Scientific publications

The recent interest in fibrillar biological attachment systems, as found in the gecko, has led to the development of micropatterned elastomer adhesion surfaces. All reported studies have been performed at ambient humidity neglecting its possible influence on adhesion. The present paper investigates, for the first time, the effect of systematic changes in ambient humidity from 2 to 90%. Adhesion measurements were performed on PDMS (Sylgard 184) surfaces possessing micropillars with flat-ended and hemispherical contact shape. The pillar radius was varied between 2.5 and 25 µm; the pillar aspect ratio was kept at 1. While the adhesion of a flat sample was not affected by humidity, we found that pillar size and shape influenced the sensitivity to humidity changes: Thinner pillars, with higher pull-off forces in the dry state, exhibited decreasing adhesion values, by up to 35 %, with increasing humidity. The effect was stronger for the hemispherical tip shape, where the positive effect of finer pillars was even reversed. Possible explanations for these effects, which may lower the reliability of biomimetic adhesion devices in the presence of humidity, are given.

We explore the morphology space of nanocomposites prepared from poly(isoprene-block-ethylene oxide) (PI-b-PEO) diblock copolymers as structure directing agents for aluminosilicate nanoparticles prepared from (3-glycidyloxypropyl)trimethoxysilane (GLYMO) and aluminum(III) sec-butoxide. The results of structural investigations of over 60 polymer−inorganic nanocomposites are reported. They are obtained from 12 different block copolymers of varying molecular weight (~10−100 kg/mol) and PEO weight fraction (fw 0.1−0.8) through addition of different amounts of inorganic components. Eight different morphologies as well as composites with biphasic character are observed. Individual block copolymers show up to five different well-defined morphologies upon addition of the inorganic sols. Differential scanning calorimetry (DSC) studies on the composites show that the addition of the inorganic components suppresses PEO crystallization when the inorganic to PEO weight fraction ratio of the composites is greater than 1.3−1.5. The eight phases are mapped out using two- and three-component morphology diagrams.

The recently emerging interest in fibrillar biological attachment systems, as those found in the gecko, has led to the fabrication of micropatterned elastomer adhesion surfaces. Reported studies have demonstrated that measurements on micropatterned surfaces are affected by experimental parameters not relevant for flat samples. The present paper investigates the influence on adhesion values of the sample stiffness, the backing layer thickness, the ambient humidity, and of repetitive measurements at the same location. Measurements were performed on PDMS (Sylgard® 184) micropatterned surfaces possessing flat-ended pillars with 10 µm diameter and 10 µm height. We find that adhesion increased with decreasing sample stiffness and decreasing backing layer thickness, whereas it dropped when several tests were carried out at exactly the same location. For ambient humidities between 2 and 90%, no influence on adhesion performance was found.

The biomimetic reproduction of adhesion organs, as found in flies, beetles and geckoes, has become a topic of intense research over the past years. Successes, however, have so far been limited. This is due to the vast range of parameters involved, including fibril size, elastic modulus, contact shape, surface roughness and ambient humidity. In previous studies, design and materials selection charts to determine the optimum materials and design combination for dry adhesive systems have been established. The effect of shape on the adhesive properties of single fibers and fiber arrays has also been a research focus. In this paper both approaches are combined to provide more advanced guidelines for the design of optimal adhesive structures.

Current semiconductor technology demands the use of compliant substrates for flexible integrated circuits. However, the maximum total strain of such devices is often limited by the extensibility of the metallic components. Although cracking in thin films is extensively studied theoretically, little experimental work has been carried out thus far. Here, we present a systematic study of the cracking behavior of 34 to 506 nm thick Cu films on polyimide with 3.5 to 19 nm-thick Ta interlayers. The film systems have been investigated by a synchrotron-based tensile testing technique and in situ tensile tests in a scanning electron microscope. By relating the energy release during cracking obtained from the stress-strain curves to the crack area, the fracture toughness of the Cu films can be obtained. It increases with Cu film thickness and decreases with increasing Ta film thickness. Films thinner than 70 nm exhibit brittle fracture, indicating an increasing inherent brittleness of the Cu films.

Geckos können sich auf nahezu allen Oberflächen bewegen, egal ob glatt oder rau. Dabei entwickeln sie Haftkräfte, die ein Vielfaches ihres Körpergewichts tragen können. Ihr Haftsystem ist selbstreinigend, rückstandsfrei und nachwachsend. Grund genug, sich dieses einmal genauer anzusehen.

Geckos can adhere to nearly every kind of surface, independent of its chemical properties or roughness. The adhesive force generated is high enough to support their own weight multiple times and allows the gecko to walk upside down. Their adhesion system is self-cleaning, renewable (self healing), and can be removed from a surface without residues. It is these unique properties that have kindled steeply rising research interest over the last decade.

During the last few years several research groups have focused on the fabrication of artificial gecko inspired adhesives. For mimicking these structures, different polymers are used as structure material, such as polydimethylsiloxanes (PDMS), polyurethanes (PU), and polypropylene (PP). While these polymers can be structured easily and used for artificial adhesion systems, the effects of repeated adhesion testing have never been investigated closely. In this paper we report on the effect of repeated adhesion measurements on the commercially available poly(dimethylsiloxane) polymer kit Sylgard 184 (Dow Corning). We show that the adhesion force decreases as a function of contact cycles. The rate of change and the final value of adhesion are found to depend on the details of the PDMS synthesis and structuring.

Recently, much work has focused on the size effect in face centered cubic (fcc) structures, however few pillar studies have focused on body centered cubic (bcc) metals. This paper explores the role of bcc crystal structure on the size effect, through compression testing of [001] and [235] Molybdenum (Mo) small-scale pillars manufactured by focused ion beam (FIB). The pillar diameters ranged from 200 nm to 5 μm. Results show that the relationship between yield stress and diameter exhibits an inverse relationship (σy ∝ d−0.22 for [001] Mo and σy ∝ d−0.34 for [235] Mo) weaker than that observed for face centered cubic (fcc) metals (σy ∝ d−0.6 to −1.0). Additional tests at various loading rates revealed that small-scale Mo pillars exhibit a strain rate sensitivity similar to bulk Mo.