An ancient Confucian philosopher once said, "I love the lotus because while growing from mud, it is unstained."

Now, almost one thousand years since Zhou Dunyi wrote these lines in China, scientists finally understand how the plant keeps itself clean and dry.

The process of solving this biological problem inspired Duke University engineers to make use of man-made surfaces resembling the lotus to improve the efficiency of modern engineering systems, such as power plants or electronic equipment, which must be cooled by removing heat through water evaporation and condensation.

For the first time, scientists were able to observe water as it condensed on the leaf's surface, and more importantly, how the water condensate left the leaf.

The trick lies in the surface of the plant's large leaves, and the subtle vibrations of nature. The leaves are covered with tiny irregular bumps spiked with even tinier hairs projecting upward. When a water droplet lands on this type of surface, it only touches the ends of the tiny hairs. The droplet is buoyed by air pockets below and ultimately is repelled off the leaf.

"We faced a tricky problem - water droplets that fall on the leaf easily roll off, while condensate that grows from within the leaf's nooks and crannies is sticky and remains trapped," said Jonathan Boreyko, a third-year graduate student at Duke's Pratt School of Engineering, who works in the laboratory of assistant professor Chuan-Hua Chen. The results of the team's experiments were published early on-line in the journal Physics Review Letters.

"Scientists and engineers have long wondered how these sticky drops are eventually repelled from the leaf after their impalement into the tiny projections," Boreyko said. "After bringing lotus leaves into the lab and watching the condensation as it formed, we were able to see how the sticky drops became unsticky."

The key was videotaping the process while the lotus leaf rested on top of the woofer portion of a stereo speaker at low frequency. Condensation was created by cooling the leaf. It turned out that after being gently vibrated for a fraction of a second, the sticky droplets gradually unstuck themselves and jumped off the leaf.

"Duke University researchers Jonathan Boreyko, left, and Chuan-Hua Chen."

Source: Duke University