Science & technology | The lotus position

A self-repairing surface that stays clean and dry

What do you get when you cross a lotus and a lizard?

THE repulsive powers of lotus leaves are the stuff of legend. Water sprayed onto them forms instantly into silvery beads (see picture) and rolls right off again—carrying any dirt on the leaf’s surface with it.

The physics behind this impressive and beautiful phenomenon is well understood. Lotus leaves repel water because they are covered with minuscule waxy nodules that stop water molecules bonding with a leaf’s surface tissues, meaning those molecules bond with each other instead. That arrangement has been replicated in several man-made materials. Unfortunately, these are easily damaged by abrasion—and, not being alive, cannot regrow and repair themselves. They are thus hard to commercialise, which is a pity, because the self-cleaning, self-drying surfaces they create could be of great value. A technique just described in Langmuir by Jürgen Rühe of the University of Freiburg, in Germany, may, however, fix this problem by giving lotus-like materials the ability to regenerate when damaged.

Dr Rühe’s approach is to mimic a second living organism—this time an animal, the lizard. As lizards grow, their scales do not grow with them. Instead, old scales are shed and replaced from below by new ones. Dr Rühe theorised that it might likewise be possible to create a stack of lotus-like layers that would flake off when damaged, revealing a pristine surface beneath.

Lotus-like man-made materials belong to a class known as nanograsses—so called because, under an electron microscope, they resemble lawns. Dr Rühe’s nanograsses have water-repellent “blades” attached to thin sheets of silicon. The task he set himself was to create a stack of these that could tell when the one at the top was compromised so badly that it needed to be replaced, and then replace it automatically.

He conceived of doing this by gluing the layers of the stack together with a water-soluble material. He reasoned that, as the top layer got worn, and water began leaking through it, this glue would start to dissolve. A small amount of damage would do no harm. But enough would weaken the glue to the point where the uppermost nanograss lawn flaked off, and the next one down took over.

Testing this idea out using an appropriate glue (a special water-soluble polymer), he found that it worked. When he scratched the top of such a stack with a scalpel and exposed it to water, it did, indeed, come loose and fall off as the water seeped into the underlying glue. Such an arrangement will not, of course, last for ever. Eventually, it will run out of layers. But if the idea can be applied to industrial practice, then long-lived, self-cleaning surfaces may at last become routine.

This article appeared in the Science & technology section of the print edition under the headline "The lotus position"

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