Rose petals hang on to tiny water droplets. Credit: GETTY

Behind the natural beauty of a rosebud covered in dew drops lies a decades-old mystery: why don't the tiny droplets fall off, even when the flower is turned upside down? Now researchers have unpicked the secrets of the rose's trick, and replicate it in a man-made material.

The beading of water droplets on natural materials is not a rare thing. But on many flowers and leaves the droplets slide off with the slightest tremble, taking dust and small insects off with them. The effect is known by biologists as 'self cleaning' and has been well studied by researchers keen to make better water-repellent materials.

The water slides off because the surfaces are very rough and spiky at the microscopic scale, and the tips of the spikes are covered in wax. The water molecules therefore come into contact with only a tiny fraction of the surface, and then only to water-repelling wax.

Lin Feng and her colleagues at Tsinghua University in Beijing found that although rose petals are coated with similar projections, they have wide, gentle-sloping troughs between the spikes, and no wax. The spikes keep the dew drops in a spherical shape, but the water 'leaks' into the troughs between spike-covered bumps, giving a bit of 'stick' and stopping a small droplet from rolling around (see diagram). Feng and colleagues report this structure in the journal Langmuir1.

Once the team realized what the rose petals were doing to hold water, they were curious whether they could replicate the effect. They put some polyvinyl alcohol onto rose petals and allowed it to set, then peeled off a thin plastic cast of the petal surface. This film, the researchers found, had the same properties as the rose petal: the film could hold droplets of between 3-5 microlitres even when held upside down.

"It is very interesting that the authors were able to make a casting of the natural petal surface that showed similar behaviour to the original surface, even though the materials were different," says Ronald Fearing, a biomimetic engineer at the University of California at Berkeley. Just as with gecko feet, it seems that the physical shape of the surface is much more important than any chemical properties of the material in creating 'stickiness'.

For a rose, this stickiness might come in handy as reflective water drops that glisten in the Sun might attract pollinating insects. In the lab, such materials might be useful for 'lab on a chip' devices that need to hold and shunt around tiny quantities of liquid without leaking or being contaminated by nearby materials. "These findings present many interesting applications in microfluid handling," says Fearing.