Self-cleaning glass is a special type of glass whose surface keeps itself free of dirt and grime.
The field of glass self-cleaning coatings is divided into two categories: hydrophobic and hydrophilic.
Both coatings clean themselves through the action of water, the former by rolling water droplets and the latter by water that carries away dirt. Hydrophilic coatings based on titanium dioxide, however, have an added property: they chemically break down dirt absorbed by sunlight. The requirements for self-cleaning hydrophobic surfaces are a very high static water contact angle θ, often quoted as θ > 160°, and a very low roll-off angle, i.e. the minimum required inclination for droplets to roll off the surface.
Several techniques are known to pattern hydrophobic surfaces through the use of molding polymers and waxes, through physical processing methods such as ion etching and compression of polymer beads, and through chemical methods such as plasma chemical roughening, all of which May result in a superhydrophobic coating.  While these surfaces are effective self-cleaning agents, they suffer from a number of drawbacks that have so far hindered widespread application. Batch processing of hydrophobic materials is an expensive and time-consuming technique, and the resulting coatings are often hazy, precluding applications on lenses and windows, and fragile materials. The second type of self-cleaning surface is a hydrophilic surface that does not rely solely on water flow to wash off dirt. These coatings chemically break down dirt when exposed to light, a process called photocatalysis. Although hydrophilic self-cleaning coatings are commercialized in numerous products, the field is far from mature; studies of the basic mechanisms of self-cleaning and characterization of new coatings are regularly published in the primary literature.
【Discovery of self-cleaning behavior】
The first self-cleaning glass is based on a thin-film titanium dioxide coating. Flakes can be chelated by spin-coating organotitanate precursors (eg, titanium isotetrapropoxide chelated by acetylacetone), followed by heat treatment at high temperature to burn organic residues and form anatase phase. In this case, sodium may diffuse from the glass to the primary titania, leading to a decrease in the hydrophilic/catalytic effect , unless preventive measures are taken. The glass self-cleans in two stages. The "photocatalytic" stage of the process uses UV light to break down organic dirt on the glass, making the glass superhydrophilic (usually glass is hydrophobic). In the next "superhydrophilic" stage, the rain washes away the dirt with little streak, because the water is evenly distributed over the superhydrophilic surface.