The lotus effect has been known for many years. The leaves of the lotus plant have a waxy covering that, when viewed under a microscope, can be seen to consist of many microscopic structures in an ordered array. When water falls on these leaves, rather than wetting the surface, it forms beads that role off, taking dust and dirt with them. Coatings that mimic the lotus effect have been developed and applied to window glass and building exterior materials, to make them self-cleaning.
Another application for the lotus effect is in aircraft. Filigree micro patterns on the aircraft skin encourage smooth airflow in flight and reduce drag; a similar effect is exploited in high-tech swimwear for elite athletes, which have "shark tooth" patterns on their surface. Researchers at the Fraunhofer Institute for Material and Beam Technology (IWS) in Dresden, working with the city's Technical University and with Airbus, have now devised a method to use lasers to etch such patterns directly into metal surfaces.
An advantage of etching nano-patterns rather than using a coating is that it has been found that such coatings age quickly and deteriorate over time; moreover, many of them use materials that do not comply with EU environmental regulations that are coming into force.
Project leader Tim Kunze, who heads up the IWS surface functionalisation group, explained that the goal is to eliminate all forms of contamination on aircraft surfaces. "But it would also be a success if we could at least reduce it considerably," he added. Another goal was to find a quick method of forming nano patterns across a wide area: the current state-of-the-art requires them to be drawn on individually with a laser, almost like drawing lines with a pencil.
The key to the technique turned out to be one that any physics student will recognise: the demonstration of wave particle duality known as the double-slit experiment. This involves shining a beam of light (or electrons) at two narrow parallel slits. The beam emerges from the opposite side of the slits in an interference pattern of light and dark bands, indicating that even if the beam is made up of individual particles they seem to travel through the slits at the same time, as though they were waves. Kunze's team has developed a technique it calls DLIP – direct laser interference patterning. It uses special optics to split a single laser beam into multiple partial beams, which recombine on the metal surface at which the optics are aimed. This creates very precise and controllable patterns of light and dark. When used on titanium, the team observed that the lasers ablated the material in the light areas, forming patterns that resemble halls of columns or corrugated iron roofs. The distance between pillars can be set between 150nm and 30µm. This creates a surface on which water cannot "grip"; instead of spreading out to form a film, it beads and rolls off as with lotus leaves.
The technique is fast. Depending on whether titanium or polymers form the surface to be treated, DLIP can pattern up to 1 m² of surface per minute. “This is a world record,” said Prof. Andrés Lasagni, who laid the cornerstone of the working group at Fraunhofer IWS up to 2017 and now holds the professorship for laser-based methods for large-scale surface structuring. “Together with our colleagues from IWS, we have developed the largest DLIP system worldwide, located at the TU Dresden today. This system, which was supported by the Excellence Initiative of the German Research Foundation (DFG), facilitates the treatment of large areas at high throughput. In addition, the DLIP laser heads can be integrated into standard industrial machines allowing even medium-sized companies to access this technology today.”
Airbus engineers are currently testing the effectiveness of the surface patterning in flight. “We have provided a titanium test surface with our columnar structure,” said Kunze. “The colleagues at Airbus have added a chemical treatment to the coating and fixed it to the leading edge of a wing.” Elmar Bonaccurso, a material scientist at Airbus Central R&T, said: “Flight-testing in various operating conditions and coating inspections at regular intervals are especially useful for investigating the durability and functionality of such water and contamination repellent coatings that have already shown to perform very well in the laboratory.”
The technology may also have other applications. Nano-structured patterned surfaces could be used to guard against counterfeiting or to improve the biocompatibility of surgical implants, as such patterning has been shown to improve the ability of cells to adhere to surfaces. “Properly structured implant screws could be better accepted by the body,” said Prof. Lasagni. “And that would mean fewer complications for the patient.”