To achieve the result the team deposited silicon nanowires with diameters of 20-50 nm randomly on a silicon surface. Then they coated the wires with a monolayer of photochromic spiropyran molecules. These organic molecules are hydrophobic until illuminated with ultraviolet light. In fact, lotus leaves make use of a similar structure to repel water droplets - they have a very fine surface structure and are coated with hydrophobic wax crystals of around 1 nm in diameter (see Lotus effect shakes off dirt).

A water droplet will move across a surface if its advancing contact angle is lower than the receding contact angle. Shining ultraviolet light with a wavelength of 366 nm onto the spiropyran molecules converted them to their polar hydrophilic form and reduced the contact angle. So you might expect ultraviolet light to cause water droplets to move on any surface coated with spiropyran.

But there's a snag. On a smooth surface the transformation of the spiropyran molecules to their hydrophilic form reduced the contact angle by 12°. That wasn't enough to cause the water droplet to move, however, as the contact angle hysteresis (the difference between advancing and receding contact angles) for the smooth surface was 37° under visible radiation.

On a nanowire surface, in contrast, the contact angle hysteresis was only 17° and the illumination of the spiropyran molecules with UV light reduced the contact angle by 23°, nearly twice as much as for the smooth surface. As a result of both these factors the advancing contact angle became lower than the receding contact angle and the water droplets moved along the surface towards the source of the UV light.

"We have been working on the problem of using light to move microscopic amounts of water around for drug delivery and microanalysis applications," said Tom Picraux of Arizona State. "Our advance came when we realized that if the surface was roughened at the nanoscale, not only would we obtain the 'lotus-leaf effect', but we could also magnify the small change in water repelling controlled by light to a level that can overcome the hysteresis, or the attraction, that causes water to stick even when a drop is pushed along."

The process is also reversible since treating the spiropyran molecules with visible radiation with a wavelength of 450-550 nm caused them to return to their closed hydrophobic form.

Moving water droplets with light avoids the need to use potentially damaging electric fields, air bubbles, which can denature proteins, or microscopic mechanical pumps, which are expensive to make and difficult to repair.

The scientists, who reported their work in Journal of Physical Chemistry B, say their findings also point the way to enhanced fluidic motion and control in electrowetting and thermowetting microfluidic systems.