A Petal's Pull On Water

When tiny raindrops land on roses, they bead up into spheres but resist rolling off the flower, even when it's held upside down. By examining the surface features of petals, researchers in China have now developed an explanation behind this phenomenon, which they have dubbed "the petal effect" (Langmuir 2008, 24, 4114). Their findings could lead to novel adhesives and coatings.

Lin Feng, a chemistry professor at Tsinghua University, was inspired to study the petal effect after a day out at the arboretum. "I was attracted by the petals with water droplets glittering under the sunshine," she says.

Feng was familiar with the "lotus-leaf effect," wherein waxy micro- and nanoscale features on the lotus leaf's surface cause water to bead up and then roll off, taking dirt and dust along with it. She wondered how a petal's surface features might influence its ability to cling to water.

Feng, along with Lei Jiang of the Chinese Academy of Sciences in Beijing, led a group that focused a scanning electron microscope on the petals of red roses, sunflowers, and Chinese Kafir lilies. They found that the micro- and nanoscale landscapes of these petals feature rough textures. For example, the surface of a rose appears to be covered with wrinkled bumps, each about 10 μm across. This roughness makes the petals superhydrophobic, prompting water to bead up.

As for the petal's adhesive properties, Feng explains that different textures contact water in different ways. Water droplets sit on top of a lotus leaf's nanoscale projections, she says. On a petal, however, the sizes of micro- and nanostructures are larger than those of the lotus leaf, and water droplets will enter these larger scale grooves. "Therefore, water droplets can easily fall off the surface of a lotus leaf, while sticking to the flower's petals," Feng notes.

Feng and Jiang's team also used the petals as molds to make polymer films with the same superhydrophobic and adhesive properties. "The textured films we have made can be used for coatings and functional fibers," Feng suggests.