Superhydrophobic surfaces are those that are extremely difficult to wet. The contact angle of a drop of water on a superhydrophobic surface can be as high as 150°. This is in contrast to most ordinary surfaces, which have a contact angle with water of less than 90°. Superhydrophobic surfaces are thus highly “water-repellent”.
The term “superhydrophobic” is sometimes used more generally to describe any material that is very difficult to wet. However, the term is properly reserved for those materials whose contact angle with water is greater than 120°. Materials with a contact angle below 120° are said to be “hydrophobic”, while those with a contact angle between 120° and 150° are said to be “ultrahydrophobic”.
Superhydrophobic surfaces have been found in nature, most notably on the leaves of the lotus plant (Nelumbo nucifera). The lotus effect was first described by German botanist Wilhelm Barthlott in 1977. Barthlott and his colleagues found that the leaves of the lotus plant were covered in tiny bumps (papillae), each one topped with a waxy substance known as cutin. These papillae caused water droplets to bead up into small spheres, which rolled off the leaf surface taking dirt and other contaminants with them. The self-cleaning properties of the lotus leaf have since been exploited in many commercial products, such as paints and coatings.
In recent years, much effort has gone into creating artificial superhydrophobic surfaces using various nanotechnology techniques. Such surfaces have potential applications in areas such as self-cleaning coatings, anti-fogging treatments and drag reduction (e.g., for ships). In addition, superhydrophobic surfaces can be used to create microfluidic devices for use in laboratories (e.g., for DNA analysis).