Fogging happens when a water vapor present in the air comes in contact with the surface that is colder than the dew point of the surrounding air-water vapor mixture. This problem is familiar to all wearing glasses but in addition to being annoying, it can even cause safety risks in some situations. One of the reasons why safety goggles are often not used for example is that people find them difficult due to fogging issues. There are several anti-fogging coatings and formulations developed to solve the issues.
How anti-fogging properties can be achieved?
Anti-fogging can be achieved in several different ways. In cars, the rear windshields are often heated, and the air is blown to the windshield to remove the fog formed on the glass. Both are quite effective but cannot be used in applications such as safety goggles. Different types of surface treatments offer another means to prevent fog. By tailoring the wetting properties of the surface, the behavior of water drops on it can be changed. Either the chemistry of the surface can be altered by the introduction of the functional groups, or the surface roughness can be enhanced with nanoscale patterning. In practice, both are needed to ensure proper anti-fogging properties.
Superhydrophilic and superhydrophobic strategies for anti-fogging
There are two approaches to anti-fogging. The surfaces can be made either superhydrophilic or superhydrophobic. Superhydrophobic surfaces mean surfaces with high water contact angles (>150°) and low contact angle hysteresis (< 5°). This can be achieved when hydrophobic surface chemistry is combined with surface roughness as stated by the Wenzel equation. When the surface is superhydrophobic, the water drops will roll off the surface instead of accumulating on it. Another, more commonly used approach is to make the surface superhydrophilic (< 5°). This is done by the combination of hydrophilic coating and surface roughness. On a superhydrophilic surface, the water drops will spread completely forming a very thin layer of water that doesn’t change the optical properties of the surface.
Contact angle measurements can be used to study both of these strategies. If you would like to read more about contact angle and how it can be measured, please download the white paper through the link below.