Functional Surfaces

Superhydrophobic Coatings

Water_SuperhrodrophobicnGimat is actively developing self-cleaning coatings for architectural glass, automotive glass, solar cells, space applications, and other specialty products. The natural cleanness observed on lotus and other plant leaves first inspired development of superhydrophobic self-cleaning surfaces. Studies of lotus leaves by scanning electron microcopy (SEM) revealed that the key features of the lotus leaf are a microscopically rough surface consisting of an array of randomly distributed micropapillae with diameters ranging from 5 to 10 mm. These micropapillae are covered with waxy hierarchical structures in the form of branch-like nanostructures with average diameters of about 125 nm. The water contact angle on a lotus leaf is higher than 160° with a rolling angle of about 2°, which would be considered a high performance superhydrophobic self-cleaning surface. For reference, the droplet on an nGimat-coated surface shown here has a contact angle of greater than 165°.

Water droplets coming in contact with a superhydrophobic surface (contact angle >150°) form nearly spherical beads. The contaminants, either inorganic or organic, on such surfaces are picked up by water droplets or adhere to the water droplet and are removed from the surface when the water droplets roll off. The combination of low surface energy and micro- and/or nano-structured features, which can certainly reduce the contact area between the surface and water droplets, form superhydrophobic surfaces.

nGimat has used its CCVD process to deposit thin films with nanostructured surfaces that mimic the lotus leaf effects but in a controllable process that allows incorporation of other properties, such as transparency. On nGimat-coated glass, metal, or plastic surfaces, the water droplets are nearly perfect spheres. Though a relatively recent addition to nGimat’s area of interest, the CCVD process has enabled rapid development of promising small scale prototype coatings. Glass coaters are evaluating these coatings for a variety of applications.

Anti-microbial Coatings

Anti-microbial coatings are used to reduce or stop growth of organisms like bacteria and fungi. Although there are many chemicals developed for anti-microbial purposes, one of the oldest methods known to combat microbes is silver, where a small amount of silver ions is often all that is needed. Unlike most other heavy metals that also exhibit anti-microbial activity, silver is not detrimental to humans, especially in the tiny quantities used for anti-microbial applications. In addition, since silver acts in multiple ways to inhibit a bacterium, resistant strain development of the organism is thought to be less likely than with other anti-bacterial products.

nGimat’s NanoSpraySM Combustion Processing technology is suited to making silver nanostructured functional surfaces. nGimat is pursuing antimicrobial surfaces by depositing a layer of adherent silver nanodots onto substrates to provide, in the presence of moisture, silver ions to inhibit growth of fungi and bacteria for applications such as food packaging.

Anti-reflective Coatings

Even though many of the packages used for consumer products are clear, seeing the product inside a clear box may still be challenging if light reflected from the container hinders the view. Anti-reflective coatings are used to reduce glare and to improve the transmission of light through the clear material by decreasing its reflectance. In the case of packaging, an anti-reflective, protective covering will allow more comprehensive inspection or presentation of the contents without having to tilt and move the product to accommodate light reflection.

nGimat is developing low-cost, functional nanostructured coatings that provide anti-reflective surfaces to various substrates. To illustrate, the graphs below show the typical transmittance and reflectance spectra of bare glass and polycarbonate substrates and of glass and polycarbonate substrates coated with a CCVD anti-reflective coating. The CCVD-coated glass (coated one side only) has a transmittance of about 2% higher than that of bare glass, and a reflectance of about 2% lower than that of bare glass. The CCVD-coated polycarbonate (two sides coated) has a transmittance of about 5% higher than that of bare polycarbonate, and a reflectance of about 5% lower than that of bare polycarbonate. These results show that the CCVD films reduce reflection in the visible range.