superhydrophobicity
Hydrophobicity and superhydrophobicity refer to as surface's ability to repel water or, in other words, are difficult to wet. Similarly to icephobic surface construction, hydrophobic surfaces are designed with nano-textures that increase the contact angle between the surface and the water droplet (over 150 degrees for superhydrophobic surfaces, known as the Lotus affect). In the context of icephobic surfaces, a common misconception is that superhydrophobic surfaces would naturally be icephobic. However, that is not the case and many researchers struggle with finding a surface design that is both superhydrophobic and icephobic as it could be applicable to objects that endure a wide range of climates.
Constructing Superhydrophobic Surfaces
As mentioned before, superhydrophobicity is achieved with certain surface roughness, often mimicking that of a lotus leaf. Essentially, the nanotexture of a lotus leaf has uncharacteristic bumps or hills on an extremely small scale. This leads to the water droplets remaining in the Cassie state when on the leaf surface. That's why, when looking at water droplets on a lotus leaf, they seem to fall, form on the leaf, and then simply roll off the leaf. This level of water repellency became the basis for hydrophobic and, with recent distinction, superhydrophobic surfaces.
lotus leaf effect
In 2004, it was discovered that the wettability, or lack of hydrophobicity, decreased when electrospun fibers were made with smaller fiber diameters. Electrospinning has now become a common fiber production method where threads can be pulled together on a nano-scale. It turns out that electrospun surfaces have shown excellent water repellency, an early sign that hydrophobic surfaces could be manufactured without unrealistic procedures. Similarly, carbon fibers (fibers made from carbon that are typically 5-10 micrometers in diameter), sometimes called "nanograss" or "nanoforests," were found to be chemically modified to become a rougher, more hydrophobic surface. To mimic the lotus leaf, carbon nanotubes (essentially carbon fibers), have begun to be applied as a secondary coating on surfaces to make the surface more hydrophobic.
So Why aren't our windows superhydrophobic?
Water repellency seems like it would be ideal for windows on cars and houses or glass on skyscrapers to inhibit condensation and larger water droplets. And, with the capability to design and manufacture transparent hydrophobic surfaces, you may be wondering why you still see water on the glass all around you. It turns out that, if the temperature difference between the surface and the environment or the humidity is great enough, immense condensation can occur on superhydrophobic transparent surfaces. Because the surface is hydrophobic, the condensation forms droplets and rolls off the surface fairly quickly. However, this can be distracting, especially when driving if the surface was the windshield. Therefore, most windows and mirrors have superhydrophilic surfaces. Superhydrophilic is the opposite of superhydrophobic and means that the surface allows wetting. For a windshield, for instance, a superhydrophilic surface would collect the condensation and create a film on the surface which happens to not decrease visibility when viewed perpendicularly.