Chemists at Utrecht University and Philips Research have developed a technique to regulate light flow through a liquid using an electric field. Based on the phenomenon of light absorption by "nanorods" -- metal particles that measure 12 to 22 nm in diameter and 40 to 730 nm long -- the technique allows scientists to select the amount of light passing through the liquid by applying an electric field to change the rods' orientation. The aqueous dispersion of gold rods has a color that is dependent upon the rods' aspect ratio, or length divided by diameter. An aspect ratio of 8.9 will yield a reddish-brown dispersion, while a ratio of 2.6 will produce a blue-gray dispersion.Patented applications exploiting nanorods' light-absorption properties have existed since the 1930s, according to Bianca van der Zande, a research scientist working on the effort sponsored by the Netherlands Organization for Scientific Research. But research since then has been complicated by scientists' inability to produce aqueous dispersions in which the metal rods do not coagulate, she said. To produce the dispersions, researchers filled a finely perforated membrane with gold, dissolved the membrane, then transferred the rods to water using ultrasonic vibration. They applied an external electric field between two electrodes within the dispersion, which caused the rods to change their orientation and absorb light. The gold rods' length-to-width ratio determines the wavelengths at which light will be absorbed, van der Zande said; e.g., a ratio of 1.8 will yield an absorption band around 520 nm and another at 650 nm, while higher aspect ratios yield an absorption band in the near-IR and one about 520 nm. "By adjusting the aspect ratio of your rods, you can practically choose which wavelength region is absorbed," she said, adding that rods with exactly the same length and diameter will produce very narrow spectral regions around the desired absorption wavelength. The researchers also found that the rods' orientation in the dispersive medium plays a key role in determining the amount of light absorbed. Random orientation produces the greatest amount of absorption, and the greatest transmission occurs when all the rods are oriented in the same direction.Although still in the research stage, van der Zande sees several potential applications for the technology, including panels that screen out light in car windows, color-enhanced displays and polarizers. She stressed, however, that more research is needed into the properties of the aqueous dispersions, including their long-term stability, sedimentation buildup and electric field behavior at long time intervals.