Ralph Spolenak

From micro to nano contacts in biological attachment devices.

Animals with widely varying body weight, such as flies, spiders, and geckos, can adhere to and move along vertical walls and even ceilings. This ability is caused by very efficient attachment mechanisms in which patterned surface structures interact with the profile of the substrate. An extensive microscopic study has shown a strong inverse scaling effect in these attachment devices. Whereas μm dimensions of the terminal elements of the setae are sufficient for flies and beetles, geckos must resort to sub-μm devices to ensure adhesion. This general trend is quantitatively explained by applying the principles of contact mechanics, according to which splitting up the contact into finer subcontacts increases adhesion. This principle is widely spread in design of natural adhesive systems and may also be transferred into practical applications.

Effects of contact shape on the scaling of biological attachments

Adhesion of biological systems has recently received much research attention: the survival of organisms ranging from single cells and mussels to insects, spiders and geckos relies crucially on their mechanical interaction with their environments. For spiders, lizards and possible other ‘dry’ adhesive systems, explanations for adhesion are based on van der Waals interaction, and the adhesion of single–contact elements has been described by the classical Johnson–Kendall–Roberts (JKR) model derived for spherical contacts. However, real biological contacts display a variety of shapes and only rarely resemble a hemisphere. Here, we theoretically assess the influence of various contact shapes on the pull–off force for single contacts as well as their scaling potential in contact arrays. It is concluded that other shapes, such as a toroidal contact geometry, should lead to better attachment; such geometries are observed in our microscopic investigations of hair–tip shapes in beetles and flies.

Scanning X-ray microdiffraction with submicrometer white beam for strain/stress and orientation mapping in thin films

Scanning X-ray microdiffraction (microSXRD) combines the use of high-brilliance synchrotron sources with the latest achromatic X-ray focusing optics and fast large-area two-dimensional-detector technology. Using white beams or a combination of white and monochromatic beams, this technique allows for the orientation and strain/stress mapping of polycrystalline thin films with submicrometer spatial resolution. The technique is described in detail as applied to the study of thin aluminium and copper blanket films and lines following electromigration testing and/or thermal cycling experiments. It is shown that there are significant orientation and strain/stress variations between grains and inside individual grains. A polycrystalline film when investigated at the granular (micrometer) level shows a highly mechanically inhomogeneous medium that allows insight into its mesoscopic properties. If the microSXRD data are averaged over a macroscopic range, results show good agreement with direct macroscopic texture and stress measurements.

Resolving the nanoscale adhesion of individual gecko spatulae by atomic force microscopy

Animals that cling to walls and walk on ceilings owe this ability to micron and nanoscale attachment elements. The highest adhesion forces are encountered in geckoes, which have developed intricate hierarchical structures consisting of toes (millimetre dimensions), lamella (400–600 μm size), setae (micron dimensions) and spatulae (∼200 nm size). Adhesion forces of setae on different substrates have previously been measured by a micro-electromechanical system technique. Here we report the first successful experiments in which the force–displacement curves were determined for individual spatulae by atomic force microscopy. The adhesion force for these smallest elements of the geckos attachment system is reproducibly found to be about 10 nN. This method sheds new light on the nanomechanisms of attachment and will help in the rational design of artificial attachment systems.

Electrical transport in pure and boron-doped carbon nanotubes

The resistivities of individual multiwalled pure and boron-doped carbon nanotubes have been measured in the temperature range from 25 to 300 °C. The connection patterns were formed by depositing two-terminal tungsten wires on a nanotube using focused-ion-beam lithography. A decrease of the resistivity with increasing temperature, i.e., a semiconductor-like behavior, was found for both B-doped and pure carbon nanotubes. B-doped nanotubes have a reduced room-temperature resistivity (7.4×10−7–7.7×10−6 Ωm) as compared to pure nanotubes (5.3×10−6–1.9×10−5 Ωm), making the resistivity of the doped tubes comparable to those along the basal plane of graphite. The activation energy derived from the resistivity versus temperature Arrhenius plots was found to be smaller for the B-doped (55–70 meV) than for the pure multiwalled nanotubes (190–290 meV).

Ultrastrong ductile and stable high-entropy alloys at small scales

Refractory high-entropy alloys (HEAs) are a class of emerging multi-component alloys, showing superior mechanical properties at elevated temperatures and being technologically interesting. However, they are generally brittle at room temperature, fail by cracking at low compressive strains and suffer from limited formability. Here we report a strategy for the fabrication of refractory HEA thin films and small-sized pillars that consist of strongly textured, columnar and nanometre-sized grains. Such HEA pillars exhibit extraordinarily high yield strengths of ∼10 GPa—among the highest reported strengths in micro-/nano-pillar compression and one order of magnitude higher than that of its bulk form—and their ductility is considerably improved (compressive plastic strains over 30%). Additionally, we demonstrate that such HEA films show substantially enhanced stability for high-temperature, long-duration conditions (at 1,100 °C for 3 days). Small-scale HEAs combining these properties represent a new class of materials in small-dimension devices potentially for high-stress and high-temperature applications.

High spatial resolution grain orientation and strain mapping in thin films using polychromatic submicron x-ray diffraction

The availability of high brilliance synchrotron sources, coupled with recent progress in achromatic focusing optics and large area 2D detector technology, have allowed us to develop an X-ray synchrotron technique capable of mapping orientation and strain/stress in polycrystalline thin films with submicron spatial resolution. To demonstrate the capabilities of this instrument, we have employed it to study the microstructure of aluminum thin film structures at the granular and subgranular level. Owing to the relatively low absorption of X-rays in materials, this technique can be used to study passivated samples, an important advantage over most electron probes given the very different mechanical behavior of buried and unpassivated materials.

Tensile testing of ultrathin polycrystalline films: A synchrotron-based technique

The mechanical properties of metallic thin films on the nanoscale acquire increasingly more importance as applications in microelectromechanical systems/NEMS as well as microelectronics have reached this size scale. Here, we present a synchrotron x-ray diffraction technique by which it is possible to characterize the evolution of mechanical stress in a metallic film thinner than 100 nm at measurement times shorter than 60 s per data point. This high data acquisition rate is achieved because no relative motions or tilting of specimen, x-ray source and detector (a large-area charge coupled device camera) are required. The technique comprises an initial “sin2 ψ” measurement to establish the absolute stress values followed by periodic “sin2 φ” measurements during straining to determine stress increments. We describe an experimental setup established at the synchrotron radiation source ANKA (Karlsruhe, Germany) which is specifically suited for monitoring the stress evolution during in situ tensile tests on thi...

Stretchable heterogeneous composites with extreme mechanical gradients

Heterogeneous composite materials with variable local stiffness are widespread in nature, but are far less explored in engineering structural applications. The development of heterogeneous synthetic composites with locally tuned elastic properties would allow us to extend the lifetime of functional devices with mechanically incompatible interfaces, and to create new enabling materials for applications ranging from flexible electronics to regenerative medicine. Here we show that heterogeneous composites with local elastic moduli tunable over five orders of magnitude can be prepared through the site-specific reinforcement of an entangled elastomeric matrix at progressively larger length scales. Using such a hierarchical reinforcement approach, we designed and produced composites exhibiting regions with extreme soft-to-hard transitions, while still being reversibly stretchable up to 350%. The implementation of the proposed methodology in a mechanically challenging application is illustrated here with the development of locally stiff and globally stretchable substrates for flexible electronics.

Ordered arrays of faceted gold nanoparticles obtained by dewetting and nanosphere lithography

Nanoparticle arrays have potential applications in many areas, such as optics and electronics. Earlier work is mostly concentrated on the synthesis of such arrays, whereas we would like to focus on the control of particle size and shape, which has a great influence on the particle properties. In this study three different types of arrays of faceted nanoparticles were synthesized; it is shown that the particle size and shape in such arrays can be modified by the annealing treatment. By taking advantage of this, a novel method for the production of lines of faceted particles was demonstrated. These structures have potential applications as plasmon waveguides.