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. This 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. They have provided the first description of the microscale surface of roses, composed of arrays of tiny, fleshy projections called micropapillae. The micropapillae form a seal with water droplets, allowing them to cling to the surface of the rose petal. Using these new insights, Feng was able to create a synthetic rose petal surface with same properties.
The team found that 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.
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," Nature quoted Ronald Fearing, a biomimetic engineer at the University of California at Berkeley, as saying.
"These findings present many interesting applications in microfluid handling," he added.
The findings appear in the journal Langmuir 1.