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- ItemDirection specific adhesion induced by subsurface liquid filled microchannels(Cambridge : Royal Society of Chemistry, 2012) Majumder, Abhijit; Mondal, Subrata; Tiwari, Anurag Kumar; Ghatak, Animangsu; Sharma, AshutoshWhile 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.
- ItemQuantitative multichannel NC-AFM data analysis of graphene growth on SiC(0001)(Frankfurt am Main : Beilstein-Institut, 2012) Held, Christian; Seyller, Thomas; Bennewitz, RolandNoncontact atomic force microscopy provides access to several complementary signals, such as topography, damping, and contact potential. The traditional presentation of such data sets in adjacent figures or in colour-coded pseudo-three-dimensional plots gives only a qualitative impression. We introduce two-dimensional histograms for the representation of multichannel NC-AFM data sets in a quantitative fashion. Presentation and analysis are exemplified for topography and contact-potential data for graphene grown epitaxially on 6H-SiC(0001), as recorded by Kelvin probe force microscopy in ultrahigh vacuum. Sample preparations by thermal decomposition in ultrahigh vacuum and in an argon atmosphere are compared and the respective growth mechanisms discussed.
- ItemBright fluorescent silica-nanoparticle probes for high-resolution STED and confocal microscopy(Frankfurt am Main : Beilstein-Institut, 2017) Tavernaro, Isabella; Cavelius, Christian; Peuschel, Henrike; Kraegeloh, AnnetteIn recent years, fluorescent nanomaterials have gained high relevance in biological applications as probes for various fluorescencebased spectroscopy and imaging techniques. Among these materials, dye-doped silica nanoparticles have demonstrated a high potential to overcome the limitations presented by conventional organic dyes such as high photobleaching, low stability and limited fluorescence intensity. In the present work we describe an effective approach for the preparation of fluorescent silica nanoparticles in the size range between 15 and 80 nm based on L-arginine-controlled hydrolysis of tetraethoxysilane in a biphasic cyclohexane–water system. Commercially available far-red fluorescent dyes (Atto647N, Abberior STAR 635, Dy-647, Dy-648 and Dy-649) were embedded covalently into the particle matrix, which was achieved by aminosilane coupling. The physical particle attributes (particle size, dispersion, degree of agglomeration and stability) and the fluorescence properties of the obtained particles were compared to particles from commonly known synthesis methods. As a result, the spectroscopic characteristics of the presented monodisperse dye-doped silica nanoparticles were similar to those of the free uncoupled dyes, but indicate a much higher photostability and brightness. As revealed by dynamic light scattering and ζ-potential measurements, all particle suspensions were stable in water and cell culture medium. In addition, uptake studies on A549 cells were performed, using confocal and stimulated emission depletion (STED) microscopy. Our approach allows for a step-by-step formation of dye-doped silica nanoparticles in the form of dye-incorporated spheres, which can be used as versatile fluorescent probes in confocal and STED imaging.
- ItemIn situ measurements with CPC micro-actuators using SEM(Bellingham : SPIE, 2014) Kaasik, Friedrich; Must, Indrek; Lust, Enn; Jürgens, Meelis; Presser, Volker; Punning, Andres; Temmer, Rauno; Kiefer, Rudolf; Aabloo, AlvoComparative measurements of carbon-polymer composite micro-actuators based on room temperature ionic liquid electrolyte were carried out in situ (1) in vacuum using a state-of-the-art scanning electron microscope, (2) in an oxygen-free atmosphere under ambient pressure, and (3) under ambient environment. The fabricated micro-actuators sustained their actuation performance in all three environments, revealing important implications regarding their humidity-dependence. SEM observations demonstrate high stroke actuation of a device with submillimeter length, which is the typical size range of actuators desirable for medical or lab-on-chip applications.
- ItemTemperature-dependent size effects on the strength of Ta and W micropillars(Amsterdam : Elsevier, 2016) Torrents Abad, Oscar; Wheeler, Jeffrey M.; Michler, Johann; Schneider, Andreas S.; Arzt, EduardThe strength of metals increases with decreasing sample size, a trend known as the size effect. In particular, focused ion beam-milled body-centered cubic (BCC) micropillars exhibit a size effect known to scale with the ratio of the test temperature to the critical temperature (Tc) of the BCC metal, a measure of how much the yield stress is governed by the lattice resistance. In this paper, this effect is systematically studied by performing high-temperature compression tests on focused ion beam-manufactured Ta and W single crystal pillars ranging in diameter from 500 nm to 5 μm at temperatures up to 400 °C, and discussed in the context of bulk strength and size dependent stresses. Both metals show larger size effects at higher temperatures, reaching values that are in the range of FCC metals at temperatures near Tc. However, it is demonstrated that size effects can be considerably affected by material parameters such as dislocation density and lattice friction, as well as by the yield criterion used. Furthermore, for W, a change from uniform wavy deformation to localized deformation is observed with increasing temperature and pillar size, further indicating that the temperature ratio strongly influences the relative motion of screw and edge dislocations.
- ItemSwitchable adhesion in vacuum using bio-inspired dry adhesives(Washington D.C. : American Chemical Society, 2015) Purtov, Julia; Frensemeier, Mareike; Kroner, ElmarSuction based attachment systems for pick and place handling of fragile objects like glass plates or optical lenses are energy-consuming and noisy and fail at reduced air pressure, which is essential, e.g., in chemical and physical vapor deposition processes. Recently, an alternative approach toward reversible adhesion of sensitive objects based on bioinspired dry adhesive structures has emerged. There, the switching in adhesion is achieved by a reversible buckling of adhesive pillar structures. In this study, we demonstrate that these adhesives are capable of switching adhesion not only in ambient air conditions but also in vacuum. Our bioinspired patterned adhesive with an area of 1 cm2 provided an adhesion force of 2.6 N ± 0.2 N in air, which was reduced to 1.9 N ± 0.2 N if measured in vacuum. Detachment was induced by buckling of the structures due to a high compressive preload and occurred, independent of air pressure, at approximately 0.9 N ± 0.1 N. The switch in adhesion was observed at a compressive preload between 5.6 and 6.0 N and was independent of air pressure. The difference between maximum adhesion force and adhesion force after buckling gives a reasonable window of operation for pick and place processes. High reversibility of the switching behavior is shown over 50 cycles in air and in vacuum, making the bioinspired switchable adhesive applicable for handling operations of fragile objects.
- ItemFlexible distributed Bragg reflectors from nanocolumnar templates(Hoboken, NJ : Wiley, 2015) Calvo, Mauricio E.; González-GarcÃa, Lola; Parra-Barranco, Julián; Barranco, Angel; Jiménez-Solano, Alberto; González-Elipe, AgustÃn R.; MÃguez, HernánA flexible distributed Bragg reflector is made by the infiltration of a nanocolumnar array with polydimethyl siloxane oligomers. The high optical reflectance displayed by the final material is a direct consequence of the high refractive index contrast of the columnar layers whereas the structural stability is due to the polymer properties.
- ItemNew insights into the structure of nanoporous carbons from NMR, Raman, and pair distribution function analysis(Washington D.C. : American Chemical Society, 2015) Forse, Alexander C.; Merlet, Céline; Allan, Phoebe K.; Humphreys, Elizabeth K.; Griffin, John M.; Aslan, Mesut; Zeiger, Marco; Presser, Volker; Gogotsi, Yury; Grey, Clare P.The structural characterization of nanoporous carbons is a challenging task as they generally lack long-range order and can exhibit diverse local structures. Such characterization represents an important step toward understanding and improving the properties and functionality of porous carbons, yet few experimental techniques have been developed for this purpose. Here we demonstrate the application of nuclear magnetic resonance (NMR) spectroscopy and pair distribution function (PDF) analysis as new tools to probe the local structures of porous carbons, alongside more conventional Raman spectroscopy. Together, the PDFs and the Raman spectra allow the local chemical bonding to be probed, with the bonding becoming more ordered for carbide-derived carbons (CDCs) synthesized at higher temperatures. The ring currents induced in the NMR experiment (and thus the observed NMR chemical shifts for adsorbed species) are strongly dependent on the size of the aromatic carbon domains. We exploit this property and use computer simulations to show that the carbon domain size increases with the temperature used in the carbon synthesis. The techniques developed here are applicable to a wide range of porous carbons and offer new insights into the structures of CDCs (conventional and vacuum-annealed) and coconut shell-derived activated carbons.
- ItemThe springtail cuticle as a blueprint for omniphobic surfaces(Cambridge : Royal Society of Chemistry, 2015) Hensel, René; Neinhuis, Christoph; Werner, CarstenOmniphobic 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.
- ItemLower nanometer-scale size limit for the deformation of a metallic glass by shear transformations revealed by quantitative AFM indentation(Frankfurt am Main : Beilstein-Institut, 2015) Caron, Arnaud; Bennewitz, RolandWe combine non-contact atomic force microscopy (AFM) imaging and AFM indentation in ultra-high vacuum to quantitatively and reproducibly determine the hardness and deformation mechanisms of Pt(111) and a Pt57.5Cu14.7Ni5.3P22.5 metallic glass with unprecedented spatial resolution. Our results on plastic deformation mechanisms of crystalline Pt(111) are consistent with the discrete mechanisms established for larger scales: Plasticity is mediated by dislocation gliding and no rate dependence is observed. For the metallic glass we have discovered that plastic deformation at the nanometer scale is not discrete but continuous and localized around the indenter, and does not exhibit rate dependence. This contrasts with the observation of serrated, rate-dependent flow of metallic glasses at larger scales. Our results reveal a lower size limit for metallic glasses below which shear transformation mechanisms are not activated by indentation. In the case of metallic glass, we conclude that the energy stored in the stressed volume during nanometer-scale indentation is insufficient to account for the interfacial energy of a shear band in the glassy matrix.