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Design of H2O repelling material inspired by nature

WASHINGTON, D.C.  — In many respects, modern science is just starting to catch up to some of the natural engineering marvels of the world.

One of these naturally occurring engineering marvels is found in leaves from the lotus plant. They have an uncanny ability to repel nearly all of the rain and dew that hits them. They reign supreme as the gold standard of waterproofing — and one that the engineers devoted to making new waterproof materials are now beginning to approach.

Using newly designed nanotechnology-based materials, physicist Chuan-Hua Chen and his team from Duke University in Durham, N.C., have created a material that repels water like the surface of the lotus leaf. High-speed videos of these materials show how they actually launch small droplets of water off their surfaces.

Scientists have long known that that microscopic spikes and fibers covering the surface of the leaves work together to prevent water from sticking. Large drops of water have trouble adhering to the leaves and roll off, while small drops are physically ejected without using any outside force.

Chen started out researching how lotus leaves use ambient air currents to shed drops of water. Drawing inspiration from the lotus adorned flag of his home county of Honghu in China, Chen was fascinated because leaves growing in the wild seemed to be better at repelling water than laying flat on a table.

Chen found that when a lotus leaf shakes, large drops of water sticking to its surface bead up and roll off of the side. He tested this by strapping a leaf to the middle of a 100 watt speaker and filming water droplets as they slid off. With the speaker playing, the water’s surface tension prevented it from clinging to the spikes and hairs on the surface of the leaf, while the water clung to the leaf when there was no sound. Based on this, Chen speculates that lotuses may have evolved their long thin stems to pick up and amplify ambient vibrations to shed water and keep their leaves dry.

In order to synthesize such a surface, Chen coated a bumpy silicon base with carbon nanotubes and water-proof compound hexadecanethiol. He left it to collect water condensation, but he was surprised to discover just a few large drops on its surface. Chen realized that there must be some natural phenomenon ejecting water droplets.

Using a high-speed camera, Chen found that when two small drops merge, a “liquid bridge” between them forms and rapidly expands out to form a single large round drop. The force of this liquid bridge hitting the surface underneath sends the water drops flying. On most surfaces, the adhesion of the water would prevent the drops from flying off, but the bumps and hairs on his synthetic surface prevented the water from bonding with the surface.

Chen said that he and his team have started looking into several potential applications for this kind of material — mainly in the garment industry. A super-water-repellant fabric would be an obvious choice for advanced rain jackets.

At the same time, his team is also thinking about large industrial applications. Rolling drops of water pick up dirt and grime more effectively than sliding drops, making for easier cleaning. Chen also thinks that the springing drops of water can be used to improve condensation heat exchangers and to reduce drag on the sides of ships.

“We are interested in getting these ideas working in a real engineering system,” Chen said, “We are looking for applications we can put into real devices, not just a paper.”

Mike Lucibella reports for Inside Science News Service.

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