Although it is a surface that repels water, the drops adhere to this surface
Materials scientists around the world have been trying for decades to answer a beautiful phenomenon of nature: the way in which spherical dew droplets adhere to the petals of the world’s most popular flower, and do not fall off., even if we put the flower upside down. This phenomenon is called the rose petal effect. Finding the solution to this enigma opens up a world of possibilities in biology and in the development of new materials.
Lotus leaf and rose petal
There are two very unique plant surfaces, of great interest to materials science due to their relationship with water: the upper part of the Lotus leaf (Nelumbo nucifera), a symbol of purity due to its self-cleaning properties (Lotus effect), and the upper part of the petal pink, due to the enormous adherence of the drops (rose petal effect).
Both surfaces are very hydrophobic (the water droplets on them are almost spherical), but on the lotus leaf, the droplets slide, while on the rose petal they stick. A first-order unknown that is not a matter of magic, but of science.
Usually, these phenomena have been interpreted considering only the roughness of the surfaces and their hydrophobic and uniform condition. This was the case in Lin Feng’s article published in 2008 in which he named this phenomenon. However, the unsolved puzzle is how the same scientific justification could explain both the repellency of water droplets by the leaves of the Lotus and the adherence of the drops to the rose petal. Something didn’t add up.
The enormous complexity of plant surfaces
We have spent more than two hundred years trying to understand how surfaces, both biological and synthetic, are “wet”. Likewise, for two centuries research has been carried out to analyze the chemical composition and structure of the surfaces of plant organs with limited success, due to their enormous complexity.
An inherent problem in the study of plant surfaces is that their structure can be altered by uprooting the plant organ. The petals are very delicate and their surface loses its natural shape shortly after being separated from the flower.
Natural petals in the laboratory
In a novel interdisciplinary study between different Spanish research centers, we have managed to use natural petals to analyze –at the nanoscale– not only their morphology (as has been the case up to now), but also the chemical properties that determine their wetting.
In the first phase, we selected a variety of roses whose petals kept water droplets attached in a similar way on their upper (beam) and lower (underside) faces. We managed to characterize both faces in natural petals, subjecting them to a treatment that would preserve their structure.
With the scanning electron microscope (SEM) we observe that both the texture and the roughness of both surfaces of the petal (the upper/beam face and the lower/underside) are very different. However, they are similarly wetted by drops of water. So the roughness does not serve to explain everything. You had to look deeper to find the key to the rose petal effect.
Atomic Force Microscope (AFM) Solved the Mystery
Atomic force microscopy (AFM) analysis gave us the answer. AFM allows the surface of the petal to be analyzed at a very fine scale, nanometers, and essentially works by very delicately “feeling” the surfaces with an extremely sharp tip. In addition to sensing roughness, AFM is capable of “noticing” chemical composition. Thus we discover that the surface of the petals at that scale has fractal roughness in the range between 5 nm and 20 µm.
We also found that at the nanometer scale the surface of the petal has an irregular tessellation, a repeating pattern of figures, that completely covers the surface.
But there is still something more striking, and that is that this intimate tile is made up of alternating hydrophilic and hydrophobic nano-areas. And with this, the mystery of why the rose petal is hydrophobic but adherent to water at the same time is solved. The existence of small hydrophilic zones interspersed with more abundant hydrophobic areas on the surface of rose petals allows water droplets (of a polar nature) to adhere, despite the fact that the surface is hydrophobic due to its great roughness and because most of the surface it is.
Oddly enough, both sides of the petal have the same fractal dimension and this explains why the water droplets interact in a similar way, despite the fact that the total roughness is about ten times greater on the upper side than on the lower side.
With these new results, the difference between the Lotus effect and the rose petal effect is explained in a natural way.
Both surfaces are extremely rough, but in the rose petal, the material that covers it (the cuticle) presents a hydrophilic/hydrophobic tessellation, while in the Lotus leaf it is homogeneously hydrophobic, with an extra coating of deposited wax nanotubes on the cuticle, which causes the droplet to detach.
Intimately, at the nanometric scale, which only the atomic force microscope allows us to access, it is possible to appreciate the difference between these two plant surfaces and understand why the dewdrops are anchored to the rose and not to the lotus leaf.
Materials of the future and biological implications
Following these results, studies of the wettability of natural or synthetic materials that are carried out in the future should assess the possible chemical heterogeneity of the surfaces.
At the level of plant surfaces, these hydrophilic zones are of great interest because they can play a fundamental role in the absorption of water and solutes deposited on the leaves, such as aerosols or foliar sprays of agrochemicals, and can also be vulnerable points for attack. of pests and diseases.
The presence, relevance, and abundance of hydrophilic zones on plant surfaces is currently Pandora’s box that we have just opened and that will predictably bring us many surprises.
Unveiling the secret of the rose petal, its great roughness, and its chemical heterogeneity will allow materials science and biomimetics to develop new highly useful surfaces. The field is open.
