Sunday, June 15, 2014

Keeping Glass Clear

Windows get "foggy" when water condenses on them. The glass of the window is colder than the surrounding air, and the air contains moisture which is gaseous water. When the gaseous water hits the surface of the cooler glass, the water turns into a liquid.
What happens next depends on the nature of the glass surface.

If the glass were perfectly flat, and perfectly clean, the strong adhesion between the water molecules and the hydrophilic glass would allow the water to spread out evenly over the surface of the glass so you would still be able to see through the glass.
If the glass is dirty it has an uneven cover of hydrophobic particles which reduces the adhesive forces between the water and the surface resulting in a multitude of small water droplets collecting on the surface which means it is harder to see through the glass.

Traditional ways of keeping the glass of car windows "fog free" include wiping the water off if it is on the outside, or turning on the heater or air-conditioner to increase the rate of evaporation if the "fog" is on the inside of the car.

From a chemistry point of view, we can see there are two possible approaches to making a coating for the glass surface so that it will stay "fog free":
  • Make an extremely hydrophobic, transparent coating, then any water that comes into contact with the surface will not adhere at all and will just run off. This is the traditional approach used to create water repellent coatings and materials.
  • Make an extremely hydrophilic, transparent coating so that any water than comes into contact with the surface will spread out evenly, making an extremely thin, transparent, layer over the surface. This is what scientists at A*STAR's Institute of Materials Research and Engineering (IMRE) have done. They have created a new technology, CleanClear, which is a durable and permanent ceramic coating that is transparent and superhydrophilic, which means it attracts water instead of repelling it.
The new transparent, superhydrophilic ceramic coating is anticipated to have many application; on car windows, glass cooking pot covers, hot food displays, it might even be used to keep your spectacles clear.

The Agency for Science, Technology and Research (A*STAR). "Creating a water layer for a clearer view." ScienceDaily. ScienceDaily, 12 June 2014.

Further Reading

Suggested Study Questions
  1. Write a balanced chemical equation to show the condensation of gaseous water on a glass surface.
  2. Write a balanced chemical equation to show how liquid water evaporates off a glass surface.
  3. Use (kinetic) particle theory of matter to explain what happens when water condenses and evaporates.
  4. Define the terms hydrophilic and hydrophobic.
  5. Define the terms adhesion and cohesion.
  6. Explain why water forms spherical droplets.
  7. Explain why glass is a hydrophilic surface.
  8. Explain why dirty glass, glass covered with oily particles, is hydrophobic.
  9. Explain how a water repellent coating on glass might work.
  10. Discuss potential problems with using CleanClear on car windows, that is, what factors might reduce its effectiveness and why.

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