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Scientists in Australia have produced a "tractor
beam" in a water tank. They can control the movement of a floating
ping-pong ball just by making a specific pattern of waves.By changing
the pattern, they moved the ball around the tank in various ways,
including pulling it closer like the famous beam from science fiction.
The findings, published in the journal Nature Physics, have the
potential to help contain oil spills or control and retrieve other
floating debris.
"No one could have guessed this result," said Dr Horst Punzmann, who led
the project at the Australian National University in Canberra.
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"We have figured out a way of creating waves that can force a floating
object to move against the direction of the wave."
Dr Punzmann and his colleagues used a very fast videocamera to record
the water movements that resulted from a range of different shapes,
plunged in and out of the water at various speeds.
They also covered the surface with floating tracer particles that
allowed them to see the direction of currents on the water surface,
which are distinct from the patterns of the waves themselves.
Bathtub science
Simply dropping a cylindrical wave-maker - a bit like a long rolling pin
- in and out of the water, gently, produces a predictable set of waves
undulating away from the disturbance. The current on the water surface
also travels away from the middle of the cylinder.
But if the size of the up-and-down movement is increased, that wave
pattern breaks up into choppier pulses. At the same time, the central
current switches direction, so that something floating near the middle
of the tank can be pulled back towards the wave-maker.
As well as this relatively simple "tractor beam" effect, the team
created and modelled various other flow changes that meant they could
effectively move a ping-pong ball around their tank at will.
"We can engineer surface flows of practically any shape," said Prof
Michael Shats, the paper's senior author. "These could be vortices,
these could be outward and inward jets - it's a variety of different
flow configurations."
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Prof Shats explained that his team started their tank experiments after
they realised just how complicated the maths involved must be.
"We realised that particle motion on the surface, determined by waves,
is far too complex to handle by any existing theory."
The researchers believe their discovery could be applied in open water.
"The applications could be numerous," said Prof Shats said. "For
example, collecting floating objects, manipulating small boats on the
surface, or maybe collecting oil spills."
"It's one of the great unresolved problems, yet anyone in the bathtub
can reproduce it," Dr Punzmann said. "We were very surprised no one had
described it before."
Recreating the tractor beam at bath time could prove a rather exhausting
challenge, however. The machine used in the lab created waves of precise
heights at a rate of between 10 and 100 every second. |