Not only are greasy fingerprints on shiny stainless steel surfaces unattractive, they also attack the surface in question. In future, a new nanocoating that is being developed by Fraunhofer researchers will prevent the annoying smudges that result from fingers touching stainless steel surfaces.
Oil drops on a superhydrophobic and oleophobic coating on stainless steel.
FEW Chemicals GmbH Wolfen
Shiny new refrigerators feature attractive stainless steel fronts. But it doesn't take long before the door is covered in dark fingerprints that are difficult to remove with only a cloth and detergent. Fingerprints like these are more than just unsightly, since the grease film also attacks the metal surface. Together with their colleagues at FEW Chemicals GmbH in Wolfen, researchers from the Fraunhofer Institute for Microstructure of Materials and Systems IMWS in Halle are now working to put an end to smudges like these. The secret lies in a coating layer containing special additives that is also water and oil repellent. This also has two other effects: when the particles integrated in the coating settle on the surface of the stainless steel, the surface becomes rougher and its surface area increases. Now, when a finger comes into contact with the refrigerator door, it only touches the raised points on the surface and the grease on the fingertip never reaches the "valleys" of the stainless steel surface. This means the surface area which actually comes into contact with the grease is minimal. In addition the refractive index of the coating has been adjusted so that it matches that of the natural oil content of the human finger. This means the light that reaches the coated stainless steel surface is reflected in roughly the same manner as by a surface that has been touched by sticky fingers. As a result, the fingerprints are hardly noticeable.
Analysis of the layer systems
While FEW Chemicals GmbH is handling the development of the coating systems, the Fraunhofer team is concentrating on the analysis of these layers. "We're investigating the layers created using not only optical microscopy, but also scanning electron microscopy and atomic force microscopy. We look at how large the individual particles in the coating system are and whether or not they are distributed homogeneously. The effect of the additives used is another focus of our analysis," says Dr Jessica Klehm, research associate at Fraunhofer IMWS. Such questions are extremely important in assessing the quality of the coating. For example, if the nanoparticles aggregate to form larger particles, the coating may lose its transparency as a result. On the other hand if the particles are too small, the surface remains too smooth, so that the grease film can adhere to it over larger areas in spite of the coating.
Furthermore the researchers are developing an automatic testing machine for the layers that uses a combination of spectrometric and optical procedures to analyse the percentage efficiency level of the coating's anti-fingerprint effect. The researchers have already found a favourite among the various coating systems they have investigated The task now is to further optimise the system. Development activities should be completed by the end of 2020, when the industrial-scale production of the coating system will be turned over to FEW Chemicals GmbH.