SAN DIEGO, March 16, 2016 -- Instead of applying a deicing agent to strip ice from an aircraft's wings before stormy winter takeoffs, airport personnel could in the future just watch chunks slide right off without lifting a finger. Scientists report they have developed a liquid-like substance that can make wings and other surfaces so slippery that ice cannot adhere. The slick substance is secreted from a film on the wing's surface as temperatures drop below freezing and retreats back into the film as temperatures rise.
The researchers present their work today at the 251st National Meeting & Exposition of the American Chemical Society (ACS). ACS, the world's largest scientific society, is holding the meeting here through Thursday. It features more than 12,500 presentations on a wide range of science topics.
The liquid-secreting materials the researchers developed are called self-lubricating organogels, or SLUGs. "The SLUGs technology has a host of formulations and applications, including in a gel form that can be encapsulated in a film coating on the surface of a wing or other device," says research director Atsushi Hozumi, Ph.D.
"We came upon this idea when we observed real slugs in the environment," Chihiro Urata, Ph.D., explains. "Slugs live underground in soils when it is daytime and crawl out at night. But we never see slugs covered in dirt. They secrete a liquid mucus on their skin, which repels dirt, and the dirt slides off. From this, we started focusing on the phenomenon called syneresis, the expulsion of liquid from a gel."
The gel and the liquid-repellent substance are held in a matrix of silicone resin. The mix is cured and applied to a surface as a nearly transparent and solid film coating, Urata explains. Both Urata and Hozumi are at the National Institute of Advanced Industrial Science & Technology (Japan).
The team examined the anti-icing properties of several types of organogels under tests at various temperatures, Urata says. The discovery of the material's thermo-responsive secretion properties was an unexpected surprise. The tests also showed that the secretion was a reversible process. The syneresis gradually starts when temperatures fall below freezing. So although ice can still form, it cannot adhere to the surface and it slips off. Once the temperature rises above freezing, the liquids return back to the film.
Urata sees potential applications for SLUGs beyond aircraft and singles out antifouling coatings in packaging, paints, ship bottoms, metal molds and more.
Their research is currently focusing on increasing the transparency of the SLUG's coating, Urata says. "We are planning a short-term project to apply the coating where transparency is essential. For example, we are just beginning a project to field-test the durability and visibility of SLUGs coating on signage in Japan's northern counties."
Their research is funded through a grant-in-aid for scientific research on innovative areas from the Japanese Ministry of Education, Culture, Sports, Science and Technology.
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Anti-Stick Coatings Using Self-Lubricating Organogels (SLUGs)
Functional coatings with exceptional surface properties, such as liquid-repellency and low-friction/adhesion, have been commonly prepared by combining textured surfaces with long-chain perfluorinated compounds. However, unfortunately, the chemical and physical effects of the LPFCs on human health and environment have been viewed lately with concern. In addition, once such artificial surfaces are physically and chemically damaged, they permanently lose their surface properties. In contrast, some living things maintain their surface properties through secretion of plant waxes and mucus. Here, we report on novel coatings inspired by such biological systems. To realize long-lasting surface properties, we have particularly focused on the syneresis of organogels, which were prepared by hydrosilylation of 2 types of silicones, and several guest organic liquids. As compatibility between guest liquids and polymer matrixes (cross-linked polydimethylsiloxane) is decreased to a certain critical point which is induced by the chemical and/or physical effects, the guest liquids begins to gradually leach out to the outmost organogel surface. Thanks to this self-lubricating property, adhesion of various objects was effectively reduced, resulting in the excellent anti-sticking properties. Viscous liquids flowed on the syneretic organogel surface more freely than that of non-syneretic organogel surface. For the purpose of anti-icing applications, we tuned the critical incompatibility point our organogels, which possess reversible thermo-responsive secretion nature. In this case, the syneresis gradually starts when the temperature is cooled (< 0°C) and the syneresis liquids returns back into the organogel again by heating to room temperature. Thanks to this smart surface property, an ice-pillar formed on the organogel at -15°C easily slid off without any additional force. Furthermore, we have successfully demonstrated regeneration of superhydrophobicity artificially mimicking lotus leaves using n-octadecyltrichlorosilane as an active guest liquid. Our technique, demonstrated here, undoubtedly shows great potential for application in dynamic, multifunctional, and self-recovering coatings.