Scientists at the Max Planck Institute for Intelligent Systems (MPI-IS) have developed a bioinspired reversible dry adhesive material that is able to repel all liquids, regardless of their surface tension.
“Our material effectively repels not only water, but any liquid. Oils, for example, which easily wet surfaces due to their low surface tension, would normally spread on and between the fibrils - the fine hairs - causing them to clump together and lose adhesion. But because of the specific tip structure we have created, our fibrils can fend off all liquids, including oils,” said Ville Liimatainen, postdoctoral researcher in the physical intelligence department at the MPI-IS.
Scientists Ville Liimatainen, Dirk Drotlef, Donghoon Son, and Metin Sitti took inspiration from the tiny fibrils on a gecko’s footpads. Dry adhesives do not require any chemicals or glue to make them stick. The adhesives are reusable, soft, bendable, and stretchable but fail in wet conditions.
New Gecko-inspired Dry Adhesives Suitable for All Wet Surfaces
By changing the shape of the fibril tips, the scientists were able to outsmart nature and make the dry adhesive super-repellent to water and other liquids. New Gecko-inspired dry adhesives would be able to stick to any wet surface without any adhesion loss, and a climbing robot using bioinspired dry adhesives would be able to climb on surfaces with any liquids. Alternately, a robot hand equipped with a layer of such an adhesive would be able to pick-and-place any object covered in liquid.
Even if a liquid can spread to the bottom corners of the fibril tips, surface tension will have an upward- pointing component. The force supports the liquid, which cannot slip down between the fibrils due to the overhanging T-shape geometry of the fibril tip, which can stop even ultralow-surface-tension liquids.
Each fibril is 40 micrometers in height and 10 micrometers thick at the narrowest point just underneath the cap, which has a diameter of 28 micrometers. This size ratio combined with the special fibril tip shape and the use of stretchable, scratch-resistant soft silicone elastomer as the building material enables durable dry adhesives with strong adhesion and extreme liquid repellency.
Fabrication of Soft Elastomeric Mushroom-like Double Re-entrant Fibril Arrays
The fabrication process of a sheet of fibrils starts with 3D-printing a rigid master mold using two-photon laser lithography. A soft silicone elastomer is spread on the master mold and cured to create a negative replica. The elastomer is then applied on the negative and cured for the final positive replica.
The potential applications of this research are manifold. Super liquid-repellent dry adhesives are useful where wet conditions have so far rendered existing fibrillar dry adhesives infeasible, including medical devices, wearable electronics, high-grip gloves, and robotic handling of wet objects.
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