American Scientist

To the Editors:

The Science Observer article “Sunburned Ferns?” (March–April) invokes a widespread scientific misconception. The error is the idea that water is repelled by some surfaces, in this case ginkgo leaf surfaces. In fact, water molecules and all hydrophobic substances attract one another, although weakly. The false belief in repulsion traces to the original use of the misleading descriptive term “hydrophobic.” A valid exposition of the interaction of water with hydrophobic substances is found in a 2002 Nature article by David Chandler. Chandler states, “the term hydrophobic (water-fearing) is commonly used to describe substances that, like oil, do not mix with water. Although it may look as if water repels oil, in reality the separation of oil and water in ambient conditions is not due to repulsion ... but to particularly favorable hydrogen bonding between water molecules.” Also widely in current use is the term “superhydrophobic,” referring to surfaces that often are described, erroneously, as repelling water. A 2006 Nano Letters paper by L. Zhai et al. deals with this topic. The authors explain, “Water repellent in the context of our work simply means that water droplets placed on the surface will roll off freely at a small angle. This is how we and many others define the superhydrophobic state (contact angle greater than 150° coupled with a small rolling angle).” Not surprisingly, the forces involved—cohesion, adhesion, and gravity—are all attractive.

J. Lee Kavanau
University of California at Los Angeles