Introduction

Water is all around us. On a wet day we need coats to keep us dry, windscreen wipers so we can see and reservoirs to collect it and keep us alive. There are few things more essential for life. Nature controls water in a myriad of ways. The Lotus leaf cleanses itself of dust when it rains, a beetle in the desert collects drinking water from an early morning fog and some spiders walk on water. We understand so little of how to mimic the adaptations to water that Nature has evolved, but if we did, we could use this natural resource so much better.

Much of our work is about how to create surfaces that are extremely water repellent and from which droplets of water simply roll off (see video 1). We want to know why water forms a pancake-like film on one surface and a rolled up ball on another. We want to know how we can use this knowledge to create better clothes, windscreens, and even miniature bio-chemical factories on the size of a credit card.

To create super water-repellent surfaces we use a combination of the sort of chemistry found on non-stick frying pans and the hairiness found on plant leaves in the garden. Imagine lying on a bed of nails (A Fakir's carpet). If the nails are close enough together it is possible to lie on the sharp points without being impaled (see video 2). Water can do the same and so skate across the tops of hairs (or the bumpiness) of a surface providing the chemistry of those hairs is water repellent. If the hairs are thin enough, then a droplet of water sits mainly on the space in-between them. We know that a drop of water hanging from a thread forms itself into a perfect sphere because of its surface tension - the skin effect of a liquid. This effect allows a small metal paperclip to 'float' on the surface of water (see video 3). The same happens for our droplet of water on the hairy surface. When it balls up, it rolls around and off the surface. As it rolls, dust sticks to the ball and so the surface not only keeps itself dry, but it self-cleans when it rains - the so-called Lotus-Effect ®.