11 Sep 2003
A polymer film that bends when it is illuminated with polarized light is revealed in this week’s Nature.
Three scientists in Japan have discovered that polymer films containing liquid crystals can be bent along any axis simply by illuminating them with linearly polarized light. The trio from Toyko Institute of Technology says this characteristic means that lasers could power micromachines instead of batteries. (Nature 425 145).
“This striking photomechanical effect results from a photoselective volume contraction,” say Tomiki Ikeda and colleagues. “It may be useful in the development of high-speed actuators for micro-scale or nano-scale applications, for example in micro-robots in medicine or optical microtweezers.”
The films contain two molecules: a liquid-crystal monomer and a diacrylate crosslinker. Crucially, both of these molecules contain azobenezene groups, which the authors single out as the component responsible for inducing the bending.
To bend the film, the researchers shine 366-nm linearly polarized laser light with an intensity of 3.5 mW/cm2 on it. This makes the film’s edges curl up towards the light source. “The film is bent towards the direction of irradiation of the light, with the bending occurring parallel to the direction of the light polarization,” says Ikeda.
Exposing the film to visible laser light longer than 540 nm in wavelength restores it to its original, flat state. According to the authors, the film takes less than 10 seconds to curl up or straighten out and, after repeated bending, does not degrade.
When the laser light hits the film, it is selectively absorbed by the azobenzene groups in the liquid crystal. These groups then align themselves along the direction of polarization of the light. “The subtle reduction in microscopic size and ordering of the liquid-crystal components gives rise to a substantial macroscopic volume contraction at the film surface,” says Ikeda.
Having demonstrated the phenomenon using a 4.5mm x 3mm x 7 µm film, Ikeda’s team now hopes to miniaturize the system and apply it to driving micromachines and nanomachines with lasers.