Laboratoire Ondes et Milieux Complexes

Earthquakes represent one of the most catastrophic natural events affecting mankind. At present, a universally accepted risk mitigation strategy for seismic events remains to be proposed. Most approaches are based on vibration isolation of structures rather than on the remote shielding of incoming waves. In this work, we propose a novel approach to the problem and discuss the feasibility of a passive isolation strategy for seismic waves based on large-scale mechanical metamaterials, including for the first time numerical analysis of both surface and guided waves, soil dissipation effects, and adopting a full 3D simulations. The study focuses on realistic structures that can be effective in frequency ranges of interest for seismic waves, and optimal design criteria are provided, exploring different metamaterial configurations, combining phononic crystals and locally resonant structures and different ranges of mechanical properties. Dispersion analysis and full-scale 3D transient wave transmission simulations are carried out on finite size systems to assess the seismic wave amplitude attenuation in realistic conditions. Results reveal that both surface and bulk seismic waves can be considerably attenuated, making this strategy viable for the protection of civil structures against seismic risk. The proposed remote shielding approach could open up new perspectives in the field of seismology and in related areas of low-frequency vibration damping or blast protection.

International audience

We present the first astronomical detection of a diatomic negative ion, the cyanide anion CN<sup>-<sup/>, and quantum mechanical calculations of the excitation of this anion by means of collisions with para-H<sub>2<sub/>. The anion CN<sup>-<sup/> is identified by observing the <i>J<i/> = 2–1 and <i>J<i/> = 3–2 rotational transitions in the C-star envelope IRC +10216 with the IRAM 30-m telescope. The U-shaped line profiles indicate that CN<sup>-<sup/>, like the large anion C<sub>6<sub/>H<sup>-<sup/>, is formed in the outer regions of the envelope. Chemical and excitation model calculations suggest that this species forms from the reaction of large carbon anions with N atoms, rather than from the radiative attachment of an electron to CN, as is the case for large molecular anions. The unexpectedly high abundance derived for CN<sup>-<sup/>, 0.25% relative to CN, indicates that its detection in other astronomical sources is likely. A parallel search for the small anion C<sub>2<sub/>H<sup>-<sup/> remains inconclusive, despite the previous tentative identification of the <i>J<i/> = 1–0 rotational transition. The abundance of C<sub>2<sub/>H<sup>-<sup/> in IRC +10216 is found to be vanishingly small, <i><<i/>0.0014% relative to C<sub>2<sub/>H.

The BASECOL2012 database is a repository of collisional data and a web service within the Virtual Atomic and Molecular Data Centre (VAMDC, http://www.vamdc.eu). It contains rate coefficients for the collisional excitation of rotational, ro-vibrational, vibrational, fine, and hyperfine levels of molecules by atoms, molecules, and electrons, as well as fine-structure excitation of some atoms that are relevant to interstellar and circumstellar astrophysical applications. Submissions of new published collisional rate coefficients sets are welcome, and they will be critically evaluated before inclusion in the database. In addition, BASECOL2012 provides spectroscopic data queried dynamically from various spectroscopic databases using the VAMDC technology. These spectroscopic data are conveniently matched to the in-house collisional excitation rate coefficients using the SPECTCOL sofware package (http://vamdc.eu/software), and the combined sets of data can be downloaded from the BASECOL2012 website. As a partner of the VAMDC, BASECOL2012 is accessible from the general VAMDC portal (http://portal.vamdc.eu) and from user tools such as SPECTCOL.

The appearance of nonlinear effects in elastic wave propagation is one of the most reliable and sensitive indicators of the onset of material damage. However, these effects are usually very small and can be detected only using cumbersome digital signal processing techniques. Here, we propose and experimentally validate an alternative approach, using the filtering and focusing properties of phononic crystals to naturally select and reflect the higher harmonics generated by nonlinear effects, enabling the realization of time-reversal procedures for nonlinear elastic source detection. The proposed device demonstrates its potential as an efficient, compact, portable, passive apparatus for nonlinear elastic wave sensing and damage detection.

In this paper we investigate the tensile properties of a natural composite material: the flax fiber. The beginning of the stress-strain curve of a flax fiber upon tensile loading appears markedly non-linear. The hypothesis of a progressive alignment of the cellulose microfibrils with the tensile axis provides a quantitative explanation of this departure from linearity. This hypothesis is confirmed by the similar behavior which characterises hemp and ramie fibers. Besides, it has long been recognized that the natural character of flax fibers induces a large scatter of their mechanical properties. This scatter is shown not to be associated with the pronounced cross-section size variation observed along the fiber profiles. Two fiber size measurement methods are compared in order to check their influence on the property scatter and the decrease of the fiber mechanical properties as a function of the fiber diameter.

Elastic plates patterned with triangular and circular holes provide the first experimental demonstration of topologically protected helical edge modes, a robust approach to manipulating vibrations, with potential applications in sensing-signal processing and wave guiding.

Modelling of molecular emission from interstellar clouds requires the calculation of rates for excitation by collisions with the most abundant species. This paper deals with the calculation of rate coefficients for rotational excitation of the HCN and HNC molecules in their ground vibrational state in collision with He. Calculations of pure rotational (de-)excitation cross-sections of HCN and HNC by He were performed using the essentially exact close-coupling method. Cross-sections for transitions among the 26 first rotational levels of HCN and HNC were calculated for energies up to 3500 cm−1. These cross-sections were used to determine collisional rate constants for temperatures ranging from 5 to 500 K. The propensity rules of both collisional systems are discussed. A propensity for even Δ&lt;it&gt;j&lt;/it&gt; transitions is observed in the case of HCN–He collisions whereas a propensity for odd Δ&lt;it&gt;j&lt;/it&gt; transitions is observed in the case of HNC–He collisions. These propensities become less pronounced at high temperature, although they do not vanish within the temperature range considered in this work. The new rate coefficients will significantly help in interpreting HCN and HNC emission lines observed with current and future telescopes. In particular, the HNC/HCN abundance ratio derived from observations would have to be revised from values &gt;1 to values ≤1.

As appreciated by those familiar with disposable diapers, polymeric hydrogels swell to absorb a remarkable amount of liquid. Although gels are widely used in many applications from moisture control to drug delivery, the mechanisms of fluid uptake and release are complex, highly nonlinear, and not well understood. The authors use modeling and experiment to show that the processes of swelling and drying are inherently transient and dynamic---and strikingly different from each other.

AOK has received funding from the European Union’s 7th Framework programme for research and innovation&#13;\nunder the Marie Skłodowska-Curie Grant Agreement No. 609402-2020 researchers: Train to Move (T2M).MM&#13;\nhas received funding from the European Union’s Horizon 2020 research and innovation programme under the&#13;\nMarie Skłodowska-Curie Grant Agreement No. 658483. NMP is supported by the European Research Council&#13;\nPoC grant 2015 SILKENE No. 693670, EU FETPROACTIVE grant 732344 ‘NEUROFIBRES’, and by the&#13;\nEuropean Commission under the Graphene Flagship (WP14 ‘Polymer Nanocomposites’, No. 604391). FB is&#13;\nsupported EU FETPROACTIVE grant 732344 ‘NEUROFIBRES’.

Flax fibers were used to process unidirectional composites by two different methods. Their mechanical properties obtained by tensile testing are discussed with respect to the properties of the fibers and those of the matrix (unsatured polyester). The similarity of the tensile curves of the composites and of the elementary fibers is attributed to the good adhesion of the fibers with the matrix. Moreover, as there is almost a linear evolution of the composite properties with the fiber volume fraction, these properties are used to estimate those of the real reinforcement material, that is, the flax bundles: the calculations lead to a fiber strength of 500-800 MPa and a fiber modulus of roughly 30 GPa, which is half the values obtained by tensile testing elementary fibers. These data may be helpful when trying to model the deformation behavior of flax fiber-reinforced composites.

Abstract. Erosion of rock cliffs has been considered to be relatively unpredictable. This perceived stochastic nature of the erosional processes often occurs through collapses along fractures in the rock-mass. The prediction of catastrophic cliff failures and collapses remains very difficult. For advancing in this field, it is important to understand the processes through which a crack is initiated, how it develops and propagates until the final failure. This paper examines the micro-seismic signals recorded 15 h prior to a rock-fall located at Mesnil-Val, France. The results lead to the hypothesis that several phases of failure mechanisms contribute to rock-fall occurrence. The most important phases were associated with micro-seismic event families identified by multiplet selection. Each event family contained one specific frequency spectrum showing a progressive decrease of the frequencies as the rock approached failure suggesting the following phases: 1) the micro-seismic events recorded 15 h before the rock-fall were characterised by the highest frequencies in a large spectrum-band, between ~100 and 1000 Hz (family 1), suggesting a crack initiation mechanism or the opening of existing fractures; 2) the micro-seismic events recorded several minutes before the rock-fall were associated with a clear decrease in the highest frequency components (family 2) suggesting that the mechanism was related to the growing and development (or coalesce) of existing micro-cracks into larger fractures; 3) micro-seismic events recorded just before the rock-fall were associated with a lower frequency spectrum than families 1 and 2, the highest frequency components were absent (family 3), the frequency emission source mechanism could be related to the shearing or opening of the existing large fractures permitting the complete detachment of the blocky rock-mass; 4) finally, micro-seismic events with a very low frequency spectrum (lower than 100 Hz) characterized the rock-fall impact on the ground. These encouraging results offer the possibility of using the micro-seismic system to monitor high risk sections of coastline and to advance understanding of cliff failure mechanisms.

The design and engineering of hybrid materials exhibiting tailored phononic band gaps are fundamentally relevant to innovative material technologies in areas ranging from acoustics to thermo-optic devices. Phononic hybridization gaps, originating from the anti-crossing between local resonant and propagating modes, have attracted particular interest because of their relative robustness to structural disorder and the associated benefit to 'manufacturability'. Although hybridization gap materials are well known, their economic fabrication and efficient control of the gap frequency have remained elusive because of the limited property variability and expensive fabrication methodologies. Here we report a new strategy to realize hybridization gap materials by harnessing the 'anisotropic elasticity' across the particle-polymer interface in densely polymer-tethered colloidal particles. Theoretical and Brillouin scattering analysis confirm both the robustness to disorder and the tunability of the resulting hybridization gap and provide guidelines for the economic synthesis of new materials with deliberately controlled gap position and width frequencies.

Exploration of the northern Peruvian subduction zone with the French submersible 'Nautile' has revealed benthlc communities dominated by new species of vesicomyid bivalves (Calyptogena spp and Ves~comya sp ) sustained by methane-nch fluid expulsion all along the continental margin, between depths of 5140 and 2630 m Videoscoplc studies of 25 dives ('Nautiperc cruise 1991) allowed us to describe the distribution of these biological conlnlunities at different spahal scales At large scale the communities are associated with fluid expuls~on along the major tectonic features (scarps, canyons) of the margln At a smaller scale on the scarps, the distribuhon of the communities appears to be controlled by fluid expulsion along local fracturatlon features such as joints, faults and small-scale scars Elght dlves were made at one particular geological structure the Middle Slope Scarp (the scar of a large debns avalanche) where numerous clam beds have been discovered The spatial dlstnbution of the chemosynthetic communities on thls scarp indicates extensive fluid expulsion, but the low clam densities and low fauna1 diversity in clam beds and the presence of dead beds suggest fluld expulsion is temporally irregular An exceptionally large clam field was observed at the bend of this scarp at the intersection of 2 faults It extends for about 1000 m2 with the biomass of b~valves as hlgh as 30 kg m-' wet weight without shells A strong and regular fluid flow, estimated at 400 m yr-l, is requlred at this locatlon to generate such a hlgh chemosynthetic pnmary production Temperature anomalies In the sediment were measured in s ~t u in the main fleld and compared to clam denslty The dlstnbution of the bivalves at the metre scale 1s likely related to local vanations In fluid flow and in fluid expulsion patterns controlled by the nature of the subsurface sediment Exceptionally large and densely dlstnbuted serpuhds (Neoverm~ha n sp ) were arranged in clumps of 20 to 30 m', and covered 200 m2 of the fleld Their abundance may be related to the filtration of chemoautotrophic free-living bacteria Another 22 non-syinbiotic species displaying varlous trophic strategies have been sampled or observed, and 5 are still undescnbed new specles The resulting community structure is more complex than In other subduction systems descnbed so far

ABSTRACT To understand how phosphorus (P)-bearing molecules are formed in star-forming regions, we have analysed the Atacama Large Millimeter/Submillimeter Array (ALMA) observations of PN and PO towards the massive star-forming region AFGL 5142, combined with a new analysis of the data of the comet 67P/Churyumov–Gerasimenko taken with the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument onboard Rosetta. The ALMA maps show that the emission of PN and PO arises from several spots associated with low-velocity gas with narrow linewidths in the cavity walls of a bipolar outflow. PO is more abundant than PN in most of the spots, with the PO/PN ratio increasing as a function of the distance to the protostar. Our data favour a formation scenario in which shocks sputter phosphorus from the surface of dust grains, and gas-phase photochemistry induced by UV photons from the protostar allows efficient formation of the two species in the cavity walls. Our analysis of the ROSINA data has revealed that PO is the main carrier of P in the comet, with PO/PN &amp;gt; 10. Since comets may have delivered a significant amount of prebiotic material to the early Earth, this finding suggests that PO could contribute significantly to the phosphorus reservoir during the dawn of our planet. There is evidence that PO was already in the cometary ices prior to the birth of the Sun, so the chemical budget of the comet might be inherited from the natal environment of the Solar system, which is thought to be a stellar cluster including also massive stars.

Spider silk is a remarkable example of bio-material with superior mechanical characteristics. Its multilevel structural organization of dragline and viscid silk leads to unusual and tunable properties, extensively studied from a quasi-static point of view. In this study, inspired by the Nephila spider orb web architecture, we propose a design for mechanical metamaterials based on its periodic repetition. We demonstrate that spider-web metamaterial structure plays an important role in the dynamic response and wave attenuation mechanisms. The capability of the resulting structure to inhibit elastic wave propagation in sub-wavelength frequency ranges is assessed, and parametric studies are performed to derive optimal configurations and constituent mechanical properties. The results show promise for the design of innovative lightweight structures for tunable vibration damping and impact protection, or the protection of large scale infrastructure such as suspended bridges.

The negative refraction of transverse elastic waves is demonstrated experimentally in a two-dimensional phononic crystal (PC) made of a square lattice of cylindrical air cavities in an aluminum matrix. Dispersion curves of elastic waves in this PC exhibit a unique branch with phase and group velocities of opposite signs in a broad frequency range. Measurement of refraction angles through prismatic PC included in an aluminum block demonstrates negative refraction of elastic transverse wave.

We present an application of recently developed, explicitly correlated, partially spin-restricted coupled-cluster RCCSD(T)-F12x (x = A/B) methods [G. Knizia, T. B. Adler, and H.-J. Werner, J. Chem. Phys., 2009, 130, 054104] for the generation of multi-dimensional potential energy surfaces (PESs) for scattering calculations. We test the method on the O(2)-He van der Waals model system by a comparison with standard orbital-based coupled-cluster techniques, employing correlation-consistent atomic basis sets (aug-cc-pVXZ, X = T, Q, 5, 6) and a complete basis set. From this comparison, it is obvious that the RCCSD(T)-F12/aug-cc-pVTZ approach is accurate enough for the description of short and long-range interactions with low computational effort. We apply this new method in studies of the interaction of the carbon-rich interstellar species C(4)(X(3)Σ) with atomic He. This PES is subsequently used in quantum close-coupling scattering calculations. The collisional excitation cross-sections of the fine-structure levels of C(4) by He are calculated at low collisional energies. The thermal dependence of rate coefficients is calculated up to 50 K. The propensity rules between fine-structure levels are studied, and it is shown that F-conserving cross sections are much larger, especially for high-N rotational levels rather than F-changing cross sections, as expected from theoretical considerations. This is the first report on the collisional rate coefficients for this system and may have important implications for the astrophysical detection of C4 and modeling of carbon-rich media.

Since flax is the most promising plant for the reinforcement of polymer-based composites in structural applications, we have chosen to investigate its hygrothermal characteristics which can be useful for the understanding of the behaviour of other plant fibres. The flax fibres were exposed to different hygrothermal conditions: in an oven at various controlled temperatures (–40 to 140℃) and measured relative humidity, in a climate chamber at 50% relative humidity for define temperatures between 25℃ and 85℃, or different determined aging conditions. The correlation of these hygrothermal conditions to the evolution of the mechanical properties gives evidence of the prominent influence of water over temperature on the microstructural changes of flax fibres. The mechanical parameters drastically decrease in usually prescribed hygrothermal aging conditions for organic matrix composite materials, the strength being particularly sensitive to the presence of water. These evolutions were correlated to the fibre microstructure modifications induced by water absorption as revealed by electron microscopy analyses. These findings could be useful for understanding the behaviour of polymer matrix biocomposites in severe hygrothermal conditions.

A set of towing test results of a free-float capable Tension Leg Platform (TLP) from the ARC-WIND project, the CENTEC-TLP, is presented. Its design process and overall response behavior are available in the literature. This work focuses on its transportation stage, characterised by shallow draft and certain aspects resembling blunt bodies such as barges. This hull form is investigated experimentally in calm water, and its added resistance in waves is evaluated. Several configurations and towing connections are considered to identify the differences and advantages of each alternative. The system’s responses and the related risk of excessive motions during transport are also discussed in two relevant sea states.