Lotus effect
The lotus effect (also known as the lotus flower effect) describes the self-cleaning behavior of certain surfaces with very low wettability (superhydrophobicity). Liquids, especially water, roll off the surface and carry dirt particles with them.
Where does the name lotus effect come from?
The term lotus effect refers to the lotus plant, which is known for its self-cleaning behavior. The term was coined by the botanist and bionic scientist Wilhelm Barthlott, who was the first to scientifically describe the effect and paved the way for its use in technical surfaces [1]. The term Lotus-Effect was trademarked by StoAG.
Where can you find the lotus effect in everyday life?
The lotus effect is used in many products to make cleaning easier or completely unnecessary. Examples include facade paints or textiles for outdoor areas, glass panes, and roof tiles. Medical devices are also provided with self-cleaning coatings so that germs do not adhere and can be easily removed.
How is the lotus effect created?
The lotus effect utilizes the surface chemical properties of a water-repellent substance (low surface free energy) in combination with a rough surface texture. In the Lotos plant, for example, a wax-like material is arranged in a nub-like nanostructure on the leaf. The low wettability of the wax and the high surface tension of the water together ensure that no water gets into the indentations, so that drops can roll off the surface like spheres.
Cassie and Baxter [2] have scientifically described the wetting state of rough, superhydrophobic surfaces.
Can the lotus effect be measured by the contact angle?
As the shape of a droplet on a surface is represented by the contact angle, measuring this variable is generally suitable for characterizing superhydrophobic surfaces with a lotus effect. From 90° a material is considered non-wettable and from 150° it is described as superhydrophobic.
More meaningful than the simple contact angle is the determination of the contact angle hysteresis, which describes the difference in the contact angle during wetting (advancing angle) and dewetting (receding angle). Surfaces with a lotus effect have an extremely low hysteresis. Hysteresis can be measured optically by drop contour analysis of dynamically enlarged and reduced drops or by tensiometric measurement using the Wilhelmy method. The latter measures the wetting force when the drop is immersed in the liquid and then pulled out.
Do all surfaces with a very large water contact angle show the lotus effect?
A very large contact angle does not guarantee that there is a lotus effect and that drops roll off the surface. There is also the opposite Rose Petal Effect [3], in which there is strong adhesion of the liquid despite a very large contact angle. In this case, the depressions between the hydrophobic elevations are filled with liquid. Which effect prevails depends on the chemical and geometric structure of the rough surface, and the wetting conditions.
Measurements of the pressure required to force the wetting of the indentations are particularly informative. For such investigations, tensiometers equipped with a camera are ideal for combining force and contact angle measurements.
Can the roll-of behavior of the lotus effect be measured directly?
The roll-off behavior of a liquid from a material can be determined using the roll-off angle. This is the tilt angle of a surface at which a dosed drop with a defined volume rolls off the surface – i.e. not a contact angle. Nevertheless, this method can be combined with contact angle measurements by placing the measuring instrument on a tilting device.
Literature
- [1] W. Barthlott (2023): The Discovery of the Lotus Effect as a Key Innovation for Biomimetic Technologies. - in: Handbook of Self-Cleaning Surfaces and Materials: From Fundamentals to Applications, Chapter 15, pp. 359-369.
- [2] A.B. D. Cassie, S. Baxter, Wettability of Porous Surfaces. In: Trans. Faraday Soc. 40, 1944, pp. 546-551.
- [3] B. Bhushan, M. Nosonovsky M. (2012). Rose Petal Effect. In: B. Bhushan (Ed.): Encyclopedia of Nanotechnology, pp. + 2265-2272.
