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