Lotus leaves shake off water

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

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

Chen and Jonathan Boreyko claim that they have found a physicalexplanation for this natural advantage. Apparently inspired hischildhood experiences, Chen recalled lotus leaves flapping vigorouslyin the wind and realized that this is due to their unusually large leafbeing supported by a long thin stem. He had the idea that the lotusleaf may use this vibration to shake off water condensate that may haveotherwise penetrated their rough surfaces.

The next big challenge was to accurately reproduce the large swingsof a lotus leaf within the laboratory setting. Fortunately for Chen,Boreyko is an intuitive experimentalist who figured out that the speedat which the lotus leaf flaps in the wind is the important parameter inthe process. In light of this, the researchers realized that they couldsimply attach the leaf to a basic audio speaker and vary the frequencyand 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 researchersapplied a mixture of water and ethanol (2:1 vol) to the lotus leaf andfixed the laboratory conditions at 21 °C with a relative humidity of51%. As the ethanol evaporated, this simulated water condensation onthe leaf surface. After 6 minutes, when more than 90% of the ethanolhad evaporated, the researchers turned on the speaker to vibrate theleaf.

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 ejectedcompletely intact from the leaf surface when vibrations were at afrequency of 80 Hz and a peak-to-peak amplitude of 0.6 mm. In caseswhere vibrations were too weak, the droplets remained on the surface;and in cases where vibrations were too strong, a sticky residue wasleft on the surface of the leaf.

The Duke University researchers intend to develop theirresearch by exploring ways to apply the findings to practicalapplications such as self-cleaning, non-sticking surfaces. A robustsuperhydrophobic surface could also help to reduce drag in a range ofplaces including condenser pipes and on ship hulls.

Source: physicsworld.comAdded: 25 November 2009