120 research outputs found
How nanoporous silicon-polypyrrole hybrids flex their muscles in aqueous electrolytes: In operando high-resolution x-ray diffraction and electron tomography-based micromechanical computer simulations
Macroscopic strain experiments revealed that Si crystals traversed by
parallel, channel-like nanopores functionalized with the muscle polymer
polypyrrole exhibit large and reversible electrochemo-mechanical actuation in
aqueous electrolytes. On the microscopical level this system still bears open
questions, as to how the electrochemical expansion and contraction of PPy acts
on to np-Si pore walls and how the collective motorics of the pore array
emerges from the single-nanopore behavior. An analysis of in operando X-ray
diffraction experiments with micromechanical finite element simulations, based
on a 3D reconstruction of the nanoporous medium by TEM tomography, shows that
the in-plane mechanical response is dominantly isotropic despite the
anisotropic elasticity of the single crystalline host matrix. However, the
structural anisotropy originating from the parallel alignment of the nanopores
lead to significant differences between the in- and out-of-plane
electromechanical response. This response is not describable by a simple 2D
arrangement of parallel cylindrical channels. Rather, the simulations highlight
that the dendritic shape of the Si pore walls, including pore connections
between the main channels, cause complex, inhomogeneous stress-strain fields in
the crystalline host. Time-dependent X-ray scattering on the dynamics of the
actuator properties hint towards the importance of diffusion limitations,
plastic deformation and creep in the nanoconfined polymer upon (counter-)ion
adsorption and desorption, the very pore-scale processes causing the
macroscopic electroactuation. From a more general perspective, our study
demonstrates that the combination of TEM tomography-based micromechanical
modeling with high-resolution X-ray scattering experiments provides a powerful
approach for in operando analysis of nanoporous composites from the
single-nanopore up to the porous-medium scale.Comment: Supplementary see ancillary file. 20 pages, 11 figure
First Evidence of Reproductive Adaptation to âIsland Effectâ of a Dwarf Cretaceous Romanian Titanosaur, with Embryonic Integument In Ovo
<div><h3>Background</h3><p>The Cretaceous vertebrate assemblages of Romania are famous for geographically endemic dwarfed dinosaur taxa. We report the first complete egg clutches of a dwarf lithostrotian titanosaur, from ToteĹti, Romania, and its reproductive adaptation to the âisland effectâ.</p> <h3>Methodology/Findings</h3><p>The egg clutches were discovered in sequential sedimentary layers of the Maastrichtian Sânpetru Formation, ToteĹti. The occurrence of 11 homogenous clutches in successive strata suggests philopatry by the same dinosaur species, which laid clutches averaging four âź12 cm diameters eggs. The eggs and eggshells display numerous characters shared with the positively identified material from egg-bearing level 4 of the Auca Mahuevo (Patagonia, Argentina) nemegtosaurid lithostrotian nesting site. Microscopic embryonic integument with bacterial evidences was recovered in one egg. The millimeter-size embryonic integument displays micron size dermal papillae implying an early embryological stage at the time of death, likely corresponding to early organogenesis before the skeleton formation.</p> <h3>Conclusions/Significance</h3><p>The shared oological characters between the HaĹŁeg specimens and their mainland relatives suggest a highly conservative reproductive template, while the nest decrease in egg numbers per clutch may reflect an adaptive trait to a smaller body size due to the âisland effectâ. The combined presence of the lithostrotian egg and its embryo in the Early Cretaceous Gobi coupled with the oological similarities between the HaĹŁeg and Auca Mahuevo oological material evidence that several titanosaur species migrated from Gondwana through the HaĹŁeg Island before or during the Aptian/Albian. It also suggests that this island might have had episodic land bridges with the rest of the European archipelago and Asia deep into the Cretaceous.</p> </div
How phyllosilicate mineral structure affects fault strength in Mg-rich fault systems
The clay mineralogy of fault gouges has important implications for the frictional properties of faults, often identified as a major factor contributing to profound fault weakness. This work compares the frictional strength of a group of Mgârich minerals common in the MgâAlâSiâO compositional space (talc, saponite, sepiolite, and palygorskite) by conducting triaxial frictional tests with water or argon as pore fluid. The studied minerals are chemically similar but differ in their crystallographic structure. Results show that fibrous Mgârich phyllosilicates are stronger than their planar equivalents. Frictional strength in this group of minerals is highly influenced by strength of the atomic bonds, continuity of water layers within the crystals, and interactions of mineral surfaces with water molecules, all of which are dictated by crystal structure. The formation and stability of the minerals studied are mainly controlled by small changes in pore fluid chemistry, which can lead to significant differences in fault strength
Nitrate Reduction Functional Genes and Nitrate Reduction Potentials Persist in Deeper Estuarine Sediments. Why?
Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are processes occurring simultaneously under oxygen-limited or anaerobic conditions, where both compete for nitrate and organic carbon. Despite their ecological importance, there has been little investigation of how denitrification and DNRA potentials and related functional genes vary vertically with sediment depth. Nitrate reduction potentials measured in sediment depth profiles along the Colne estuary were in the upper range of nitrate reduction rates reported from other sediments and showed the existence of strong decreasing trends both with increasing depth and along the estuary. Denitrification potential decreased along the estuary, decreasing more rapidly with depth towards the estuary mouth. In contrast, DNRA potential increased along the estuary. Significant decreases in copy numbers of 16S rRNA and nitrate reducing genes were observed along the estuary and from surface to deeper sediments. Both metabolic potentials and functional genes persisted at sediment depths where porewater nitrate was absent. Transport of nitrate by bioturbation, based on macrofauna distributions, could only account for the upper 10 cm depth of sediment. A several fold higher combined freeze-lysable KCl-extractable nitrate pool compared to porewater nitrate was detected. We hypothesised that his could be attributed to intracellular nitrate pools from nitrate accumulating microorganisms like Thioploca or Beggiatoa. However, pyrosequencing analysis did not detect any such organisms, leaving other bacteria, microbenthic algae, or foraminiferans which have also been shown to accumulate nitrate, as possible candidates. The importance and bioavailability of a KCl-extractable nitrate sediment pool remains to be tested. The significant variation in the vertical pattern and abundance of the various nitrate reducing genes phylotypes reasonably suggests differences in their activity throughout the sediment column. This raises interesting questions as to what the alternative metabolic roles for the various nitrate reductases could be, analogous to the alternative metabolic roles found for nitrite reductases
Dual EFPI sensors to extend absolute measurable strain range
A demodulation technique for measuring large absolute strain ranges using multiple collocated extrinsic Fabry-Perot interferometric (EFPI) sensors is presented. The reaearch and discussion focuses on proving the feasibility of measuring absolute strain using multiple collocated EFPI sensors. --Abstract, page iii
Molecular dynamics of open systems: construction of a mean-field particle reservoir
The simulation of open molecular systems requires explicit or implicit reservoirs of energy and particles. Whereas full atomistic resolution is desired in the region of interest, there is some freedom in the implementation of the reservoirs. Here, we construct a combined, explicit reservoir by interfacing the atomistic region with regions of point-like, non-interacting particles (tracers) embedded in a thermodynamic mean field. The tracer molecules acquire atomistic resolution upon entering the atomistic region and equilibrate with this environment, while atomistic molecules become tracers governed by an effective mean-field potential after crossing the atomistic boundary. The approach is extensively tested on thermodynamic, structural, and dynamic properties of liquid water. Conceptual and numerical advantages of the procedure as well as new perspectives are highlighted and discussed
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