Micromotors swim toward and away from the light

Since the first molecular motors were synthesized in the late 1990s, the catalog has expanded to include self-propelling micromotors that roll, walk, or swim. Building such devices is challenging enough, but controlling their direction of motion has also proved difficult. Now chemist Baohu Dai and his colleagues at the University of Hong Kong have created a tree-shaped micromotor that exploits the motion of charges inside and outside the device to swim toward or away from a light source. Each device is about 10 μm long and consists of titanium dioxide nanowires (yellow in the illustration) sprouting from one end of a silicon wire (pink). Ordinarily the microswimmers succumb to Brownian motion (see the article by Dean Astumian and Peter Hänggi, Physics Today, November 2002, page 33). But the randomness is overcome when the miniature trees are immersed in an aqueous solution of hydrogen peroxide and illuminated with...