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).
If the SLIPS are gently angled, the drops roll off, leaving nothing behind. You can see that in the images below. Drops of oil and blood leave no traces as they roll over the SLIPS, but they form big stains as they travel over the middle Teflon layers. Ice won’t form on the slips either – the second the crystals come together, they slide off. Nor can insects get a grip – an ant, climbing after a dollop of jam, slips off just as it would on the rim of a pitcher plant (with the jam quickly following).
Wong’s SLIPS are around ten times as slippery as the next best synthetic ones. They are smoother, they work under high pressures, and they can be made transparent. They can also heal themselves. When Wong damaged the solid structure, the liquid part simply refills the affected area within less than a second. Best of all, they’re easy to make. The materials for the solid part are widely available and can easily be shaped into the right structures. For the liquid part, a wide variety of chemicals can be used and tailored to the chosen solids.
There are many possible applications. A wall coated in SLIPS would be impossible to graffiti. Medical devices or instruments covered in SLIPS would be hard to contaminate. The SLIPS are stable under a range of temperatures and pressures, which makes them useful for transporting fluids from crude oil to biofuels, or for exploring the deep ocean. They’re ice-resistant, and could be used to coat instruments in polar conditions. They are transparent and self-cleaning, so you could used them to make lenses, sensors, solar cells or night-vision devices.
This is not the first time that a naturally liquid-repellent surface has inspired the design of man-made ones. Lotus leaves are famous for their ability to repel water and clean themselves. Like the pitcher plant’s rim, they also have a microscopically uneven surface, with rows upon rows of tiny studs. Drops of water sit on top of these studs and as they roll off, they pick up dirt and other particles. Many scientists have mimicked the lotus’s structure to create water-repellent, self-cleaning surfaces.
But these lotus-inspired materials, unlike the pitcher-based ones, are fragile, sensitive and limited in their use. “They only work against water,” says Joanna Aizenberg, who led the Wong’s study. Other complex liquids, such as oils, can easily force their way into the air pockets between the studs and ruin their ability to repel water. Water itself can also do this under high pressure; a heavy rainstorm is enough. The studs can also be easily damaged; every new defect threatens to hold drops of liquid in place and prevent them from rolling off. “These surfaces are still not robust enough for many standard applications, let alone harsh conditions,” says Aizenberg.
These problems can be overcome, but at great difficulty and expense. Wong opted for a different approach by trading the lotus’s empty bumps for the pitcher plant’s liquid-filled ones. Walter Federle from the University of Cambridge, who discovered the structure of the pitcher plant’s peristome, says, “It’s really exciting to see that this principle has inspired the authors and allowed them to develop something that could prove extremely useful.” However, he adds, “I am curious whether it will be possible to make these surfaces survive over long periods of time under demanding outdoor conditions.”
Wong has designed the SLIPS so that their film of liquid lubricant stays in place. However, Aizenberg says, “Certain conditions such as the extremely high shear forces encountered by a high-speed jet could potentially deplete the liquid.” It’s incredible that those are the types of forces that would ruin the material, but the team sees this as a weakness nonetheless. They are now working on tweaking the properties of the liquid layer so that it can withstand even “high-flow or turbulent environments”.
Update: Neil Withers on Twitter says, “You’re SUCH a biologist – 3 paras on bloody plants and no mention of what the “posts”/lube actually are!” Heh. It’s a fair cop. The solid bit is either “an array of nanoposts functionalized with a low-surface energy polyfluoroalkyl silane” or “a random network of Teflon nanofibres” and the liquid is “low-surface-tension perfluorinated liquids (for example, 3M Fluorinert FC-70, or DuPont Krytox oils)”. So basically, congealed pixie dust…
Reference: Wong, Kang, Tang, Smythe, Hatton, Grintha & Aizenberg. 2011. Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity. Nature http://dx.doi.org/10.1038/nature10447
Images: Pitcher plant by Thomas Gronemeyer; all others from Nature