Experimental demonstration of negative refraction with 3D locally resonant acoustic metafluids.

Negative refraction of acoustic waves is demonstrated through underwater experiments conducted at ultrasonic frequencies on a 3D locally resonant acoustic metafluid made of soft porous silicone-rubber micro-beads suspended in a yield-stress fluid. By measuring the refracted angle of the acoustic beam transmitted through this metafluid shaped as a prism, we determine the acoustic index to water according to Snell's law. These experimental data are then compared with an excellent agreement to calculations performed in the framework of Multiple Scattering Theory showing that the emergence of negative refraction depends on the volume fraction [Formula: see text] of the resonant micro-beads. For diluted metafluid ([Formula: see text]), only positive refraction occurs whereas negative refraction is demonstrated over a broad frequency band with concentrated metafluid ([Formula: see text]).

Collapse and cavitation during the drying of water-saturated PDMS sponges with closed porosity.

In this paper, we study the drying of water-saturated porous polydimethylsiloxane (PDMS) elastomers with closed porosity in which the evaporation of water is possible only via the diffusion across PDMS. Starting from water/PDMS emulsions, we fabricate soft macroporous samples with different pore diameter distributions and average diameters ranging from 10 to 300 µm. In these materials, the drying may lead to either a collapsed state with low porosity or the cavitation and reopening of a fraction of the pores. Using optical microscopy and porosity measurements, we showed the influence of the pore diameters and interactions on the result of drying. At pore diameters lower than 30 µm, the majority of pores remain collapsed. We attribute the permanence of the collapse of most small pores to a low probability of cavitation and to the adhesion of the pore walls. Pores with diameters larger than 100 µm reopen via cavitation of the water they contain. The behavior of pores with diameters ranging from 30 to 100 µm depends on the porosity and drying temperature. We also visualize collective cavitation upon the drying of sponges initially saturated with sodium chloride solution. In this case, the cavitation in the largest pores leads to the reopening of small pores in a neighboring zone of the sample. To our knowledge, our results present the first experimental proof of the pore-size-dependent and cooperative nature of the response of soft sponges with closed porosity to drying.

Tuning the sound speed in macroporous polymers with a hard or soft matrix.

In this paper, we investigate the factors affecting the sound speed in air-filled macroporous polymer materials at ultrasound frequencies. Due to the presence of large proportion of gas, these porous materials present high compressibility and, as a consequence, low sound speed which may fall down to values as low as 40 m s-1. Using an emulsion-templating method, we synthesize macroporous samples with similar porous structures but with three different matrices, i.e. a hard poly(styrene-divinylbenzene (DVB)) matrix, a soft epoxy-modified polydimethylsiloxane (PDMS) matrix and a very soft polyaddition PDMS matrix. We characterize the matrix mechanical properties by measuring both the bulk modulus K0 and the shear modulus G0. Next, we compare the sound speed measured in porous samples with porosity varying from 0 to 50%. We show that, in agreement with theoretical predictions, the sound speed is mainly controlled by two parameters, the porosity value and the K0/G0 ratio of the polymer matrix. These parameters may be used to control the sound propagation in porous polymers, which opens the way to the realization of gradient-index materials.

Soft porous silicone rubbers with ultra-low sound speeds in acoustic metamaterials.

Soft porous silicone rubbers are demonstrated to exhibit extremely low sound speeds of tens of m/s for these dense materials, even for low porosities of the order of a few percent. Our ultrasonic experiments show a sudden drop of the longitudinal sound speed with the porosity, while the transverse sound speed remains constant. For such porous elastomeric materials, we propose simple analytical expressions for these two sound speeds, derived in the framework of Kuster and Toksöz, revealing an excellent agreement between the theoretical predictions and the experimental results for both longitudinal and shear waves. Acoustic attenuation measurements also complete the characterization of these soft porous materials.

Binary and ternary recombination of H2D(+) and HD2(+) ions with electrons at 80 K.

The recombination of deuterated trihydrogen cations with electrons has been studied in afterglow plasmas containing mixtures of helium, argon, hydrogen and deuterium. By monitoring the fractional abundances of H3(+), H2D(+), HD2(+) and D3(+) as a function of the [D2]/[H2] ratio using infrared absorption observed in a cavity ring down absorption spectrometer (CRDS), it was possible to deduce effective recombination rate coefficients for H2D(+) and HD2(+) ions at a temperature of 80 K. From pressure dependences of the measured effective recombination rate coefficients the binary and the ternary recombination rate coefficients for both ions have been determined. The inferred binary and ternary recombination rate coefficients are: αbinH2D(80 K) = (7.1 ± 4.2) × 10(-8) cm(3) s(-1), αbinHD2(80 K) = (8.7 ± 2.5) × 10(-8) cm(3) s(-1), KH2D(80 K) = (1.1 ± 0.6) × 10(-25) cm(6) s(-1) and KHD2(80 K) = (1.5 ± 0.4) × 10(-25) cm(6) s(-1).

Tailoring of the porous structure of soft emulsion-templated polymer materials.

This paper discusses the formation of soft porous materials obtained by the polymerization of inverse water-in-silicone (polydimethylsiloxane, PDMS) emulsions. We show that the initial state of the emulsion has a strong impact on the porous structure and properties of the final material. We show that using a surfactant with different solubilities in the emulsion continuous phase (PDMS), it is possible to tune the interaction between emulsion droplets, which leads to materials with either interconnected or isolated pores. These two systems present completely different behavior upon drying, which results in macroporous air-filled materials in the interconnected case and in a collapsed material with low porosity in the second case. Finally, we compare the mechanical and acoustical properties of these two types of bulk polymer monoliths. We also describe the formation of micrometric polymer particles (beads) in these two cases. We show that materials with an interconnected macroporous structure have low mechanical moduli and low sound speed, and are suitable for acoustic applications. The mechanical and acoustical properties of the materials with a collapsed porous structure are similar to those of non-porous silicone, which makes them acoustically inactive.

Incorporation of negatively charged iron oxide nanoparticles in the shell of anionic surfactant-stabilized microbubbles: The effect of NaCl concentration.

We report on the key effect of NaCl for the stabilization of nanoparticle-decorated microbubbles coated by an anionic perfluoroalkylated phosphate C10F21(CH2)2OP(O)(OH)2 surfactant and negatively charged iron oxide nanoparticles. We show that hollow microspheres with shells of 100-200 nm in thickness can be stabilized even at high pH when a strong ionic force is required to screen the negative charges. Due to the more drastic conditions required to stabilize the hollow microspheres, they appear to be stable enough to be deposited on a surface and dried. That can be a simple way to fabricate porous ceramics.

Hollow magnetic microspheres obtained by nanoparticle adsorption on surfactant stabilized microbubbles.

We report on the stabilization of nanoparticle-decorated microbubbles for long periods of time using a synergism between a soluble surfactant and nanoparticles. The soluble surfactant is the perfluoroalkyl phosphate C8F17(CH2)2OP(O)(OH)2 (labeled F8H2Phos) and the nanoparticles (NPs) are 20-25 nm cobalt ferrite (CoFe2O4). The NP-F8H2Phos system has been studied by dynamic light scattering, dynamic magnetic susceptibility measurements and thermal gravimetric analysis. Microbubbles with diameters in the 1-20 µm range have been stabilized in 0.1 M NaCl brine. Its presence is crucial for the long-term stabilization. The surfactant adsorbs rapidly on bubbles and slows down the bubble shrinkage. Thus, the NPs can attach to the bubble and form a hollow sphere with a rigid shell. The charge screening by NaCl favors the attachment of NPs to the bubble surface. The coverage of the bubbles by the CoFe2O4 nanoparticle layer is confirmed by thermally induced inflation-deflation experiments and the control of bubbles with a magnetic field.

Super-elastic air/water interfacial films self-assembled from soluble surfactants.

We show that water-soluble monosodic salts of F-alkyl phosphates C(n)F(2n+1) (CH2)2OP(O)(OH)2, with n=8 and 10 (F8H2Phos and F10H2Phos) form Gibbs films with exceptionally high dilational viscoelastic modules E that reach ~900 mN m(-1) in the condensed phases. These E values are up to one order of magnitude larger than those recorded for phospholipid, protein and polymer films commonly considered as highly viscoelastic. F8H2Phos.1Na undergoes a transition between a liquid-expanded and a liquid-condensed phase. In the case of F10H2Phos.1Na, a transition occurs between a gas phase of surface domains, in which the molecules are densely packed, and a liquid-condensed phase.

pH-controlled microbubble shell formation and stabilization.

We report on microbubbles with a shell self-assembled from an anionic perfluoroalkylated surfactant, perfluorooctyl(ethyl)phosphate (F8H2Phos). Microbubbles were formed and effectively stabilized from aqueous solutions of F8H2Phos at pH 5.6-8.5. This range overlaps the domains of existence of the monosodic and disodic salts. The shell morphology of microbubbles formed spontaneously by heating aqueous solutions of F8H2Phos was monitored during cooling, directly on the microscope's stage. At pH 5.6, the shell collapses through nucleation of folds, as typical for insoluble surfactants. At pH 8.5, no folds were seen during shrinking. At higher pH, the microbubbles rapidly adsorb on the glass. The effect of pH (from 5.6 to 9.7) on adsorption kinetics of F8H2Phos at the air/water interface, and on the elasticity of its Gibbs films, was determined. At low pH, F8H2Phos is highly surface active. The interfacial film undergoes a dilute-to-condensed phase transition and a dramatic increase of elastic module, leading to extremely high values (up to 500 mN m(-1)). At high pH, the surfactant's adsorption is quasi-instantaneous, but interfacial tension lowering is limited, leading to very low elastic module (∼5 mN m(-1)). At pH 5.6 and 8.5, the interfacial tension of F8H2Phos adsorbed on millimetric bubbles and compressed at a rate similar to that exerted on micrometric bubbles during deflation is lower than the equilibrium interfacial tension. Langmuir monolayers of F8H2Phos are highly stable at low pH and feature a liquid expanded/liquid condensed transition; at high pH, they do not withstand compression. Both mono- and disodic F8H2Phos salts are needed to effectively stabilize microbubbles: the rapidly adsorbed disodic salt stabilizes a newly created air/water interface; the more surface active monosodic salt then replaces the more water-soluble disodic salt at the interface. During deflation, the surfactant shell undergoes a transition toward a highly elastic phase, which further contributes to bubble stabilization.