Lighting the Way for Water: New strategy for steering drops with finesse

Several years ago, a team of researchers in Japan used a beam of light to move drops of oil around on a surface. They could not do the same thing with water drops, however. Now, with inspiration from lotus leaves, a second team has succeeded in manipulating water with a beam of ultraviolet light. That could open new routes for controlling biochemical reactions, the scientists say.

PHOTO SWITCH. Ultraviolet light can move a water drop along a rough photosensitive surface (left), whereas a drop on a smooth photosensitive surface (right) stays put and spreads out. R. Rosario

The difficulty of moving drops of water with light stems from the way water molecules interact with surfaces. In previous experiments, Antonio Garcia and his colleagues at Arizona State University in Tempe tried to sidestep that challenge by manipulating the drops on very smooth surfaces. Yet, while the front end of the drop would move toward the light, the back end would stick to the surface.

Inspired by the extremely water-repelling, or superhydrophobic, nature of lotus leaves, the researchers decided to make a rough surface. The wax-coated microtexture of the lotus leaf causes water to bead up into nearly perfect spheres that roll off (SN: 11/1/03, p. 278: Available to subscribers at Water Repellency Goes Nano: Carpet of carbon nanotubes cleans itself).

To emulate the leaf’s coarse surface, the Arizona team grew a thin carpet of silicon nanowires, each 20 to 50 nanometers in diameter, on a silicon substrate. Next, the researchers coated the carpet with the photosensitive compound spiropyran, which assumes a hydrophilic, or water-loving, configuration when exposed to light.

When the investigators placed water drops on the surface and shone UV light on a small spot next to one of the drops, the spiropyran molecules snapped into their hydrophilic shape. As a result, the drop of water edged toward the illuminated spot. Because the back side of the drop still rested on the hydrophobic portion of the surface, it followed the rest of the drop “like a tank tread,” says Garcia. The study will appear in an upcoming Journal of Physical Chemistry.

The new water-controlling tactic could improve microfluidic chips that many researchers are designing for applications ranging from medical diagnostics to environmental monitoring. In such devices, microscopic valves and pumps direct tiny amounts of fluid through specified channels and into specific microchambers (SN: 9/28/02, p. 198: Available to subscribers at Liquid Logic: Tiny plumbing networks concoct and compute).

Using light to move fluids on a flat surface would have a number of advantages over these conventional schemes, says Garcia. Miniature valves and pumps are tricky to fabricate, and leakage of fluid between a chip’s channels can ruin a reaction. A light-driven mechanism would do away with all those mechanical elements and also give researchers greater control over reactions because they could steer drops in any direction.

“This is very impressive and very elegant work,” says Manuel Marquez, a chemist at the Los Alamos National Laboratory in New Mexico. The technology could have applications beyond microfluidics, he adds. For instance, says Marquez, a beam of light could drive nanoparticles to aggregate in solution, forming a capsule that could store medicines and eventually release them in a patient’s body.

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