Anti-Fogging Glass

Car windows, eyeglasses, camera lenses, even our bathroom mirrors are all victims of the frustrating effects of fogging. Fogging can pose hazard when it inflicts automobile windscreens; glass fogs up when warm, moist air comes into contact with it and cools to form thousands of tiny water droplets on the glass. The droplets scatter light, reducing the visibility through the glass.

Existing Technology

Fog is caused when steam condenses on a cool surface and then forms minuscule water droplets due to the water's surface tension. Water molecules are more attracted to each other than to air molecules and form a spherical shape to maximise contact between water molecules, which leaves as few as possible exposed to the air.

But water is also attracted to glass, and if this attraction is enhanced, it can overcome the surface tension. Previous anti-fog coatings have capitalised on this using titanium dioxide surfaces which increase the attraction between the water and glass. This overcomes surface tension so the water spreads out in sheets.

However, these coatings first need to be charged by UV light which means they do not work for long in the dark. And they tend to stop working altogether after three months.

Latest Anti-Fogging Glass

Super-hydrophillic

Super-hydrophillic - water loving - coating is composed of nano-particles made of silica, the same material that glass is made from, to create a coating with a rough surface, although it looks smooth to the naked eye. Polymer chains are used to assemble these very tiny particles of Glass onto a surface. A polymer chain is a long chain-like molecule with positive charge and the glass particles with negative charge are taken. So positive to negative attractive force can be used to build these layers up onto the surface. The net result is we create a very porous coating, that is, a coating that has lots of holes in it.

The silica particles form layers of tiny pores, each a thousand times smaller than the width of a human hair. The pores attract the tiny droplets of water that make up the foggy surface. Stacked ten to twenty layers thick, with air pockets in between, these pores create what's called a "wicking" effect, which forms the water droplets into a uniform sheet. When a droplet is dropped on that surface, the water is drawn into these pores instantaneously and wicked away into a uniform sheet. The result is no water droplets on the surface that can scatter light and a nice transparent lens in this case.

This cheaply produced technology added benefit of increasing the clarity of unfogged glass. It reduces the glare and allows more than 99 percent of light to pass through the glass, compared to untreated glass that scatters between four and eight percent of light. Because of the thin film coating that is filled with holes, it means that the coating also acts as an anti-reflection coating. That is, it will allow more light to pass through.

The super-hydrophilic materials with self-cleaning action are currently in use for side view mirrors of vehicles and exterior materials of buildings.

Super-hydrophobic

Super-hydrophobic - water-fearing – coating is made by adding a second, ultra-thin layer of water-repelling molecules. Then the large surface area created by the roughness of the surface has the opposite effect. It increases the repulsion between the water and glass, causing the water to form droplets.

These could be used to form self-cleaning surfaces, where water that lands on the surface is snapped up into droplets that grow larger and larger until they finally roll away, taking the dirt with them. This is the method a lotus leaf naturally uses to clean itself.

Application

• Windshields and windows

• Solar panels

• Lenses for cameras, endoscopes, laparoscopes, and other optical devices

• Eyewear – goggles and glasses

• Dental mirrors

Advantages

• Stable over time

• Inexpensive

• Does not require UV-light activation

• Excellent optical properties (high transmittance; low reflectance and refractive index)