This story is a supplement to the feature "Self-Cleaning Materials: Lotus Leaf-Inspired Nanotechnology" which was printed in the August 2008 issue of Scientific American.

The lotus leaf’s remarkable ability to stay clean inspired development of self-cleaning materials.

Water rolls across a leaf without sticking at all and carries away dirt (left). Microscopic bumps (a few microns in size) all across the leaf’s surface (right) hold the key to its water-repelling properties. A rough coating of nanoscopic wax crystals on these bumps further increases the effect.

Physics of the Lotus
The lotus’s self-cleaning effect stems from its surface being extremely hydrophobic (water-repelling). Whether a material is hydrophobic or hydrophilic (water-attracting) depends on the contact angle between the material and the surface of water.

Contact Angles    
Hydrophilic surface:
less than 30 degrees
Hydrophobic surface:
greater than 90 degrees

Superhydrophobic surface:
greater than 150 degrees

The large angle results from bumps that trap air between water and the surface, minimizing contact with the surface.

How the Lotus Cleans Itself  
On a typical surface (one not extremely hydrophilic or hydrophobic), a drop of water slides across and leaves most dirt particles sticking to the object. On a superhydrophobic surface, a drop rolls across, picking up dirt and carrying it away. The water and dirt have greater affinity for each other than either does for the surface.


A Lotus Lesson: One Way To Stay Clean
Companies have made textiles that can shrug off water and food spills by being superhydrophobic like a lotus leaf (top). Modifications to the material’s individual cotton fibers produce the effect. In one scheme (bottom), particles form bumps several hundred nanometers in size on the fibers. Many other products, such as facade paint and roofing tiles, have a microscopic or nanoscopic rough finish to induce the lotus effect.