Berkeley Lab Develops Antifogging Coating With Nanostructured Materials
A coating consisting of nanoparticles of titanium dioxide can keep surfaces such as solar panels, windows, and safety goggles free of fog and dust particles with no additional inputs of chemical catalysts or energy. The coating was developed by Sam Mao, Vasileia Zormpa, and Xiaobo Chen of the Environmental Energy Technologies Division (EETD).
Keeping the surfaces free of fog and dirt is a surprisingly difficult technical problem to solve effectively. The antifogging technology currently in the marketplace is either temporary, such as spray or liquid coating that eventually washes away, or requires additional inputs such as chemical catalysts or ultraviolet radiation to work.
Laboratory testing has shown that the EETD-developed Nanostructured Antifogging Coating is continuously self-cleaning. It prevents water droplets from forming on the surface it covers (Figure 1) without reducing the transparency of its substrate material. The testing also suggests that it resists scratching and does not lose its ability to prevent fogging.
This self-cleaning coating can keep photovoltaic solar panels and solar thermal collectors free of dirt, maximizing their energy conversion efficiency. Desert areas where solar facilities are being planned are known to be subject to windstorms that scatter considerable dust.
In concentrating solar power plants that are built in deserts and other dusty areas, wind deposits dust onto collectors, and can reduce their efficiency to the point where the plant generates insufficient heat to provide any power at all.
In these power plants, the working fluid, when vaporized, moves through the system under pressure and turns a turbine to generate electricity. If enough fog or dust is coating the mirror, it can prevent the working fluid (such as water) from reaching the threshold temperature required to vaporize it. Therefore, a self-cleaning film allows solar energy-related applications to harvest energy to their maximum potential.
The core of the Nanostructured Antifogging Coating is the use of titanium dioxide (TiO2) nanoparticles that are formed into self-similar porous clusters. (The self-similar surface is one in which the nanoparticles have the same rough geometry at the level of the aggregate clusters, the smaller porous clusters, and the nanoparticles themselves.) These porous clusters are, in turn, formed into aggregate clusters, which are used to coat a substrate material such as glass, plastic, mirrored surfaces, or any other solid substrate for which antifogging properties are desired.
The self-similar hydrophilic nanoparticles provide water droplets with a surface of angled particles at several different order-of-magnitude length scales¹ that form low angles with the walls of water droplets. As a result, these droplets do not retain their shape for very long, collapsing and flattening out into the surface in fractions of a second.
The geometry of the particles provides the mechanism of the antifogging effect—no activation process is required. The angular nanostructure of the particles also provides the coating with its self-cleaning properties. Because nanoparticles extend out from the surface at low angles everywhere, water vapor can infiltrate the spaces formed by nanoparticle overhangs, dislodging dust and grime from the surface.
The initial spreading stages of a water droplet on a glass coated with the self-similar porous TiO2 and on an untreated glass surface are shown in Figures 2a and 2b. The water droplet penetrates into the recessed areas and spreads out, within a couple hundred milliseconds, on the surface. The self-similar porous TiO2 surface exhibits a contact angle that vanishes to zero, while the contact angle for water droplets on a glass substrate is approximately 34 degrees.
The coating consists of TiO2 formed into a coating of superhydrophilic nanostructures. Figure 3 a, b, and c depicts a group of aggregate clusters, in the form of ovals (although in the actual material, each cluster could have any one of several different shapes). The dimensions of the aggregate clusters may be within the range of about 150 nanometers (nm) to 5 micrometers (μm). Figure 3 d, e, and f is a set of electron micrographs of a single aggregate cluster, which consists of a set of porous clusters that may be spherically shaped, as shown, or have other shapes or including a mixture of shapes. These particles are in the range of 50 to 600 nm.
The technology's antifogging property can be applied to surfaces that need to be free of fog to reduce safety hazards. Possibilities include sport goggles, auto windshields, windows in public transit vehicles, ships, railway locomotives, sun-wind-dust goggles, laser safety eye protective spectacles, chemical/biological protective face masks, ballistic shields for explosive ordnance disposal personnel, and vision blocks for light tactical vehicles. It can also be used to help keep the solar collectors used in concentrating solar power plants and solar photovoltaic panels clear of dust and water, and maintaining the highest efficiency of solar radiation reflection and transmission.
For more information, contact:
- The Technology Transfer Department
This technology is available for licensing through Berkeley Lab's Technology Transfer Department.
- LBNL Technology Transfer and Intellectual Property Management webpage
- Published patent application US2010/033927 available at the WIPO website.
¹ Hundreds of nanometers, tens of nanometers, and under ten nanometers in length. One nanometer equals one-billionth of a meter.