by Chris Woodford. Last updated: December 4, 2013.
It's a disaster! They've invented windows that can clean themselves! If, like me, you're one of the miniscule minority who actually enjoy cleaning your windows, this new technological development will fill you with horror. For the rest of humanity—the millions of people who loathe wobbling up ladders, bucket and soapy squeegee in hand—the prospect of windows that keep themselves sparkling automatically, using nothing but the sun and the rain, will seem like nothing short of a miracle. How can a simple piece of glass stay clean all by itself? Let's take a closer look!
Photo: Say goodbye to this tiresome chore! Self-cleaning glass means you can throw away your window-cleaning kit—or does it? You'll see it sold under brand names such as Pilkington Activ™ and Cardinal Neat® Glass.
What is self-cleaning glass?
The first thing to note about self-cleaning windows is that they're not, in fact, "simple pieces of glass." They have a very thin outer coating of titanium dioxide, a white, powdery titanium compound best known for giving that dazzling gleam to paint, toothpaste, and all kinds of other bright white things. Now if titanium dioxide is, essentially, the white in white paint, it might seem ludicrous to splash it all over a window—something we naturally want to be transparent. But the coating really is ultra-ultra thin. We're talking about putting a layer 10–25 nanometers deep on glass that might be 4mm thick, which is like sitting a dime on top of the Empire State Building! It reduces the light passing through the glass by no more than about 5 percent.
Photo: Titanium dioxide, an important part of self-cleaning windows, is also the ingredient that puts the "white" into white paint.
How does self-cleaning glass work?
The titanium dioxide coating cleans through a double-whammy, two-stage process: it's photocatalytic (light-activated) and hydrophilic (water-loving)—but let's not get bamboozled by the jargon. Let's find out more...
Titanium dioxide is a photocatalyst: it's a material that makes chemical reactions happen when the right kind of light shines on it. The right kind of light for titanium dioxide is ultraviolet (UV), the super-blue, high-energy part of sunlight that our eyes can't see, but that nevertheless can give us sunburn even on a cloudy day. When ultraviolet light hits the titanium dioxide coating of a self-cleaning window, electrons are generated. These turn water molecules from the air into hydroxyl radicals that make chemical oxidation and reduction reactions take place on the coating. In effect, the hydroxyl radicals attack organic (carbon-based) dirt and chop it up into smaller pieces that are much easier for rain to wash away. Since the reactions happen on the titanium coating, on the very surface of the glass, they attack the lowest layers of the dirt, loosening encrusted muck from the glass very effectively by chipping it away from the inside out (the opposite of normal window cleaning, where you effectively scrub the dirt from the outside in). All this is illustrated in the diagram in the box below.
Photo: It takes energy to shift dirt from windows, even if they clean themselves. The ultraviolet part of sunlight—the part that gives you sunburn—is the power behind self-cleaning glass.
Glass is usually hydrophobic or "water hating": water dropped onto glass tends to "bead" (form droplets), while rain runs down windows in noticeable rivulets, leaving dirty streaks as it goes. The titanium dioxide coating changes all that: the hydroxyl radicals produced by photocatalysis make the glass hydrophilic or "water loving." Instead of staying in drops, water molecules spread out evenly across the glass in a very even sheet. So when rain hits a dirty self-cleaning window, it spreads across it like a great big cloth. Since the window is most likely vertical or mounted at an angle, the sheet of water wipes down it neatly and evenly, a bit like a rubber squeegee, and the glass dries without any streaks or smears. Magic!