HAL Portal ESPCI (Ecole Supérieure de Physique et de Chimie Industrielles)
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Nonmonotonic Radiative Heat Transfer in the Transition from Far Field to Near Field
No full textInternational audienc
Emergence of wind ripples controlled by mechanics of grain–bed impacts
No full textInternational audiencePeriodic sediment patterns have been observed on Earth in riverbeds and sand and snow deserts, but also in other planetary environments. One of the most ubiquitous patterns, familiar wind or 'impact' ripples, adorns sand beaches and arid regions on Earth. The observation of aeolian impact ripples on Mars the same size as their terrestrial counterparts despite a thinner atmosphere raises questions about their formation. Here we show in a numerical simulation that the emergent wavelength of impact ripples is controlled by the mechanics of grain-bed impacts and not the characteristic trajectories of grains above the bed. We find that the distribution of grain trajectories in transport is essentially scale-free, invoking the proximity of a critical point and precluding a transport-related length scale that selects ripple wavelengths. By contrast, when a grain strikes the bed, the process leading to grain ejections introduces a collective granular length scale that determines the scale of the ripples. We propose a theoretical model that predicts a relatively constant ripple size for most planetary conditions. In addition, our model predicts that for high-density atmospheres, such as on Venus, or for sufficiently large sand grains on Earth, impact ripples propagate upwind.Although wind-tunnel and field experiments are needed to confirm the existence of such 'antiripples', we suggest that our quantitative model of wind-blown sediment transport may be used to deduce geological and environmental conditions on other planets from the sizes and propagation speeds of impact ripples.</p
Three-dimensional deformations in single-layer antimonene and interaction with a Au(111) surface from first principles
No full textUsing density functional theory, we investigate the electronic structure of the alpha phase of an antimony monolayer in its isolated form and in contact to the (111) surface of gold. We demonstrate that the isolated single-layer actually displays a slightly modulated puckering that stabilizes the monolayer, not a uniform one as often assumed. Moreover, it has dramatic consequences on the electronic band structure: the material is a semiconductor with low-dispersing bands near the Brillouin zone center. By further application of about 12% strain on the armchair direction, a double-cone features develops wherein an electronic bandgap of about 21~meV is found. When in contact with a Au(111) surface, a strong interaction with gold arises, as it appears clearly from (i) substantial atomic displacements compared to the isolated form, and (ii) hybridization of Sb and Au orbitals. The latter profoundly modifies the electronic band structure by strengthening the spin-orbit splitting of hybridized bands and spoiling the double-cone feature whose manipulation through substrate-induced strain appears therefore questionable, at least in the simulated epitaxial implementation
A travelling-wave strategy for plant–fungal trade
No full textInternational audienceAbstract For nearly 450 million years, mycorrhizal fungi have constructed networks to collect and trade nutrient resources with plant roots 1,2 . Owing to their dependence on host-derived carbon, these fungi face conflicting trade-offs in building networks that balance construction costs against geographical coverage and long-distance resource transport to and from roots 3 . How they navigate these design challenges is unclear 4 . Here, to monitor the construction of living trade networks, we built a custom-designed robot for high-throughput time-lapse imaging that could track over 500,000 fungal nodes simultaneously. We then measured around 100,000 cytoplasmic flow trajectories inside the networks. We found that mycorrhizal fungi build networks as self-regulating travelling waves—pulses of growing tips pull an expanding wave of nutrient-absorbing mycelium, the density of which is self-regulated by fusion. This design offers a solution to conflicting trade demands because relatively small carbon investments fuel fungal range expansions beyond nutrient-depletion zones, fostering exploration for plant partners and nutrients. Over time, networks maintained highly constant transport efficiencies back to roots, while simultaneously adding loops that shorten paths to potential new trade partners. Fungi further enhance transport flux by both widening hyphal tubes and driving faster flows along ‘trunk routes’ of the network 5 . Our findings provide evidence that symbiotic fungi control network-level structure and flows to meet trade demands, and illuminate the design principles of a symbiotic supply-chain network shaped by millions of years of natural selection
“Magnetic Marshmallows” for Soft Robotics: Magneto-Mechanical Characterization and Application in Switchable Adhesion Structures
No full textInternational audienceSoft magnetic composites exhibit fast and programmable macroscopic deformations in magnetic fields, making them promising for applications in soft untethered robotics or in designing surfaces with switchable adhesion or wetting properties. However, due to the incompressible nature of soft, non-porous actuators, their compression or elongation leads to important shape change due to lateral expansion or compression, respectively. In practice, bending and folding remain preferred actuation modes. Here, we explore the potential of magnetic elastomer “marshmallows” as compressible actuators with low Poisson’s ratio. Using a sacrificial salt pellet template method, we fabricate polydimethylsiloxane foams with open porosity filled with carbonyl iron particles. The obtained foams exhibit strong reversible compression under the influence of a magnetic field gradient. We reveal the significance of stress accumulation in the direction of the field gradient due to magnetic body forces and the key role of the foam thickness in magnetic strain. We propose a simple analytical model based on the action of a magnetic body force that explains these effects. Finally, we demonstrate the development of a novel switchable adhesion structure, in which the magnetic foam covered with a pressure-sensitive adhesive serves as a compressible actuator able to switch between adhesive and non-adhesive states
CORRELATING FORMULATION AND PERFORMANCE IN ALL-ACRYLIC WATERBORNE PRESSURE SENSITIVE ADHESIVE LATEXES
No full textInternational audienceThis work investigates the correlation between the formulation of waterborne acrylic pressuresensitive adhesives (PSAs) and their resulting adhesive performance. A library of n-butyl acrylate-based latexes was synthesized via miniemulsion polymerization. Controlled variations in the composition of these polyacrylates -such as the inclusion of rigid comonomers, latent crosslinkers and chain transfer agents -enabled precise control over key network structures in the particles leading then to a range of physical parameters in the cast PSA films such as the glass transition temperature, entanglement and crosslink density. The viscoelastic and mechanical behaviors of the resulting networks were carefully characterized using a combination of techniques, including linear rheology, dynamic mechanical analysis, and tensile testing. These properties were then correlated with adhesive performance, as measured by probe tack and shear holding time tests. This comprehensive approach allowed for the identification of the optimal formulation balance that delivers the best compromises of adhesive performance, directly correlating these outcomes with the macromolecular design
Steady-state neuron-predominant LINE-1 encoded ORF1p protein and LINE-1 RNA increase with aging in the mouse and human brain
No full textInternational audienceRecent studies have established a reciprocal causal link between aging and the activation of transposable elements, characterized in particular by a de-repression of LINE-1 retrotransposons. These LINE-1 elements represent 21% of the human genome, but only a minority of these sequences retain the coding potential essential for their mobility. LINE-1 encoded proteins can induce cell toxicity implicated in aging and neurodegenerative diseases. However, our knowledge of the expression and localization of LINE-1-encoded proteins in the central nervous system is limited. Using a novel approach combining atlas-based brain mapping with deep-learning algorithms on large-scale pyramidal brain images, we unveil a heterogeneous, neuron-predominant and widespread ORF1p expression throughout the murine brain at steady-state. In aged mice, ORF1p expression increases significantly which is corroborated in human post-mortem dopaminergic neurons by an increase in young LINE-1 elements including those with open reading frames. Mass spectrometry analysis of endogenous mouse ORF1p revealed novel, neuron-specific protein interactors. These findings contribute to a comprehensive description of the dynamics of LINE-1 and ORF1p expression in the brain at steady-state and in aging and provide insights on ORF1p protein interactions in the brain
Enhancing the Infrared Emission from Silver Chalcogenide Quantum Dots through Microcavity Coupling
No full textInternational audienceNanocrystals (NCs) appear as a promising platform for cost effective infrared optoelectronics, while offering a simplified coupling to the CMOS platform. However, this perspective is slowed down by toxicity concerns, the most effective materials being based on Pb and Hg. There is currently a large effort to bring forth new platforms that are active in the infrared with a reduced heavy metal content. Here, we focus on silver chalcogenides, which, thanks to a combination of inter-and intraband transitions, appear quite suited to cover both short and mid wave infrared ranges. However, this material being less mature, the achieved photoluminescence (PL) appears quite broad which is problematic for device integration. Here, we demonstrate that a strong control over the PL spectrum can be obtained through integration into a dielectric cavity which magnifies the electric field by a factor 20 and narrows the PL full width at half maximum down to 15 nm for an emission at telecom wavelength. The PL is also highly directional to avoid most waveguiding effects which is of utmost interest for enhancing the efficiency of NC based LEDs
Swelling and evaporation determine surface morphology of grafted hydrogel thin films
No full textInternational audienceWe experimentally study the formation of surface patterns in grafted hydrogel films of nanometer-to-micrometer thicknesses during imbibition-driven swelling followed by evaporation- driven shrinking. Creases are known to form at the hydrogel surface during swelling; the wavelength of the creasing pattern is proportional to the initial thickness of the hydrogel film with a logarithmic correction that depends on microscopic properties of the hydrogel. We find that, although the characteristic wavelength of the pattern is determined during swelling, the surface morphology can be significantly influenced by evaporation-induced shrinking. We observe that the elastocapillary length based on swollen mechanical properties gives a threshold thickness for a surface pattern formation, and consequently an important change in morphology
Acceleration of acoustic aggregation of particles in acoustic levitation by collective effects. Application to cost-effective ultrasonic harvesting of microalgae
No full textInternational audienceAcoustic aggregation of particles in acoustic levitation is a well-known phenomenon. However, it is only well described in the dilute regime, i.e. when particles do not interact with each other. In this study, we first demonstrate that the acoustic focusing speed can be significantly increased when dealing with particle aggregates. These results suggest that acoustic aggregation can be more efficient when dealing with relatively dense suspensions. This principle is successfully applied to microalgae suspensions. It is shown that microalgae harvesting can be accelerated without power increase using acoustically induced flocculation if the initial concentration is sufficiently high. This demonstrates that a cost-effective industrial process for acoustic microalgae harvesting is possible