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- ItemStrong Wet and Dry Adhesion by Cupped Microstructures(Washington, DC : American Chemical Society, 2019) Wang, Y.; Kang, V.; Arzt, E.; Federle, W.; Hensel, R.Recent advances in bio-inspired microfibrillar adhesives have resulted in technologies that allow reliable attachment to a variety of surfaces. Because capillary and van der Waals forces are considerably weakened underwater, fibrillar adhesives are however far less effective in wet environments. Although various strategies have been proposed to achieve strong reversible underwater adhesion, strong adhesives that work both in air and underwater without additional surface treatments have yet to be developed. In this study, we report a novel design - cupped microstructures (CM) - that generates strong controllable adhesion in air and underwater. We measured the adhesive performance of cupped polyurethane microstructures with three different cup angles (15, 30, and 45°) and the same cup diameter of 100 μm in dry and wet conditions in comparison to standard mushroom-shaped microstructures (MSMs) of the same dimensions. In air, 15°CM performed comparably to the flat MSM of the same size with an adhesion strength (force per real contact area) of up to 1.3 MPa, but underwater, 15°CM achieved 20 times stronger adhesion than MSM (∼1 MPa versus ∼0.05 MPa). Furthermore, the cupped microstructures exhibit self-sealing properties, whereby stronger pulls lead to longer stable attachment and much higher adhesion through the formation of a better seal. © 2019 American Chemical Society.
- ItemNanopillar diffraction gratings by two-photon lithography(Basel : MDPI, 2019) Purtov, J.; Rogin, P.; Verch, A.; Johansen, V.E.; Hensel, R.Two-dimensional photonic structures such as nanostructured pillar gratings are useful for various applications including wave coupling, diffractive optics, and security features. Two-photon lithography facilitates the generation of such nanostructured surfaces with high precision and reproducibility. In this work, we report on nanopillar diffraction gratings fabricated by two-photon lithography with various laser powers close to the polymerization threshold of the photoresist. As a result, defect-free arrays of pillars with diameters down to 184 nm were fabricated. The structure sizes were analyzed by scanning electron microscopy and compared to theoretical predictions obtained from Monte Carlo simulations. The optical reflectivities of the nanopillar gratings were analyzed by optical microscopy and verified by rigorous coupled-wave simulations. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
- ItemSwitchable double-sided micropatterned adhesives for selective fixation and detachment(Amsterdam : Elsevier, 2019) Tinnemann, V.; Arzt, E.; Hensel, R.Micropatterned dry adhesives are promising candidates for the development of innovative adhesive platforms. Their reversible adhesion to various materials and surfaces has been reported over more than a decade. Switching between a strong and a weak adhesive state can be introduced by elastic buckling instabilities of the microstructure. In this work, we report on novel adhesive pads that exhibit micropatterned pillars on both sides. In double-sided PDMS micropatterns, the dimensions of the pillar structures were tuned by modulating the critical force for buckling during compressive loading. In this way, selective detachment of glass substrates was induced from one side of the pad. Our results indicate a significant switching efficiency of up to 83% between the strong and weak adhesive state. The new structures have high potential for emerging applications where temporary, double-sided fixations in combination with a predetermined detachment location are required. © 2018