One can imagine the technique being deployed in the open sea using special wave-generating floats, basically creating a hydraulic vacuum cleaner. In the case of an oil spill, typically contained nowadays using floating barriers and somewhat sketchy chemicals that break down oil molecules, you can see the potential.
By the by, if the reader does happen to get caught in such a tractor beam, perhaps the devilish creation of some future surf-villain, don't try to swim against it. Either paddle at a right angle to the current, exiting to its side, or just chill until it dissipates, which it eventually will. A rip current can only survive so long as its being fed by breaking waves.