Scientific publications

The size, morphology and species-specific texture of mollusc shell biominerals is one of the unresolved questions in nature. In search of molecular control principles, chitin has been identified by Weiner and Traub (FEBS Lett. 1980, 111 : 311-316) as one of the organic compounds with a defined co-organization with mineral phases. Chitin fibers can be aligned with certain mineralogical axes of crystalline calcium carbonate in a speciesspecific manner. These original observations motivated the functional characterization of chitin forming enzymes in molluscs. The full-length cDNA cloning of mollusc chitin synthases identified unique myosin domains as part of the biological control system. The potential impact of molecular motors and other conserved domains of these complex transmembrane enzymes on the evolution of shell biomineralization is investigated and discussed in this article.

High power electrochemical double layer capacitors (also called supercapacitors) rely on highly conductive electrode materials with a large specific surface area, which is easily accessible for ions. Exohedral materials with a large ion-accessible outer surface and little or no porosity within the particles are particularly attractive for supercapacitor electrodes because they decrease mass transport limitations and enable very high charge/discharge rates. This study focuses on the investigation of charge induced expansion effects of spherical exohedral carbons, that is, onion-like carbons (OLC, diameter: 5–7 nm) and carbon black (diameter: ≈40 nm). Employing electrochemical in-situ dilatometry we studied the expansion behavior within ±1 V potential window versus carbon in an organic electrolyte (tetraethylammonium-tetrafluoroborate in acetonitrile). The expansion had a very small amplitude (<0.2% at ±0.08 C·m-2 accumulated charge; i.e., approximately ±1 V versus carbon) and was fully reversible. This was explained by ion adsorption on the exohedral carbons. Molecular dynamics (MD) simulations were employed to calculate the solvation shell of the respective cation and anion and the results were used to further evaluate the measured expansion. In summary, the experiments and simulations revealed that ion intercalation or ion sieving, which are commonly found in microporous (endohedral) carbons, were absent. Finally, low sweep rates resulted in a slight electrode compaction on a cycle-by-cycle basis, which can be explained by charge-induced restructuring of the nanoparticle network.

Sierpinski carbon: Macroporous carbide-derived carbon monoliths (DUT-38) were synthesized starting from SiC-PolyHIPEs, resulting in a hierarchical micro-, meso-, and macroporous structure. The high specific surface area and high macropore volume renders PolyHIPE-CDC an excellent adsorbent combining high storage capacity with excellent adsorption rates in gas storage and air filtration.

The nanoscale interactions of room temperature ionic liquids (RTILs) at uncharged (graphene) and charged (muscovite mica) solid surfaces were evaluated with high resolution X-ray interface scattering and fully atomistic molecular dynamics simulations. At uncharged graphene surfaces, the imidazolium-based RTIL ([bmim+][Tf2N-]) exhibits a mixed cation/anion layering with a strong interfacial densification of the first RTIL layer. The first layer density observed via experiment is larger than that predicted by simulation and the apparent discrepancy can be understood with the inclusion of, dominantly, image charge and π-stacking interactions between the RTIL and the graphene sheet. In contrast, the RTIL structure adjacent to the charged mica surface exhibits an alternating cation-anion layering extending 3.5 nm into the bulk fluid. The associated charge density profile demonstrates a pronounced charge overscreening (i.e., excess first-layer counterions with respect to the adjacent surface charge), highlighting the critical role of charge-induced nanoscale correlations of the RTIL. These observations confirm key aspects of a predicted electric double layer structure from an analytical Landau-Ginzburg-type continuum theory incorporating ion correlation effects, and provide a new baseline for understanding the fundamental nanoscale response of RTILs at charged interfaces.

While pressure sensitive adhesives in general consist of a layer of viscoelastic glue sandwiched between two adherents, we explore here the design of an adhesive embedded with microchannels which remain either open to atmosphere or pressurized to different positive and negative pressures. We subject these layers to indentation by a rigid cylinder such that in addition to adhesion between the indenter and the adhesive surface, the inner walls of the channels too self-adhere; during retraction of the indenter, these surfaces debond, but at a different load, thus resulting in hysteresis. When these channels are pressurized to different extents, the contact areas of various interfaces vary, so also the resultant hysteresis. For experiments with constant depth of indentation, the hysteresis increases and attains maxima at an intermediate value of the internal pressure inside the channels. The hysteresis increases also with the skin thickness of the adhesive over the channels. These results show that subsurface channels in an adhesive allow active manipulation of adhesion over a large range via coupled effect of geometry of channels, their surface characteristics, and the pressure inside.

In this work, the adhesion of biomimetic polydimethylsiloxane (PDMS) pillar arrays with mushroom-shaped tips was studied on nano- and micro-rough surfaces and compared to unpatterned controls. The adhesion strength on nano-rough surfaces invariably decreased with increasing roughness, but pillar arrays retained higher adhesion strengths than unpatterned controls in all cases. The results were analysed with a model that focuses on the effect on adhesion of depressions in a rough surface. The model fits the data very well suggesting that the pull-off strength for patterned PDMS is controlled by the deepest dimple-like feature on the rough surface. The lower pull-off strength for unpatterned PDMS may be explained by the initiation of the pull-off process at the edge of the probe, where significant stress concentrates. To micro-rough surfaces, pillar arrays showed a maximum in adhesion for a certain intermediate roughness, while unpatterned controls did not show any measurable adhesion. This effect can be explained by the inability of micropatterned surfaces to conform to very fine and very large surface asperities.

Flow of a liquid inside a helical tube is composed of axial and circumferential components, the latter arising because of its specific geometry. For a gas-liquid two-phase flow inside a helical tube, the coupled effect of these two flow components leads to a variety of flow patterns, for example, slug, bubble, and stratified flow. We present here a novel triple-helical microchannel, in which, the two-phase flow is found to engender several additional flow patterns not observed with the conventional geometries, for example, the parallel and oscillating annular flow and even simultaneous occurrence of several such patterns. We show that the transition between these patterns depends not only on the fluid rates of the two liquids but also on the helix angle. We have presented detailed phase diagrams to elaborate these effects. We have examined also the effect of channel geometry on the specific features of these flow patterns.

In their comment (M. Lagos et al., Scripta Mater. (2012), http://dx.doi.org/10.1016/j.scriptamat.2012.04.018), Lagos et al. propose a two-dimensional plasticity model based on grain boundary sliding to explain the deformation behavior of ultrathin Cu and Ta/Cu film systems on polyimide substrates. Here, we critically discuss their comments and include new results obtained by peak profile analysis of the original in situ diffraction data; they strongly suggest that the deformation behavior of the different film systems is very likely not self-similar as claimed by Lagos et al.