A biaxial apparatus for the study of heterogeneous and intermittent strains in granular materials

We present an experimental apparatus specifically designed to investigate the precursors of failure in granular materials. A sample of granular material is placed between a latex membrane and a glass plate. A confining effective pressure is applied by applying vacuum to the sample. Displacement-controlled compression is applied in the vertical direction, while the specimen deforms in plane strain. A Diffusing Wave Spectroscopy visualization setup gives access to the measurement of deformations near the glass plate. After describing the different parts of this experimental setup, we present a demonstration experiment where extremely small (of order $10^{-5}$) heterogeneous strains are measured during the loading process

Synchronized diffusive-wave spectroscopy: Principle and application to sound propagation in aqueous foams

International audienceWe present an experimental method to measure oscillatory strains in turbidmaterial. Thematerial is illuminatedwith a laser, and the speckle patterns are recorded. The analysis of the deformations of the optical path lengthshows that the speckle patterns are modulated at the strain frequency. By recording those patterns synchronouslywith the strain source, we are able to measure the amplitude and the phase of the strain. This method is testedin the specific case of an aqueous foam where an acoustic wave propagates. The effects of material internaldynamics and heterogeneous deformations are also discussed

Sound propagation in liquid foams: Unraveling the balance between physical and chemical parameters

International audienceWe present experimental results on the propagation of an ultrasonic wave (40 kHz) in liquid foams, as a function of the foam physical and chemical parameters. We have first implemented an original setup, using transducers in a transmission configuration. The foam coarsening was used to vary the bubble size (remaining in the sub-millimeter range), and we have made foams with various chemical formulations, to investigate the role of the chemicals at the bubble interfaces or in bulk. The results are compared with recently published theoretical works, and good agreement are found. In particular, for all the foams, we have evidenced two asymptotic limits, at small and large bubble size, connected by a non-trivial resonant behavior, associated to an effective negative density. These qualitative features are robust whatever the chemical formulation; we discuss the observed differences between the samples, in relation to the interfacial and bulk viscoelasticity. These results demonstrate the rich and complex acoustic behavior of foams. While the bubble size remain here always smaller than the sound wavelength, it turns out that one must go well beyond mean-field modeling to describe the foam acoustic properties

Stroboscopic Diffusive Wave Spectroscopy as a probe of acoustic deformations in complex systems

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Data from: Water vapor transport across an arid sand surface - non-linear thermal coupling, wind-driven pore advection, subsurface waves, and exchange with the atmospheric boundary layer

These files contain data and other outputs supporting all results reported in Louge et al. Water vapor transport across an arid sand surface - non-linear thermal coupling, wind-driven pore advection, subsurface waves, and exchange with the atmospheric boundary layer. In Louge et al., we found: Deserts inhale and exhale water vapor through their surface. Although this process affects the water balance over vast sand seas, it is poorly understood for want of sensitive instruments. We discover how it operates using a new probe that detects tiny amounts of moisture on sand grains. Our analysis reveals that vapor infiltration is considerably slower in dry sand, and that wind flowing over a dune creates weak internal air currents contributing to the transport of moisture. Their strength depends on dune location, wind speed and direction. When wind is strong enough to let dry sand meander over a dune, the resulting rapid variation in surface moisture sends evanescent waves of humidity downward. An analysis of these waves implies that water evaporation from individual sand grains behaves like a slow chemical reaction. The exchange of moisture with the atmosphere is not always driven by the difference between humidity at the dune surface and in the ambient, as current models assume, and it is weaker than they predict. In future, the new probe can be used as ground truth to calibrate satellite observations over deserts, explore extra-terrestrial environments holding scant water, and detect moisture contamination in pharmaceutical products.This paper was made possible by the support of NPRP grants 09-546-2-206 and 6-059-2-023 from the Qatar National Research Fund, and by a Qatar Foundation Research Excellence Award

Water Vapor Transport Across an Arid Sand Surface-Non-Linear Thermal Coupling, Wind-Driven Pore Advection, Subsurface Waves, and Exchange With the Atmospheric Boundary Layer

International audienceAlthough vapor exchanged across hyper-arid surfaces without free liquid affects the water budget of sand seas, its mechanism is poorly documented for want of accurate instruments with fine spatial resolution. To rectify this, we report bulk density profiles and spatiotemporal variations of vapor mass fraction just below the surface of a mobile dune, acquired with a multi-sensor capacitance probe sensitive to tiny water films adsorbed on sand grains. We also record wind speed and direction, ambient temperature and relative humidity, net radiation flux, and subsurface temperature profiles over 2 days. The data validate a non-linear model of vapor mass fraction. Unlike heat, which conducts through grains, vapor percolates across the interstitial pore space by advection and diffusion. On time scales longer than evaporation, adsorbed films equilibrate with their surroundings and hinder molecular diffusion. Their non-linear coupling with subsurface temperature generates inflections in vapor profiles without counterpart in simpler diffusive systems. Pore advection arises as wind induces subtle pressure variations over the topography. During periods of aeolian transport, flowing sand dehydrates the surface intermittently, triggering evanescent vapor waves of amplitude decaying exponentially downward on a characteristic length implying an adsorption rate governed by a kinetic-limited activated process. Finally, the probe yields diffusive and advective exchanges with the atmospheric boundary layer. During the day, their combined flux is smaller than expected, yet nearly proportional to the difference between vapor mass fraction at the surface and aloft. Under stabler stratification at night, or during aeolian sand transport, this relation no longer holds

Mild head injury and attention deficit hyperactivity disorder in children

International audiencePurpose - Post-concussion syndrome is a well-described complication following moderate and severe head trauma but whether it occurs after mild head injury in children remains unclear. The aim of this study was to evaluate whether exposure to mild head injury with potential additional risk factors (non-surgical lesion on computed tomographic, high kinetic trauma, or Glasgow Coma Scale <15) is associated with attention deficit hyperactivity disorder (ADHD) after the head trauma. Methods - This study was performed in an emergency department on children admitted between 2009 and 2013. It compared victims of mild head injury aged 6-16 years with matched children presenting isolated non-surgical forearm fracture (ratio1/2). ADHD was assessed using Conners' Global Index-Parent short version 3-40 months after the trauma. The patients were compared using chi-square test or Fisher's exact test, t test or u-test as appropriate with a p value set at 0.05. Results - During the study period, 676 patients were admitted for mild head injury. Among them, 34 (5 %) fulfilled the inclusion criteria and were compared with 64 matched patients admitted for a forearm fracture. The groups were comparable. ADHD was observed in both groups (18 % in the mild head injury group, 11 % in the control group) with no significant differences between groups. The prevalence was high when compared to an expected frequency of 3.5-5.6 % in children aged 6-12 years in the general population. Conclusions - These results suggest that pre-existing ADHD may have contributed to injury proneness in both groups and does not argue for a specific risk of ADHD induced by mild head injury. The diagnosis of ADHD should be evoked at admission of children aged 6-16 years presenting with a trauma

Structure of liquid films of an ordered foam confined in a narrow channel.

International audienceA bamboo foam is the simplest case of an ordered foam confined in a narrow channel. It is made of a regular film distribution, arranged perpendicularly to the channel. Our work consists of studying the structural properties of several films taken in a drained foam. X-ray experiments highlighted the equality of the equilibrium thickness for each film within a foam. The same thickness was found as by measurements of disjoining pressure isotherms, proving as well that films of a bamboo foam behave like isolated ones. The refinement of X-ray data by a simple model of specular reflectivity showed a significant variation of the electronic distribution of the surfactant layer for a common black film forwarding from one equilibrium state to another. A discussion on the organization of the surfactant molecules to the gas/liquid interface and film is proposed

Cascading photoinduced, elastic, and thermal switching of spin states triggered by a femtosecond laser pulse in an Fe(III) molecular crystal

International audienceIn this paper we put on solid ground the optical time-resolved study of spin-state photoinduced switching in an Fe(III) compound whose structural dynamics have been recently unveiled. We provide the experimental evidence of complex dynamics, occurring in succession, and spanning ten decades in time. We show that in addition to the ultrafast photoinduced molecular switching occurring on the subpicosecond timescale, there exists two additional switching processes significantly affecting the fate of the macroscopic material: one driven by elastic interactions as volume expansion occurs on a tens of nanoseconds timescale, and another one associated with heat diffusion and thermal switching taking place on a tens of microseconds timescale