Hydro is Greek for water while phobos means fear. The surface of a material can react either in a water-loving (hydrophilic) or water-hating (hydrophobic) manner. Hydrophobic glass has many applications. In eyewear, water resistance is achieved through a transparent titania (titanium dioxide) coating of between 20 and 30 nanometer. Hydrophobic coating is also found in nature in the wings of many insects and on some leaves.
Other than eyewear, hydrophobicity concepts are also widely used in paints, textiles, and coatings for different items. As an example, scuba diving gear is often hydrophobic.
Some Technical Facts about Hydrophobic Glass and Hydrophobic Coating
Today, top eyeglass brands have hydrophobic coating. The technology is popular because water resistance means sweat and rain, which can form optical distortion, do not remain on the lens.
Other than water resistance, hydrophobic coating is also anti-static. This is important because it leaves the lens free from dirt particles and dust.
Hydrophobic coating repels grease and stains from our fingers, meaning you don’t have to clean lenses regularly and when you do, the process is much quicker. The coating also repels oil and a host of aqueous solutions.
Hydrophobic coating works by weakening the surface bonding between the water drop and the surface of the lens.
Like any other coating, hydrophobic coating can get damaged. You should, therefore, clean the lenses regularly with water to keep corrosive chemicals away.
There are many types of coating other than titania, but titanium dioxide coating has the added property of chemically breaking down absorbed dirt in sunlight.
The surface of hydrophobic glass has a high static water contact angle θ, with the condition that is often quoted being θ>160°, and a very low roll-off angle
How Hydrophobic Coating is made
Material scientists have for many years worked on different chemicals to alter the surface properties of different surfaces to make them hydrophobic. The leaves of lotus, which have wax nanocrystals, have particularly been inspirational in the formation of hydrophobic materials.
Like in the lotus, hydrophobic glass coating causes water to come into contact with a large fraction of air, which forces the water to form a bead shape and to slide off the surface. As the water slides off, it drags surface dirt with it, rendering the surface water-free and clean.
Two approaches are used to create hydrophobic materials today. One is coating a surface with a hydrophobic coat and the other is using nanoengineering to come up with a unique, nanopatterned textured surface. These nanopatterns are made up of small bumps that have a width of approximately 10 µm.
Today’s research is driven towards the development of nanopatterned hydrophobic materials with thin layers of lubricant. This further enhances the hydrophobicity of the surface. The gaps between the nanopattern bumps exert a precise amount of capillary force which helps to hold the lubricant in place.
The popular titania coating is applied by spin coating of organo-titanate chelated precursor such as titanium iso-tetrapropoxide that is chelated by acetylacetone. The material then undergoes heat treatment at elevated temperatures to burn the organic residues in the anatase phase. The material goes through a photocatalyst stage where UV is used to make the glass superhydrophilic. It is in this superhydrophilic stage that water is able to wash away dirt, leaving no streak on the surface.