Hydrophobic anti-soiling coatings allow water to bead up and roll off the surface given sufficient inclination, collecting soil particles as droplets are shed. In stark contrast, hydrophilic anti-soiling coatings form a stable film of water over the entire surface, which then allows excess water to wash away soil particles. These differing mechanisms determine which environments each type of coating is most effective in preventing soiling. For instance, hydrophobic anti-soiling coatings repel a broader range of soil particles and organic contaminants, making them more popular in the majority of applications, particularly drier and/or more polluted scenarios. Hydrophilic anti-soiling coatings are a relatively new competitor to hydrophobic coatings for applications where heavy rainfall is a regular occurrence.
Solar panels are a popular application of such anti-soiling coatings because even a small increase in the percentage of light absorbed by the panel (in keeping it clear of soil particles) provides significant output benefits over the lifetime of the installation. Such use cases illustrate the need to consider hydrophobic vs. hydrophilic coatings for preventing soiling with respect to their different cleaning mechanisms. To qualify for such an application, anti-soiling coatings must be UV-resistant, harder than common sand and soil particles, smooth enough for water to carry particles off the surface, stable in harsh environments such as coastal salt-spray, and very thin (less than 1 µm) to minimize light reflection. Two companies currently developing such coatings for solar applications are Lotus Leaf Coatings and Enki Technology. Both companies use sol-gel based coatings containing silica for durability but differentiate their offerings by adding various functional groups to the sol-gel chemistry. This results in a hydrophilic contact angle below 5° for Lotus Leaf Coatings and a neutral to hydrophobic contact angle of 40° to 105° for Enki Technology’s coatings.
Another application that is ripe for employment of anti-soiling coatings, and also requires balancing the pros and cons of hydrophobic and hydrophilic solutions is consumer optical lenses. For example, Aculon offers hydrophilic coatings for eyewear that prevent soiling and enable consistent wet viewing conditions thanks to the transparency of a single film of water rather than individual droplets. Hydrophobic coatings such as those offered by American Computer Optics, Inc., may offer better anti-soiling properties because most types of soiling are better repelled by low surface energy, but cannot compete with hydrophilic coatings on transparency in wet conditions.
Clients should consider adopting anti-soiling coatings for these kinds of applications generally, and consider hydrophilic coatings specifically as a competitive alternative in environments with consistent exposure to water. Other companies to consider besides Enki Technology and Lotus Leaf Coatings include SLIPS Technologies and LiquiGlide. The liquid-infused mechanism used by these companies may have particular benefits for anti-soiling due to a claimed better contact angle hysteresis than non-liquid infused coatings, a property that may aid in anti-soiling beyond surface energy extremes.
Dayton Horvath is a Research Associate at Lux Research, which provides strategic advice and on-going intelligence for emerging technologies. For more information, visit the Lux Research site.