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In the application where infinitely-variable lens without any moving parts is needed, liquid lenses are not a new thing to implement. Conventional liquid lenses use two immiscible fluids, each with a different refractive index, to create variable-focus lenses of high optical quality as small as 10 µm (microns). There are two ways to manipulate the two fluids, electrically and mechanically, but both methods take advantage of the surface tension of the liquid. The electric method uses a novel property called electrowetting (full review on electrowetting: J. Phys.: Condens. Matter, 2005, 17, R705-R774) to modify the surface tension, while the mechanical method takes advantage of surface tension to physically change the shape of the lens (more details of liquid lenses introduced by Alix Paultre).

In the recent issue of Nature (442, 551-554, 3 August 2006) a letter by Hongrui Jiang et al. reported an interesting design of an adaptive liquid lens which is controlled by stimuli-responsive hydrogels (links of the figure and video resources below are made from the headline of College of Engineering, University of Wisconsin-Madison).

A water-oil interface forms his group's lens, which resides atop a water-filled tube with hydrogel walls. The tube's open top, or aperture, is thin polymer. The researchers applied one surface treatment to the aperture walls and underside, rendering them hydrophilic, or water-attracting. They applied another surface treatment to the top side of the aperture, making them hydrophobic, or water-repelling. Where the hydrophilic and hydrophobic edges meet, the water-oil lens is secured, or pinned, in place (See this figure). When the hydrogel swells in response to a substance, the water in the tube bulges up and the lens becomes divergent; when the hydrogel contracts, the water in the tube bows down and the lens becomes convergent (illustration video). “The smaller the focal length, the closer you can look,” says Jiang. If hydrogels with different responsive behaviors, say, reverse swelling behavior against pH condition, are arranged in a suitable array, mimicry to the insect's compound eyes which can monitor different areas in space may be available (illustration video).