Many plants are extremely water-repellent owing to their rough textures, which can trap air to provide a waterproof cushioning. In some cases, plant leaves are so repellent that no droplets can stick at all; instead, they simply bounce and roll off. A lotus leaf is an example of a natural material that possesses this "superhydrophobicity" and a pair of physicists in the US are proposing that natural vibrations lie at the heart of the phenomenon.

If the effect can be mimicked by materials scientists, it could lead to a range of novel applications. "Vibrations are everywhere, they are there as you walk, they are associated with the computer fan, they are in your automobile and spacecraft," said Chuan-Hua Chen, one of the researchers at Duke University, North Carolina. "As long as you know how to harvest the essentially waste energy from environmental vibrations, you can achieve superhydrophobicity".

A flowery youth

In the past few decades researchers have had a lot of success at mimicking rough surfaces in nature in order to make water-repellent materials. One key limitation, however, is that engineered rough surfaces do not retain water repellency when water condenses on the surface, rather than landing as water droplets. Some structures in nature, such as the lotus leaf, do not suffer from this limitation and always maintain their water-repellency.

Chen and Jonathan Boreyko claim that they have found a physical explanation for this natural advantage. Apparently inspired his childhood experiences, Chen recalled lotus leaves flapping vigorously in the wind and realized that this is due to their unusually large leaf being supported by a long thin stem. He had the idea that the lotus leaf may use this vibration to shake off water condensate that may have otherwise penetrated their rough surfaces.

To test this hypothesis, Chen and his colleague decided to study a vibrating lotus leaf in fine detail in the laboratory using high-speed microscopic imaging. They first confirmed that leaf motion does play a role by fixing the lotus leaf to a cold plate and observing that it loses its superhydrophobicity. In this set-up the water condensate could easily penetrate into the cavities of the surface texture, displacing the air pockets.

Lotus music

The next big challenge was to accurately reproduce the large swings of a lotus leaf within the laboratory setting. Fortunately for Chen, Boreyko is an intuitive experimentalist who figured out that the speed at which the lotus leaf flaps in the wind is the important parameter in the process. In light of this, the researchers realized that they could simply attach the leaf to a basic audio speaker and vary the frequency and amplitude to mimic the effect of wind.

In order to observe a transition in the leaf from a "sticky" state to the "non-sticky" water-repellent state, the researchers applied a mixture of water and ethanol (2:1 vol) to the lotus leaf and fixed the laboratory conditions at 21 °C with a relative humidity of 51%. As the ethanol evaporated, this simulated water condensation on the leaf surface. After 6 minutes, when more than 90% of the ethanol had evaporated, the researchers turned on the speaker to vibrate the leaf.

Using a video camera attached to a long-distance microscope, Chen and Boreyko altered the vibrations until they captured a complete "de-wetting" of the lotus leaf. All water droplets were ejected completely intact from the leaf surface when vibrations were at a frequency of 80 Hz and a peak-to-peak amplitude of 0.6 mm. In cases where vibrations were too weak, the droplets remained on the surface; and in cases where vibrations were too strong, a sticky residue was left on the surface of the leaf.

The Duke University researchers intend to develop their research by exploring ways to apply the findings to practical applications such as self-cleaning, non-sticking surfaces. A robust superhydrophobic surface could also help to reduce drag in a range of places including condenser pipes and on ship hulls.

This research appears in Physical Review Letters.