Tak-Sing Wong from Harvard University has created a synthetic material so slippery that it makes a duck’s back look like a sponge. It is “omniphobic” – it repels everything. All manner of liquids, from water to blood to crude oil, roll straight off it. Ice cannot form on it. It even heals itself when damaged. It’s an extraordinary material and it was inspired by the lips of a flesh-eating plant.
The pitcher plant kills and eats animals. Some of its leaves are shaped like deep pitchers, and their rims, known as peristomes, are exceptionally slippery. Insects that explore the rim, looking for nectar, soon lose their footholds and fall in. They soon drown, and are broken down by the pitcher’s digestive fluids. (There are some exceptions – see slideshow at the bottom).
Under the microscope, the secret to the peristome’s slipperiness is clear. It is lined with cells that overlap one another, creating a series of step-like ridges and troughs. The plant secretes nectar onto this uneven surface. The troughs collect the nectar, and the ridges hold it in place, preventing it from draining away. The result is an extremely smooth, stable and slippery surface that repels the oils on the feet of insects. Any bug that walks on this frictionless zone falls to its doom.
Wong has mimicked these structures to create SLIPS – slippery liquid-infused porous surfaces – that are more slippery than either their natural counterparts, or other man-made materials. They are made of either stacks of tiny posts, each a thousand times thinner than a human hair, or a random network of similarly thin fibres. These provide a rough structure, which Wong filled with a lubricant, just as the pitcher plant saturates its rough cells with nectar. The lubricant mixes with neither water nor oils, and it barely evaporates.
The SLIPS are like sponges – solid blocks that trap liquids – but they are designed to firmly hold the liquid in place, while keeping its surface smooth and flat. This combination allows them to to repel a far greater range of liquids than any other man-made surface. Drops of water, blood and crude oil sit on the SLIPS as spheres. The angles between the drops and the SLIPS are usually no greater than 2 degrees (the angle would be 0 for a perfect sphere).