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Bio-inspired Engineering

Most of us are familiar with the self-cleaning lotus effect. The lotus grows in muddy ponds but shows no trace of grime. Water beads up on the lotus leaf's waxy hydrophobic surface, maximizing the contact angle between the water and leaf. Microscopic bumps on the leaf's surface further maximize contact angle. And finally, a layer of air trapped between the bumps acts as a cushion to keep the water from wetting the surface. As the beads of water roll off the leaf, they clean the plant of dirt and microbes. The lotus effect has been reproduced and commercialized in products such as self-cleaning paints and anti-stain clothing.

Most developments of synthetic liquid-repellant surfaces are based on the lotus effect. However, it was another Asian plant that inspired Tak-Sing Wong and colleagues. As a post-doctoral researcher at Harvard, Wong was part of a team in the lab of Joanna Aizenberg that developed a new type of material based on the slick surface of the Southeast Asian pitcher plant. The slippery quality of the plant's rim is due to a thin layer of water that is captured and held on the surface microstructure after rain. Because water doesn't mix with the oily coating on the feet of ants that helps them to climb, the ants slide on the surface of the water layer into the plant interior where they are eaten.

The bio-inspired invention, called slippery liquid-infused porous surfaces, or SLIPS, won the inventors a 2012 R&D 100 Award. Both the Harvard group and Wong's group at Penn State continue to develop the technology. And because there is so much left to develop, so many pathways to pursue, Wong expects SLIPS research to be one of the major focuses of his lab for the next several years.

In both the pitcher plant and SLIPS, the coating of water does not run off even if the surface is turned upside down. The reason? It's the microstructure, Wong explains. "At large scale, water is dominated by gravity. But if you have the same water, and reduce it to very small sizes, other forces come into play, surface forces."

Take a glass of water, he says, turn it upside down, and the water spills out – that's gravity. But if you cover the glass with cardboard, and make a hole in the cardboard as large as one millimeter, the water will stay in the glass held by surface tension. The microscale structure of the pitcher plant is on the order of 20 microns, but in SLIPS, the pores are even smaller, in the nanometer range. At this size scale, even more exotic molecular forces come into play. "We can use forces that oppose gravity," Wong remarks.

For the full story, see our Focus on Materials issue Summer 2013: Inspiration at the Intersection between Disciplines