Liquid-repellent adhesive material developed
The ingenious design of the numerous hair tips is the key to a material that even resists oils. The bio-inspired surface design of the adhesive material opens up a wide range of applications, as it repels any kind of liquid droplets and the adhesion is kept up permanently.
Scientists at the Max Planck Institute for Intelligent Systems (MPI-IS) in Stuttgart have developed a bioinspired reusable adhesive material that is able to repel all liquids regardless of their surface tension. While such materials have been actively researched and applied for two decades, no adhesive material is known to repel all liquids.
"Our material not only repels water, but any liquid effectively. Oils, for example, which quickly wet surfaces due to their low surface tension, would normally spread on and between the fine hairs, causing them to clump and lose their adhesion. But because of the specific mushroom-shaped nub structure that we have developed, our hairs can repel all liquids, including oils," explains Ville Liimatainen, highlighting the main property of the special adhesive material. Liimatainen is a post-doctoral fellow in the Department of Physical Intelligence at the MPI-IS and the main author of the publication "Liquid-Superrepellent Bioinspired Fibrillar Adhesives", recently published in Advanced Materials. The co-author of the publication is Metin Sitti, Director at MPI-IS and Head of the Department of Physical Intelligence. Sitti is a pioneer in the research field of gecko-inspired surfaces. He is also the founder of nanoGriptech Inc. a start-up that has brought nature-inspired surfaces to market.
The research of the Department of Physical Intelligence takes nature as a model - as in this project. Ville Liimatainen, Dirk-Michael Drotlef, Donghoon Son and Metin Sitti were inspired by the tiny structures on the soles of a gecko's feet. Each one is covered with hundreds of thousands of tiny mushroom-shaped hairs that enable the animal to climb effortlessly on virtually any surface. Imitations of these adhesive systems, perfected by nature, have been researched for decades, and their performance sometimes exceeds that of geckos. Such materials require no chemicals or adhesives to make them adhere to almost any surface. Moreover, they are reusable, residue-free, soft, flexible and stretchable. They fail, however, when they get wet - just like the bales of a gecko: Under wet conditions, the animal's ability to stick to slippery surfaces decreases dramatically.
Scientists outwit nature
By changing the shape of the so-called adhesive hairs, the scientists were able to show nature how to do it better and make the material super-repellent to water and other liquids. "Gecko-inspired surfaces are now able to adhere to any wet surface without loss of function. A climbing robot would be able to climb up a wet glass pane by using such an adhesive material. Or, to give another example, a robot hand coated with the material would be able to grip any object covered with liquid and put it down again," adds Sitti.
"The rejection of all liquids is due to the overhanging T-shape of the hair tips, which can hold even liquids with very low surface tension," explains Liimatainen. As can be seen in the article’s figure, the arrows represent the vectors of surface tension. Even if a liquid were to spread to the lower corners of the hair tips, the surface tension would have an upward component, as indicated by the arrows. This force prevents the liquid from sliding down between the hairs.
Each hair is 40 microns in height and 10 microns thick at the narrowest point directly under the cap, which is 28 microns in diameter. This size ratio, combined with the special shape of the hair tips and the use of stretchy, scratch-resistant, soft silicone elastomer as a building material, makes it possible to create a durable, dry material with strong adhesion and extreme liquid repellency.
The manufacturing process of this hair-covered surface begins with the 3D printing of a solid master form using two-photon laser lithography. A soft silicone elastomer is applied to the master mould and cured to produce a negative replica. The same elastomer is then applied to the negative and cured to produce the final positive replica.
The potential applications of this research are many and wide-ranging. Super liquid-repellent dry surfaces are useful where wet conditions have rendered previous adhesive materials useless, e.g. medical devices, portable electronic equipment, special gloves or when gripping objects wetted with liquid.
Source: Chemie.de - Flüssigkeitsabweisendes Haftmaterial entwickelt
Original article: Advanced Materials - Liquid-Superrepellent Bioinspired Fibrillar Adhesives
Image source: Advanced Materials - Liquid-Superrepellent Bioinspired Fibrillar Adhesives