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A STRETCHY liquid crystal is intriguing physicists with its bizarre tendency to swim away from light. This unique property could one day be used to make robot submarines propelled by light beams, its inventor suggests.

Peter Palffy-Muhoray of Kent State University in Ohio and his colleagues at the University of Freiburg in Germany and New York University were experimenting with various types of rubber-like liquid crystal, which they already knew would change shape when placed in a beam of laser light. This happens because the molecules in a liquid crystal tend to line up in one direction and in the intense electric field of a laser they tilt like long grass in the wind. This compresses the surface of the material in contact with the light, and if the crystal is stretchy enough makes its edges curl towards the beam.

Palffy-Muhoray’s team found the material bent much more when they added a tiny amount of an orange dye to the liquid crystal. The dye molecules fold up when they absorb light, which flexes the neighbouring liquid crystal molecules even more. But the researchers were astonished when they placed a disc cut from a thin sheet of this extra-bendy material in a beaker of water: within milliseconds of laser light shining on it, the disc sped across the surface to escape the light, they told the American Physical Society meeting in Austin, Texas, earlier this month. The disc was a strong swimmer too, forcing its way through viscous liquids like ethyl alcohol. Turning up the laser’s intensity made the disc swim faster. And irregular shapes cut from the same material behaved in the same way.

The researchers videoed the crystal throughout the experiment. The footage showed that the edges of the disc curled upwards by 90 per cent before it swam off. The researchers think this surprising behaviour happens because the centre of the disc is forced deeper into the water as the disc bends, creating an upwards force that tends to make the disc flatten out (see Graphic). But since the disc cannot flatten while it is under the light, it shoots sideways across the surface of the water to escape the laser beam.

Palffy-Muhoray says that the phenomenon could be used to pump liquids through micrometre-sized chambers, or make boats and submarines with flexible hulls that are propelled by light pulses.