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The springtail cuticle as a blueprint for omniphobic surfaces
Omniphobic surfaces found in nature have great potential for enabling novel and emerging products and technologies to facilitate the daily life of human societies. One example is the water and even oil-repellent cuticle of springtails (Collembola). The wingless arthropods evolved a highly textured, hierarchically arranged surface pattern that affords mechanical robustness and wetting resistance even at elevated hydrostatic pressures. Springtail cuticle-derived surfaces therefore promise to overcome limitations of lotus-inspired surfaces (low durability, insufficient repellence of low surface tension liquids). In this review, we report on the liquid-repellent natural surfaces of arthropods living in aqueous or temporarily flooded habitats including water-walking insects or water spiders. In particular, we focus on springtails presenting an overview on the cuticular morphology and chemistry and their biological relevance. Based on the obtained liquid repellence of a variety of liquids with remarkable efficiency, the review provides general design criteria for robust omniphobic surfaces. In particular, the resistance against complete wetting and the mechanical stability strongly both depend on the topographical features of the nano- and micropatterned surface. The current understanding of the underlying principles and approaches to their technological implementation are summarized and discussed.
Multivalent bonds in self-assembled bundles of ultrathin gold nanowires
Ultrathin gold nanowires are unusual colloidal objects that assemble into bundles with line contacts between parallel wires. Each molecule in the contact line interacts with many ligand and solvent molecules. We used X-ray scattering and electron microscopy to study how these interactions control assembly.
Heat-to-current conversion of low-grade heat from a thermocapacitive cycle by supercapacitors
Thermal energy is abundantly available, and especially low-grade heat is often wasted in industrial processes as a by-product. Tapping into this vast energy reservoir with cost-attractive technologies may become a key element for the transition to an energy-sustainable economy and society. We propose a novel heat-to-current converter which is based on the temperature dependence of the cell voltage of charged supercapacitors. Using a commercially available supercapacitor, we observed a thermal cell-voltage rise of around 0.6 mV K-1 over a temperature window of 0 °C to 65 °C. Within our theoretical model, this can be used to operate a Stirling-like charge-voltage cycle whose efficiency is competitive to the most-efficient thermoelectric (Seebeck) engines. Our proposed heat-to-current converter is built from cheap materials, contains no moving parts, and could operate with a plethora of electrolytes which can be chosen for optimal performance at specific working temperatures. Therefore, this heat-to-current converter is interesting for small-scale, domestic, and industrial applications.
Dynamic effects in friction and adhesion through cooperative rupture and formation of supramolecular bonds
We introduce a molecular toolkit for studying the dynamics in friction and adhesion from the single molecule level to effects of multivalency. As experimental model system we use supramolecular bonds established by the inclusion of ditopic adamantane connector molecules into two surface-bound cyclodextrin molecules, attached to a tip of an atomic force microscope (AFM) and to a flat silicon surface. The rupture force of a single bond does not depend on the pulling rate, indicating that the fast complexation kinetics of adamantane and cyclodextrin are probed in thermal equilibrium. In contrast, the pull-off force for a group of supramolecular bonds depends on the unloading rate revealing a non-equilibrium situation, an effect discussed as the combined action of multivalency and cantilever inertia effects. Friction forces exhibit a stick-slip characteristic which is explained by the cooperative rupture of groups of host-guest bonds and their rebinding. No dependence of friction on the sliding velocity has been observed in the accessible range of velocities due to fast rebinding and the negligible delay of cantilever response in AFM lateral force measurements.
Water desalination via capacitive deionization: what is it and what can we expect from it?
Capacitive deionization (CDI) is an emerging technology for the facile removal of charged ionic species from aqueous solutions, and is currently being widely explored for water desalination applications. The technology is based on ion electrosorption at the surface of a pair of electrically charged electrodes, commonly composed of highly porous carbon materials. The CDI community has grown exponentially over the past decade, driving tremendous advances via new cell architectures and system designs, the implementation of ion exchange membranes, and alternative concepts such as flowable carbon electrodes and hybrid systems employing a Faradaic (battery) electrode. Also, vast improvements have been made towards unraveling the complex processes inherent to interfacial electrochemistry, including the modelling of kinetic and equilibrium aspects of the desalination process. In our perspective, we critically review and evaluate the current state-of-the-art of CDI technology and provide definitions and performance metric nomenclature in an effort to unify the fast-growing CDI community. We also provide an outlook on the emerging trends in CDI and propose future research and development directions.
Direction specific adhesion induced by subsurface liquid filled microchannels
While directional effects in adhesion and locomotion have in general been generated by creating symmetry breaking topographic features on the surface of a soft bodied object, here we present a novel method for imparting this effect to thin adhesive layers by embedding liquid filled microchannels arranged in pairs with specific intra and inter pair distances. The adhesive exhibits uniform adhesion in classical peel tests when both the channels are filled with either air or a wetting liquid. But the asymmetric effect shows up when only one of the channels in the pair is filled with the liquid. The liquid alters the surface tension of the inner wall of the channel, which results in bulging deformation of the thin skin of the adhesive over the channel. The bulging however remains asymmetric, the extent of asymmetry depending on the intra-pair spacing between the channels. Besides the bulging effect, filling in one channel of a pair with liquid also leads to an asymmetric variation in its modulus. As a result, when an adherent is peeled off the adhesive from two opposite directions, significantly different adhesion strengths result. A similar directional effect also results when channels of two different diameters are used in the pair, thus opening up the possibility of generating several different adhesion strengths simply by altering the geometric features of the embedded microstructure and its filling status. We show also that for both channels in a pair filled with liquid, the adhesion strength increases significantly, by over 60 times of what is achieved for a smooth, featureless, adhesive layer.