We often experience transparent materials like windows or glasses become foggy due to humidity or steam. When this happens, it is not only inconvenient but also dangerous at the same time as it may lead to a serious accident.
This fogging occurs when the hydrophobic glass surface forms water droplets, which cause the light to diffuse, refract, and reflect. On the other hand, the contact angle of water droplets becomes smaller on hydrophilic surfaces. This causes moisture to spread into an even layer on the surface as the continuous phase and does not generate fogging.
Glasses or plastics become fogged up by the diffused light reflection when the surface temperature falls below the dew point and moisture in the air forms tiny water droplets. Methods listed in [Table 1] are adopted as the means to prevent fogging.

[Table 1] Anti-fogging method

For example, a formula made primarily of waterproof cross-linked polymer and polyether-polyol mixed with surfactants is available as anti-fog coatings. The hydrophilic characteristics added by polyether-polyol prevent the film from fogging by absorbing moisture. Even at the moisture-absorbing saturation point, the surfactants spread water droplets to form a wet surface and keep the surface fog-free.
Because surfactants are generally water-soluble, the anti-fog performance may become degraded as they evaporate with water. To prevent this drawback, several types of different surfactants are mixed together.
To evaluate the anti-fog effects, perform the followings: [1] Move the sample from the thermo-hygrostat bath at -10℃ to another bath kept at 30℃ and 65%RH to check if the sample becomes fogged up; [2] Move the sample from the thermo-hygrostat bath at 10℃ and 65%RH to another bath kept at 40℃ and 65%RH to check if the sample becomes fogged up; [3] Expose the sample to 90℃ steam and check to see if an interference pattern of water appears.