The superhydrophobic properties of rose petals

A water droplet stuck to a rose petal

While Geckos have been a very popular source of inspiration for new materials, a recent study in Langmuir suggests that we should not overlook the humble rose petal. Rose petals, like many plant surfaces, exhibit a curious property known as superhydrophobicity.

Superhydrophobicity is defined by the contact angle between a water droplet and the surface of another material. A superhydrophobic material will have a contact angle that is greater than 150 degrees. This means that a water droplet will ball up on the surface instead of spreading out. On a molecular level, this is a product of the fact that individual water molecules would rather bond to another water molecule than to the material in question.

Many plants exhibit superhydrophobicity; the lotus leaf is a classic example. On a lotus leaf, water will ball up on the surface and, when the leaf is tilted as little as five degrees, the droplet will readily slide off. This process cleans the leaf of dust that might otherwise build up on the surface. But a drop of water on a rose petal doesn't act quite the same way. When a drop of water is placed on a rose petal, it balls up, but it does not readily slide off. In fact, a rose with a small drop of water on it can be turned completely upside down and the droplet will stick tightly to the petal.

To investigate this, the research team looked at a rose petal under a scanning electron microscope. They found that the petal was covered with microscopic bumps, approximately 16µm in diameter and seven micrometers high. Each of these bumps was in turn covered with folds approximately 730nm wide.

These two features have opposite effects on a water droplet. Because the chemical composition of the rose petal is slightly hydrophobic to begin with, water cannot get into the nanometer scale folds. This leads to the superhydrophobic state.

In contrast, the micrometer scale structures are spaced far enough apart for water to get in between the bumps and be held to the surface. A lotus leaf uses a similar multi-scale technique to achieve superhydrophobicity, but the microscale bumps are too close together for water to get in between them, and thus water slides off of it very easily.

Beyond just being cool, this has important implications for our ability to mimic these surfaces. Previous work has been able to mimic the superhydrophobic state of the lotus leaf, but only using thermoplastic materials, heating, and solidification. In the present study, the authors were able to create a superhydrophobic surface using a cast made by pouring polyvinyl alcohol on a rose petal and letting it dry. They proceeded to make a new superhydrophobic material from that cast by drying a polystyrene chloroform solution in it. All of this was done with very little equipment and at room temperature. They were unable to create a cast out of the lotus leaf with this technique because the superhydrophobic nature of the leaf prevented their cast from filling in some of the spaces on the lotus leaf.

Langmuir, 2008. DOI: 10.1021/la703821h