Browsing by Author "Greiner, Christian"

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Open Access
Size and shape effects in bioinspired fibrillar adhesives
(2007) Greiner, Christian; Arzt, Eduard (Prof. Dr. phil.)
Over the last years, the striking ability of geckos and several insects to cling to walls and ceilings has inspired large research efforts. The reasons for this interest is the strong and completely reversible adhesion of these dry adhesive systems, which is based on van der Waals interactions and thus universal on almost any kind of surfaces. Therefore the structures are not only interesting for fundamental science, but also for industrial application. Theoretical contact mechanical treatment of fibrillar systems has shown that the main physical principle behind their adhesion abilities is that of contact splitting, which states that the adhesion force increases upon splitting up one large contact into many small ones. Theory has also shown that the tip shape of the contacting fibers is of great importance for adhesion performance. Both influences – the one of size and the one of shape – were experimentally and theoretically treated in the present thesis. In order to allow for systematic and controlled adhesion experiments, elastomeric model systems were fabricated by soft molding techniques based on photolithographic master structures in the micrometer regime. Taking advantage of the filling mechanisms of the molding step and the viscosity of polydimethylsiloxane, it was not only possible to generate flat punch, but also hemispherical, concave, spatula- and mushroom-shaped pillars, and structures which resembled a flat punch, but with rounded edges. Systematic adhesion testing revealed that the adhesion strength increased with decreasing pillar diameter, as was theoretically predicted. This increase was found to be strongest for the mushroom-shaped pillars which also showed the highest overall adhesion values reaching the performance of geckos. The second highest gain upon contact splitting was found for spatular structures followed by hemispherical and flat punch-like tip shape. Also, the adhesion forces increased with increasing pillar aspect ratio. The “adhesion design maps” presented in this thesis, which were developed for different contact shapes, together with the conclusions drawn from the experimental data, will allow for a more strategic approach when designing fibrillar attachment systems in the future.