Strengthening of foamed composite materials

We investigate the shear elastic modulus of soft polymer foams loaded with hard spherical particles and we show that, for constant bubble size and gas volume fraction, strengthening is strongly dependent on the size of those inclusions. Through an accurate control of the ratio $\lambda$ that compares the particle size to the thickness of the struts in the foam structure, we evidence a transition in the mechanical behavior at $\lambda \approx 1$. For $\lambda < 1$, every particle loading leads to a strengthening effect whose magnitude depends only on the particle volume fraction. On the contrary, for $\lambda > 1$, the strengthening effect weakens abruptly as a function of $\lambda$ and a softening effect is even observed for $\lambda \gtrsim 10$. This transition in the mechanical behavior is reminiscent of the so-called "particle exclusion transition" that has been recently reported within the framework of drainage of foamy granular suspensions [Haffner B, Khidas Y, Pitois O. The drainage of foamy granular suspensions. J Colloid Interface Sci 2015. In Press.]. It involves the evolution for the geometrical configuration of the particles with respect to the foam network, and it appears to control the mechanics of such foamy systems

Capture-induced transition in foamy suspensions

International audienceWe investigate the drainage behaviour of foamy granular suspensions. Results reveal large fluctuations in the drainage velocity as bubble size, particle size and gas volume fraction are varied for a given particle volume fraction. Particle capture is proved to control the overall drainage behaviour through the parameter lambda, which compares the particle size to the size of passage through constrictions within the foam pore space. lambda highlights a sharp transition: for lambda 1 particles are trapped and the resulting drainage velocity is strongly reduced. A phenomenological model is proposed to describe this behaviour

The drainage of foamy granular suspensions

International audienceFoam-based materials are promising micro-structured materials with interesting thermal and acoustical properties. The control of the material morphology requires counteracting all the destabilizing mechanisms during their production, starting with the drainage process, which remains to be understood in the case of the complex fluids that are commonly used to be foamed. Here we perform measurements for the drainage velocity of aqueous foams made with granular suspensions of hydrophilic monodisperse particles and we show that the effect of particles can be accounted by two parameters: the volume fraction of particles in the suspension (φ_p) and the confinement parameter (λ), that compares the particle size to the size of passage through constrictions in the foam network. We report data over wide ranges for those two parameters and we identify all the regimes and transitions occurring in the φ_p-λ diagram. In particular, we highlight a transition which refers to the included / excluded configuration of the particles with respect to the foam network, and makes the drainage velocity evolve from its minimal value (fully included particles) to its maximal one (fully excluded particles). We also determine the conditions (φ_p,λ) leading to the arrest of the drainage process

Stabilité des suspensions fortement aérées

We study the drainage of granular suspensions foams. Our control parameters are the gas fraction, the bubble size, the particles size and the interstitial particle fraction. First, we measure the proportion of liquid and particles retained in the foam network as function of the above mentioned parameters. These measurements are performed when the drainage is over, they are essential for the description of drainage velocity. We show that certain combinations of our study parameters lead to the jamming of the three-phase system : gas, liquid, solid. Secondly, we highlight different regimes of drainage velocity, we show that is controlled by two parameters : (i) lambda, the ratio of the particle size and constriction size, (ii) the fraction of particles in the interstitial network : phi. The key to understand these regimes is the trapping of particles in the foam : (i) the jamming, which may occur for surprisingly low fractions due to the geometry of the pore network, (ii) the particles captured by the foam network when they become larger than the constrictions network. Finally, larger particles excluded from the network increase the drainage velocity, as a consequence the minimum for the velocity corresponds to the individual capture. The granular fraction of the suspension in the foam network is the other key parameter. Especially, the drainage can be stopped for sufficiently high fractions for certain values of lambda. This work offers promising outlook for the stability of three-phase materialsNous étudions le drainage des mousses de suspension granulaire. Nos paramètres de contrôle sont : la fraction en gaz, la taille des bulles, celle des particules et la fraction volumique de celles-ci dans la phase interstitielle. En premier lieu, nous mesurons la proportion de liquide et de particules retenus dans le réseau de la mousse en fonction des paramètres cités précédemment. Ces mesures réalisées une fois le drainage terminé apportent des éléments de compréhension indispensables à la description des vitesses de drainage. Nous montrons également que certaines combinaisons de nos paramètres d'étude conduisent au blocage du système gaz, liquide, solide. Dans un second temps, nous avons identifié différents régimes de cinétique de drainage, nous montrons qu'ils sont contrôlés par deux paramètres : (i) le rapport lambda de la taille des particules et de la taille des constrictions du réseau, (ii) la fraction en particules dans la phase interstitielle phi. Le point clé pour comprendre ces régimes est le piégeage des particules dans la mousse qui peut avoir deux origines : (i) par piégeage collectif (jamming) qui peut survenir pour des fractions étonnamment basses à cause de la géométrie du réseau interstitiel, (ii) la capture individuelle des particules par la mousse lorsque leur taille devient supérieure à celle des constrictions du réseau interstitiel. Des particules encore plus grosses sont exclues du réseau et participent à une remontée de la vitesse de drainage, faisant apparaître un minimum pour le régime correspondant à la capture individuelle. La fraction granulaire de la phase interstitielle est aussi essentielle, le drainage pouvant être stoppé pour des fractions suffisamment élevées lorsque lambda est judicieusement choisi. Ce travail propose des pistes prometteuses pour la stabilité des matériaux triphasique

Ionized Gas in the Smith Cloud

We present WHAM observations of Halpha, [N II], and [S II] in the Smith Cloud. A map of Halpha emission from the cloud shows ionized gas coincident with the brightest H I emission, but nearly-as-bright Halpha in some regions with faint H I. The ionized mass of the cloud is at least as large as the neutral mass, > 10^6 M_sun. Ionized gas in the core of the Smith Cloud has an electron temperature 6000 K < T < 16000 K. The observed ratio [N II] / Halpha = 0.39 \pm 0.09 shows that the cloud has a non-primordial nitrogen abundance, 0.1 - 1 times solar.Comment: 4 pages, 2 figures. To appear in the proceedings of "The Role of Disk-Halo Interaction in Galaxy Evolution: Outflow vs Infall?", EAS Publication Serie

Photoionization of High Altitude Gas in a Supernova-Driven Turbulent Interstellar Medium

We investigate models for the photoionization of the widespread diffuse ionized gas in galaxies. In particular we address the long standing question of the penetration of Lyman continuum photons from sources close to the galactic midplane to large heights in the galactic halo. We find that recent hydrodynamical simulations of a supernova-driven interstellar medium have low density paths and voids that allow for ionizing photons from midplane OB stars to reach and ionize gas many kiloparsecs above the midplane. We find ionizing fluxes throughout our simulation grids are larger than predicted by one dimensional slab models, thus allowing for photoionization by O stars of low altitude neutral clouds in the Galaxy that are also detected in Halpha. In previous studies of such clouds the photoionization scenario had been rejected and the Halpha had been attributed to enhanced cosmic ray ionization or scattered light from midplane H II regions. We do find that the emission measure distributions in our simulations are wider than those derived from Halpha observations in the Milky Way. In addition, the horizontally averaged height dependence of the gas density in the hydrodynamical models is lower than inferred in the Galaxy. These discrepancies are likely due to the absence of magnetic fields in the hydrodynamic simulations and we discuss how magnetohydrodynamic effects may reconcile models and observations. Nevertheless, we anticipate that the inclusion of magnetic fields in the dynamical simulations will not alter our primary finding that midplane OB stars are capable of producing high altitude diffuse ionized gas in a realistic three-dimensional interstellar medium.Comment: ApJ accepted. 17 pages, 7 figure

Warm Ionized Medium throughout the Sagittarius–Carina Arm

Wisconsin H-Alpha Mapper (WHAM) observations of H-Alpha and [S II]$\lambda6716$ emission are used to trace the vertical distribution and physical conditions of the warm ionized medium (WIM) along the Sagittarius-Carina arm. CO emission, tracing cold molecular gas in the plane of the Galaxy, is used as a guide to isolate H-Alpha and [S II] emission along individual spiral arms. Exponential scale heights of electron density squared (or emission measure) are determined using H-Alpha emission above (below) the midplane to be $330 \pm 80$ pc ( $550 \pm 230$ pc) along the near Sagittarius arm, $300 \pm 100$ pc ($250 \pm 30$ pc) along the near Carina arm, and $>1000$ pc along the far Carina arm. The emission measure scale height tends to increase as a function of Galactocentric radius along the Sagittarius-Carina arm for $R_G > 8$ kpc. Physical conditions of the ionized gas are analyzed using the [S II]/H-Alpha line ratio, which more closely traces H-Alpha Intensity than height above the plane, z, suggesting a stronger relationship with the in-situ electron density. We interpret this result as further evidence for the majority of the observed diffuse emission originating from in-situ ionized gas as opposed to scattered light from classical H II regions in the plane.Comment: 18 pages, 19 figures, 2 tables, accepted for publication in Ap

The Turbulent Warm Ionized Medium: Emission Measure Distribution and MHD Simulations

We present an analysis of the distribution of H-alpha emission measures for the warm ionized medium (WIM) of the Galaxy using data from the Wisconsin H-Alpha Mapper (WHAM) Northern Sky Survey. Our sample is restricted to Galactic latitudes |b| > 10. We removed sightlines intersecting nineteen high-latititude classical H II regions, leaving only sightlines that sample the diffuse WIM. The distribution of EM sin |b| for the full sample is poorly characterized by a single normal distribution, but is extraordinarily well fit by a lognormal distribution, with = 0.146 +/- 0.001 and standard deviation 0.190 +/- 0.001. drops from 0.260 +/- 0.002 at Galactic latitude 10<|b|<30 to 0.038 +/- 0.002 at Galactic latitude 60<|b|<90. The distribution may widen slightly at low Galactic latitude. We compare the observed EM distribution function to the predictions of three-dimensional magnetohydrodynamic simulations of isothermal turbulence within a non-stratified interstellar medium. We find that the distribution of EM sin |b| is well described by models of mildy supersonic turbulence with a sonic Mach number of ~1.4-2.4. The distribution is weakly sensitive to the magnetic field strength. The model also successfully predicts the distribution of dispersion measures of pulsars and H-alpha line profiles. In the best fitting model, the turbulent WIM occupies a vertical path length of 400-500 pc within the 1.0-1.8 kpc scale height of the layer. The WIM gas has a lognormal distribution of densities with a most probable electron density n_{pk} = 0.03 cm^{-3}. We also discuss the implications of these results for interpreting the filling factor, the power requirement, and the magnetic field of the WIM.Comment: 16 pages, 13 figures, ApJ in press. Replacement reflects version accepted for publicatio