Publikationen

In quill embroidery, leather goods are beautified with patterns stitched with stripes of the cortex of peacock feathers. Our new structural, geometric and mechanical data for the tail cover feathers of the peacock allow an understanding of the mechanisms of embroidery. This study serves as an example of how biological materials can be analyzed like engineering materials, giving insight into the optimization processes during evolution.

A second-order nonlinear elastic model is developed for biogels, whereby the elastic constants are estimated from the free energy densities for polymer networks and polymer-water mixtures. For typical values of the constants, important physical insights are revealed: a biogel stiffens under tension, contracts longitudinally under torsion (inverse Poynting effect), and experiences a negative normal stress under simple shear. This approach of extracting nonlinear elastic constants works for any constitutive law, and has the advantage of exploring physical phenomena without recourse to computationally intensive simulations.

To mimic the adhesive effects of gecko toes, artificial surfaces have been manufactured recently using polydimethylsiloxanes (PDMS). However, the effects of repeated contacts on the adhesive properties remain largely unexplored. In this paper we report on the effect of repeated pull-off force measurements on the adhesion behavior of PDMS (polymer kit Sylgard 184, Dow Corning) tested with a borosilicate glass probe. A decrease in pull-off force with increase in number of test cycles is found until a plateau is reached. The initial value and the rate of change in pull-off force strongly depend on the sample preparation procedure, including curing time and cross-linking. It is proposed that the behavior is due to steady coverage of the probe with free oligomers. The results are crucial for developing reusable, durable, and residue-free bioinspired adhesives.

Porous metal thin films have high potential for use in applications such as catalysis, electrical contacts, plasmonics, as well as energy storage and conversion. Structuring metal thin films on the nanoscale to generate high surface areas poses an interesting challenge as metals have high surface energy. In this communication, we demonstrate direct access to nanostructured metal nanoparticle hybrid thin films with high nanoparticle loadings through spin coating of a mixture of block copolymer and ligand stabilized platinum and palladium nanoparticles. Plasma cleaning to remove the organics results in a conductive metal thin film. We expect that the methods described here can be generalized to other metals, mixtures of metal nanoparticles, and intermetallics. We report on direct access to nanoporous metal thin films using block copolymer self-assembly. Nanostructured metal nanoparticle hybrid thin films with high nanoparticle loadings were generated through spin coating of a mixture of block copolymer and ligand stabilized platinum and palladium nanoparticles. Plasma cleaning to remove the organics results in a conductive metal thin film. We expect that the methods described here can be generalized to other metals, mixtures of metal nanoparticles and intermetallics.

The INM – Leibniz Institute for New Materials in Saarbruecken engages in fundamental and applied materials research – from a chemical, physical and biological perspective. The cover highlights examples of INM’s research which is presented in this special issue. The front cover shows biomineralization using the example of crystals embedded in the outer tissue of onion bulbs (courtesy of Birgit Heiland, INM). The back side demonstrates an in situ adhesion experiment in a scanning electron microscope (courtesy of Andreas S. Schneider/Anika Weber, INM).

The amazing adhesion of gecko pads to almost any kind of surfaces has inspired a very active research direction over the last decade: the investigation of how geckos achieve this feat and how this knowledge can be turned into new strategies to reversibly join surfaces. This article reviews the fabrication approaches used so far for the creation of micro- and nanostructured fibrillar surfaces with adhesive properties. In the light of the pertinent contact mechanics, the adhesive properties are presented and discussed. The decisive design parameters are fiber radius and aspect ratio, tilt angle, hierarchical arrangement and the effect of the backing layer. Also first responsive systems that allow thermal switching between nonadhesive and adhesive states are described. These structures show a high potential of application, providing the remaining issues of robustness, reliability, and large-area manufacture can be solved.

While shape memory alloys such as NiTi have strong potential as active materials in many small-scale applications, much is still unknown about their shape memory and deformation behavior as size scale is reduced. This paper reports on two sets of experiments which shed light onto an inconsistent body of research regarding the behavior of NiTi at the nano- to microscale. In situ SEM pillar bending experiments directly show that the shape memory behavior of NiTi is still present for pillar diameters as small as 200 nm. Uniaxial pillar compression experiments demonstrate that plasticity of the phase transformation in NiTi is size independent and, in contrast to bulk single crystal observations, is not influenced by heat treatment (i.e., precipitate structure).

Nanoindentation testing was used to investigate the mechanical properties of PDMS focusing on two aspects: Firstly, the results give a comprehensive overview of the mechanical behavior of PDMS Sylgard 184 which guides the property tailoring for many different application fields, where the mechanical behavior needs to be considered, but is not readily accessible. Secondly, the ability to measure the slight differences in the mechanical properties of these PDMS samples emphasized the high sensitivity of this technique and underlined its reliability for application on very compliant materials. The indentation results obtained in this study did not deviate from complementary tensile test data more than 5%. It was found that the elastic moduli of differently crosslinked samples cover a range of 2 orders of magnitude; for one and the same PDMS the modulus may increase 10-fold as a function of curing time.

The high temperature low cycle fatigue resistance and the creep–fatigue interaction (CFI) behaviour in terms of the effects of prior fatigue exposure on the subsequent creep behaviour are evaluated and reported for a short alumina fibre (Saffil) reinforced aluminium alloy (Al-12Si-CuMgNi) matrix composite at 623 K. The prior fatigue to study the CFI behaviour was imparted in the form of low cycle fatigue loading in a fully reversed, total strain controlled loading up to a quarter of fatigue life at a total strain amplitude of 0∙006 (the plastic strain amplitude at half-life is 0∙004), corresponding to a plastic strain energy per cycle value of 0∙46 MJ m-3. Subsequently, isothermal tensile creep tests were conducted at 623 K to evaluate the minimum creep rate, rupture time and strain to failure as a function of applied creep stress. Also examined were the fracture features as well as the nature and extent of damage that occurs during low cycle fatigue and creep-fatigue loading. The results obtained on the composite material are compared with those of the matrix aluminium alloy to bring out the effects of reinforcement. The results showed that the reinforcement causes significant loss in high temperature low cycle fatigue resistance in terms of fatigue ductility and cyclic energy parameters. Prior fatigue loading was seen to cause a small but consistent decrease in the creep resistance, which is attributed to the combined effects of mechanical loading and microstructural damage from prior fatigue loading.

Focused ion beam machined compression pillars created from [1 1 1], [0 0 1] and [2 1 0] NiTi demonstrate that orientation plays a dominant role in determining dislocation flow stress in stress-induced martensite. This is in contrast to bulk NiTi in which martensite strength is primarily dictated by precipitate size. Post-mortem transmission electron microscopy and Laue microdiffraction measurements reveal respectively dense dislocation structures and stabilized martensite consistent with bulk observations in heavily deformed NiTi.