A team of chemists from Tsinghua University in Beijing, China figured out why tiny water droplets seem to get stuck to petals of red roses. Not unexpectantly, the mechanism, known as the Cassie impregnating wetting state, is a result of nanostructures (“hierarchical micropapillae” and “nanofolds”) on the surface of petals. What does it all have to do with medicine? Well, the usual: possible future medical materials, or other technologies.
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 Langmuir.
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…
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.
Read: Raindrops on roses…
Abstract: Petal Effect: A Superhydrophobic State with High Adhesive Force Langmuir, 24 (8), 4114 -4119, 2008.
Image credit: . . . captured in a rose by fmarq on Flickr