Water beads up on a surface with the coating of nanoparticles.
Yao Lu
Every day we come into contact with a large number of surfaces, ranging from soft fabrics to hard metals. This inevitably means that we have to spend our time cleaning them. A class of self-cleaning materials would certainly make our lives easier.
Previous efforts to make these materials focused on developing surfaces that are rough and waxy; these cause water droplets to roll across the surface, picking up dirt and dust along the way. These materials have been relatively successful, but they have many limitations. For example, they wear off easily and are easily contaminated by oils such as those found on our skin. Fortunately, scientists have now developed a waterproof, self-cleaning coating that can be applied to materials as soft as cotton and as hard as glass.
This coating is composed of titanium dioxide (TiO2) nanoparticles with two size distributions, supplied as a suspension in ethanol containing the chemical perfluorooctyltriethoxysilane. After application, the ethanol is allowed to evaporate for 180 seconds before the coating is ready for use. This coating can be sprayed, dipped or painted onto a surface and retains its performance after various types of damage.
Scientists have shown that these materials exhibited extreme water repellency; water on the surface bubbles up into nearly spherical droplets. In effect, these surfaces eliminate wetting, meaning scientists could see water droplets bouncing off the surface.
Water droplets are actively repelled from the surface.
The coating also has good self-cleaning properties (meaning the water takes all the dust with it when it is repelled) when applied to softer materials such as cotton and paper. Scientists tested the self-cleaning properties by pouring water over spray-coated filter paper contaminated with artificial dust. The soil was completely removed from the spray coated filter paper, but it remained on the control filter paper. These coatings maintained their performance after exposure to various types of damage, including smearing to contaminate surfaces with oil, knife scratches, and multiple sanding cycles with sandpaper.
The researchers also checked whether these self-healing properties remained intact after being contaminated with oil. Usually, such water repellency is eliminated by oil contamination, which has a lower surface tension that allows oil to penetrate. For oil-contaminated samples, the self-cleaning properties were intact. But that wasn’t enough. Scientists took it a step further and showed that these self-cleaning properties were maintained even when the surface was completely submerged in oil.
The rough surfaces of these coatings are critical to their functionality and they are easy to sand, negating these self-cleaning properties. To overcome these limitations, these scientists developed a method of bonding the coatings to hard surfaces using adhesive. They showed that when the coating was applied to double-sided tape, it could withstand finger movement without destroying its self-cleaning properties.
These studies demonstrate a new class of robust, self-cleaning coatings that can be widely used for a variety of soft and hard surfaces. Despite their rough surface, they are a prime candidate for use in lubricating bearings and gears, which require a coating that can remain water-repellent in the presence of lubricating oils.
Science2015. DOI: 10.1126/science.aaa0946 (About DOIs).