The science behind self cleaning glass is quite interesting.
The first thing to say is that windowwashers isn't quite right in his statement that self-cleaning glass is meant to repel dirt. If that were the case, it would mean that it has a sort of non-stick surface. The surface will accept dirt, but the glass should then clean itself in the rain. This is how it works:
The glass is given a very thin layer of titanium dioxide (TiO
2) which performs a dual function.
First of all, it is somehow activated by UV light (and I can't begin to understand how) and breaks down organic matter.
Secondly, the TiO
2 also makes the surface very hydrophilic, so water sheets down (as Simon noted) rather than runs down in rivulets (as it does on hydrophobic glass). Just as when we wash windows with 000, the rainwater (which is typically 002 to 006 ppm tds) picks up dust and loosened organic deposits and carries them away. As the surface is hydrophilic, the water sheets down and leaves the glass to dry from the top down - which is just what we observe happening on similarly hydrophic glass.
So, provided that the sun shines enough to keep the TiO
2 activated and it rains hard enough to cause the water to sheet down the glass, it should be perfectly self-cleaning.
Of course, this combination doesn't happen very often - and Pilkington knows this and actually says so on their web site.
The coating on self-cleaning glass normally breaks down even the heaviest deposits of organic dirt in time - but if the surface is so dirty that daylight cannot reach the glass, the self-cleaning action will not take effect. In such cases, clean the glass with warm soapy water and a soft cloth - in a few days the self-cleaning process will have re-activated. During very dry spells you may need to clean your windows, as there will have been no rain to wash loosened dirt away. This is easily done by simply hosing down with water - which should remove most traces of dirt. Note: organic dirt will not necessarily be removed by hosing, so some dirt may still be seen when the glass dries after the hosing treatment.Taken from:
www.pilkingtonselfcleaningglass.co.uk/howitworks;jsessionid=D4F0480D2017F993791A8FC146CACA99#2We window cleaners DO need to be informed about these things and tell the customers the truth:
Self-cleaning glass will only work if it is kept clean! We are not about to be redundant just yet.