Air Mediated Reversible Underwater Adhesion: from Beetles to Bioinspired Materials
Biomaterials
Final Report Abstract
Many insects are able to attach to a wide variety of surfaces with help of adhesive pads on their legs: the surfaces can be hydrophilic like smooth rocks or hydrophobic, like many wax-coated leaves in nature. Most surfaces are rough and in addition, often covered in dust. Some surfaces can get wet or even flooded after heavy rain. A previous study by Hosoda and Gorb (2012) has shown that the terrestrial ladybird beetle (Coccinella septempunctata) can adhere to completely submerged substrates. It was hypothesised that a trapped bubble underneath the adhesive pads might help to free the contact area from water and thus mediate the attachment. The goals of our study were to first investigate the adhesive mechanism and the contribution of the bubble itself in alive beetles in more detail. Second, to develop a simple model of the adhesive mechanism where we can simulate other factors affecting adhesion and thus extend our knowledge to attachment systems of other animals and to technical adhesives. Third, fabricate structures with the right configuration to mimic the adhesion mechanism. Very few attempts have been made so far to develop a reversible ’sticky tape’ which can adhere even on wet surfaces. The findings from our studies are as follows: beetles adhere well to both dry hydrophilic and hydrophobic surfaces. When submerged, the pads show higher adhesion forces to hydrophobic surfaces as the dewetting of the surface with the oily adhesive fluid is facilitated here. Furthermore, the trapped bubble helps a little bit on hydrophilic surfaces but does not seem to be crucial for the attachment. Although our model simplified many aspects of the adhesive pad in beetles, it reflects very well the general trends we see in the adhesion to different surfaces. For a hydrophobic substrate, the adhesion was similar regardless of whether the contact occurred in air or underwater, with or without a trapped bubble. Our theoretical calculations showed that the bubble by itself had a negligible capillary contribution (less than 3%) to the net underwater adhesion of the pad. Direct force measurement of a single similarly sized bubble making contact with a hydrophobic substrate showed a maximum adhesion of not more than 50µN, which further validated that the bubble’s contribution is insignificant for underwater adhesion. Our first fabricated mimics show promise for underwater adhesion. We used simple round pillar structures (width-to-height ratio of 1) from PDMS and infused the material in oil to mimic the adhesive fluid in beetles. Our findings are still preliminary but show that the mineral-oil infused samples adhere best in air and underwater to hydrophobic surfaces. However, drainage of the oil is less ideal since this can lead to contamination of the surrounding media and the need to replace the oil in repeating steps. We hope that our studies can contribute to a better understanding on capillary-based adhesion in animals which in turn can lead to smart technical adhesives which perform in a similar way to what animals can do.
Publications
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2021: Wetting of the tarsal adhesive fluid determines underwater adhesion in ladybird beetles; Journal of Experimental Biology, 224 (20)
Sudersan, P. and Kappl, M. and Pinchasik, B-E. and Butt, H-J. and Endlein, T.
