Monday May 14, 2018 Q&A

Water rolls off lotus leaves because they are covered in:

Réponses

1 microscopic papillae coated with wax crystals.

2 cells that produce a water-repellent magnetic field.

3 different sized hairs that trap air and shed water droplets.

Correct answer: 1, lotus leaves are covered in microscopic papillae coated with wax crystals

It is widely known that lotus leaves repel water more effectively than the leaves of any other plant. When a water drop hits a leaf, instead of spreading out, it forms a nearly spherical bead that easily rolls towards the leaf’s center or edge. This unusual property is based on a physical phenomenon known as superhydrophobicity – also called the lotus effect. Let’s look at how it works.

Micro-roughnesses

To the naked eye, the large leaves of lotus plants appear perfectly smooth. But if you analyze one of these leaves under an electron microscope, you’ll see a complex microstructure. The surface of the leaves is covered in small clusters of cells called papillae, each measuring around ten microns. “The papillae in turn are covered with a dense layer of wax crystal tubules,” says Nathan Dupertuis, a PhD student at EPFL's Laboratory for Fundamental Biophotonics. “Because they are very short and thin, the tubules cover the surface of the papillae without any gaps.” These two layers of texture create a bumpy surface on the leaf. The effect on the drop of water is a little like when someone lies on a bed of nails: because of the very limited area of contact, it remains on the surface.

 

(a) Lotus leaves (b) Upper surface of a leaf as seen with a scanning electron microscope (SEM) (c) Close-up of the wax tubules

Photo credits: © 2011, Ensikat et al; licensee Beilstein-Institut.

Superhydrophobic surfaces demonstrate several other interesting features as well. “The contact angle – which is formed by the solid surface and the tangent at the surface of the drop – must be above 150°. This means, for example, that at an angle of 180° the droplet is perfectly spherical,” says Yves Leterrier of EPFL’s Laboratory for Processing of Advanced Composites. “What's more, the water droplet rolls very easily when the surface is tilted slightly.”

 

Self-cleaning

Superhydrophobicity gives the lotus plant an evolutionary edge. When water slides off a leaf, it carries away dust and dirt, leaving behind a pristine surface with no impurities – ideal for photosynthesis.

Lotus leaves are the prime – and best-known – example of natural superhydrophobicity, but similar properties have been observed in more than 200 plants, such as water lilies, nasturtiums and ginkgo bilobas.

The lotus effect was first described at the end of the 1970s by botanist Wilhelm Barthlott. Since then, bioinspired researchers and engineers have used it to create protective coatings, paints and “self-cleaning” textiles. Their aim is to create surfaces able to rid themselves of any type of contamination, not just water. Potential applications include car bodywork and seats, biomedical equipment and windows.

“In our lab, we are developing superhydrophobic surfaces that will be used in new types of food packaging to prevent the contents from sticking to the wrapper,” says Leterrier. “It may also be easier to recycle this type of packaging.” The project is being run in conjunction with the Lausanne University of Art and Design (ECAL) and funded by EPFL's Food and Nutrition Center. The lab’s approach is based on microscopic replication, where the leave’s physical features are printed in a resin that is then hardened using UV light. At this point, around ten plants have been analyzed, with the help of the Lausanne botanical garden. “We are currently working on rose petals. We were very surprised to see that yellow roses are superhydrophobic while red ones aren’t.”

For more information:

Superhydrophobicity in perfection: the outstanding properties of the lotus leaf. Hans J. Ensikat, Petra Ditsche-Kuru, Christoph Neinhuis and Wilhelm Barthlott. Beilstein J. Nanotechnol. (2011), 2, 152–161.

A Facile in Situ and UV Printing Process for Bioinspired Self-Cleaning Surfaces. González Lazo M.A., Katrantzis I., Dalle Vacche S., Karasu F., Leterrier Y., Materials, 9, 738 (2016). DOI: 10.3390/ma9090738.

Thanks to Nathan Dupertuis, a PhD student in EPFL’s Laboratory for Fundamental Biophotonics, and Yves Leterrier, at EPFL’s Laboratory for Processing of Advanced Composites.