Helium-4 atoms are bosons, with the capability to turn superfluid at very low temperature. Remarkably, this property is conserved even when the thickness of the Helium film is reduced down to few atoms thick only. The study of 2D helium films has led to several breakthrough in condensed matter physics including the study of third sound and topological phase transitions, the latter being rewarded by the 2016 Nobel Prize.In most experimental studies helium was adsorbed on large scale substrates, such as mm2 scale grafoil plates or Mylar. Recent advances in the field of optomechanics and nanomechanics now opens up the possibility to study fluids and superfluids of smaller dimensions.In this talk, we present our recent experiments on helium films probed through the mechanical vibrations of a carbon nanotube. We observed a strong discontinuity in the adsorption of He on the nanotube surface, that we attributed to a layering transition. In addition, the low-temperature dependence of the mechanical mode of the nanotube exhibit a mode softening. Thanks to the tunability of the nanotube resonator, we confirmed the spring nature of this effect and drawn a link with the propagation of third sound in He 2D films.

When cooling Helium-4 atoms at very low temperature the system becomes superfluid, a quantum state of matter. Remarkably, this property is conserved even when the thickness of the Helium film is reduced down to few atoms thick only. The study of 2D helium films has led to several breakthrough in condensed matter physics including the study of third sound and topological phase transitions, the latter being rewarded by the 2016 Nobel Prize.Up to now, most experimental studies focused on large scale substrates, such as mm2 scale grafoil plates or Mylar. Recent advances in the field of optomechanics and nanomechanics now opens up the possibility to study fluids and superfluids of smaller dimensions.I will present our recent experiments on helium films growed on individual carbon nanotube, and probed through the mechanical vibrations of the nanotube. We observed a strong discontinuity in the adsorption of He on the nanotube surface, that we attributed to a layering transition. In addition, the low-temperature dependence of the mechanical mode of the nanotube exhibit a mode softening. Thanks to the tunability of the nanotube resonator, we confirmed the spring nature of this effect and drawn a link with the propagation of third sound in He 2D films.

Gas flow through porous media in the slip flow regime is of great significance for the exploitation of undergroundnatural gas and oil. The gas slip effect in porous media plays a very important role in different engineeringfields, because gas slippage is involved in the determination of gas apparent permeability. In thisstudy, the gas slip effect was investigated in model microporous materials: composite silica aerogels. We measuredgas and water permeability in sets of nano composite silica aerogels, fractal materials with a high porevolume and tortuosity, and low permeability. The observed difference in gas and water permeability was analyzedfrom the point of view of the slip regime (Klinkenberg correction), b) and transition regime (Knudsencorrection). The effect of structural parameters of porous media (pore volume, tortuosity, fractal features) on theKlinkenberg and Knudsen corrections are discussed and the different models proposed in the literature aretested. Experimental results showed that: (1) gas permeability is almost two orders of magnitude greater thanwater permeability, (2) Klinkenberg and Knudsen corrections increase with decreasing permeability, (3) theKnudsen corrections calculated from the literature models were almost one order of magnitude lower thanexperimental data, (4) we also tested the Klinkenberg approach for tortuous and fractal porous media, the fractalmodel with second-order slip improved the accuracy of the prediction and agree with the experimental results.

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Structure par diffusion de neutrons aux petits angles