Superhydrophobicity — The Lotus Effect
The lotus flower is a symbol of purity in many Asian cultures. The plant is "self-cleaning" due to its superhydrophobic leaves.Copyright Jakub Halun (left) and Michael Gasperl (right),Wikimedia Commons http://commons.wikimedia.org/wiki/File:20090809_Lotus_flower_2736.jpg http://commons.wikimedia.org/wiki/File:Tropaeolum-majus%28Lotus-oben%29.jpg
Students are introduced to superhydrophobic surfaces and the "lotus effect." Water spilled on a superhydrophobic surface does not wet the surface, but simply rolls off. Additionally, as water moves across the superhydrophobic surface, it picks up and carries away any foreign material, such as dust or dirt. Students learn how plants create and use superhydrophobic surfaces in nature and how engineers have created human-made products that mimic the properties of these natural surfaces. They also learn about the tendency of all superhydrophobic surfaces to develop water droplets that do not roll off the surface but become "pinned" under certain conditions, such as water droplets formed from condensation. They see how the introduction of mechanical energy can "unpin" these water droplets and restore the desirable properties of the superhydrophobic surface.
Inspired by nature, the self-cleaning superhydrophobic products being engineered today are destined to become important and novel products in our everyday lives. Some products include: coatings that reduce the water drag on boats and protect surfaces and equipment exposed to fresh or salt water, building materials that remain clean with little maintenance and are better protected from wear from the elements, and textiles that are resistant to staining and/or remain dry when submerged in water.
After this lesson, students should be able to:
- Describe situations in which a superhydrophobic lotus-effect surface would be useful.
- Explain the difference between Cassie-Baxter and Wenzel wetting and how it applies to superhydrophobic surfaces.
- Demonstrate the transition between Wenzel and Cassie-Baxter wetting states.