A staple in most sci-fi movies, tractor beams can be used to rein in broken down starships, assist another vessel in docking, or prevent your enemies from running away. While the movies usually depict a focused beam of gravity, a team of Australian physicists have generated a tractor beam by manipulating the surface of water. The research was led by Horst Punzmann and Michael Shats of Australian National University and the paper has been published in Nature Physics, though it has been made available on arXiv.org as well.
Using a system of vertically-oscillating plungers, the team was able to generate quasi-standing Faraday waves on the surface of the water in order to manipulate the position of a drifting object. This doesn’t just apply to bringing the object toward the source of the waves; the item can be moved in any direction. For this experiment, the researchers used a ping pong ball.
“We have figured out a way of creating waves that can force a floating object to move against the direction of the wave,” Punzmann said in a press release. “No one could have guessed this result.”
In order to steer the object around in any direction they chose, they were able to manipulate the size and frequency of the waves. Particle trackers revealed that the waves create currents on the surface, based upon the wave’s shape. Plungers of various shapes were used in order to manipulate the pattern of the current’s flow.
“We found that above a certain height, these complex three-dimensional waves generate flow patterns on the surface of the water,” Shats explained. “The tractor beam is just one of the patterns, they can be inward flows, outward flows or vortices.”
The researchers suggest that this technology could be used to contain oil spills, which would expedite cleanup and could minimize environmental impact. Additionally, it could improve the understanding of rip currents, which cause 80% of the rescue responses by beach lifeguards.
While the average person may think that waves on the surface of water are fairly straight forward with a constant motion, the object is really displaced in a pattern more like this:
Image via Australian National University
This patterning is exceedingly complex, and there isn’t even a mathematical theory that explains how this works yet. There are overlapping forces on the surface from flows pushing inward and outward, gravity, currents from underneath, and other forces that need to be taken into account.
“It’s one of the great unresolved problems, yet anyone in the bathtub can reproduce it. We were very surprised no one had described it before,” Punzmann concluded.
