[Reading Science] The Secret of Fogging on Glasses and Vehicles

Even the president cannot prevent fogging on glasses. What causes fog to form on glasses or car windows?
[Photo by YouTube screenshot]

[Asia Economy Reporter Kim Jonghwa] Have you ever had trouble with fogged-up windshields while driving on cold winter mornings? Especially for those who wear glasses, you might have experienced being unable to move forward on the bus because your glasses fogged up, drawing annoyed looks from the people behind you.

So, what causes fog to form on car windows and glasses? Fog consists of tiny water droplets. These droplets form due to the temperature difference between the inside and outside. When one side is warm and humid while the other side is dry and cold, moisture condenses into liquid form.

Invisible water vapor exists in the air. Water vapor in its gaseous state is a component of air, and its molecules are so small that they are invisible. However, when people get into a car and turn on the heater, the interior temperature rises. The warm air inside then cools upon contact with the cold air outside through the car windows, causing fog to form.

Matter exists in solid, liquid, and gas states, each with different volumes and temperatures. Solids vibrate in place, liquids move around, and gases move chaotically. Especially gases expand as temperature rises, increasing their kinetic energy.

Assuming the warm interior temperature of the car is 20℃ and the outside temperature is 0℃, the energy of water vapor inside the car trying to move is about 150, while outside it is about 50. The surface temperature of the windshield is set at approximately 2℃ on the outside and 3℃ on the inside.

The water vapor inside the car, energized and moving actively, collides with the glass surface. Because there is a 1℃ temperature difference on the glass surface, the water vapor with energy 150 releases 90 of that energy through the glass as gas, but the remaining 60 energy cannot escape as gas and remains as liquid, forming fog.

The fogging of glasses, the droplets forming on the outside of a glass filled with cold water in summer, and the droplets on the surface of a cola can taken out of the refrigerator all follow the same principle. Warm water vapor outside loses heat and changes from gas to liquid, condensing as droplets on surfaces colder than the ambient temperature.

Turning on the air conditioner when the car windows fog up is for this reason. It aims to eliminate the temperature difference between the inside and outside of the vehicle. Instead of lowering the overall interior temperature, the defogging button directs cold air directly onto the glass surface to equalize the outside and inside surface temperatures.

Anti-fog sprays and agents work by forming a hydrophilic film on the surface. Since there is already a water-based film on the surface, small droplets slide off this film, preventing fog from forming. An advantage of anti-fog agents is that dust also slides off, preventing contamination.

However, the hydrophilic film created by sprays is not permanent as it can be worn off by external stimuli. Permanently fog-free glass is coated with titanium dioxide (TiO2), which is also used as a glaze for ceramics.

Titanium dioxide absorbs ultraviolet light and produces reactive oxygen species with strong oxidizing power in the air. These reactive oxygen species have excellent chemical reactions that prevent contamination, purify air, provide antibacterial effects, and act as eco-friendly photocatalysts. Recently, TiO2 coatings are mainly used for car glass, road reflectors, and lenses.