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.
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.
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.
• Windshields and windows
• Stable over time
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