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Etude optique de la transition liquide-gaz de l'hélium
confiné dans les aérogels de silice.

Abstract : The present experimental work intends to study by optical means the liquid-gas transition of helium confined in silica aerogels. Previous studies, based on adsorption isotherms, of helium in aerogels led to contradictory conclusions. On the one hand, a genuine phase separation on a pressure plateau below the bulk vapor pressure (Psat), involving two phases respectively heavier and lighter than the bulk gas and liquid. On the other hand, the existence of a pressure hysteresis between adsorption and desorption combined with a finite compressibility was more in accordance with a traditionnal capillary condensation phenomenon. In this case, the low value of the `liquid' density is seen as resulting from a partial filling of the sample at Psat. The aim of optical observation is to determine which mecanism occurs in the helium-aerogel system. In the present work, we describe the construction of the optical cryostat and the apparatus. The interest of such apparatus is to combine simultaneously the measurement of thermodynamics parameters, essential for the adsorption/desorption isotherms, with the confined helium's optical signal scattered at several angles. The study has been performed on two different samples. They have the same porosity but their microstructures are different. They exhibit a fractal structure from few nanometers up to their two different gel correlation lengths (20 and 50~nm). From our measurements we conclude to the existence of hysteresis between the filling and the emptying of the aerogels. This is surprisingly consistant with a capillary condensation model of porous media with cylindrical pores with a narrow diameter distribution centered on the gel correlation length. These observations can also be compared to another new and different theoretical approche where the disorder, and not its geometry, is the key factor. Both the measure of the density of helium confined and the optical observations lead to the conclusion that when the pressure reaches the bulk vapor pressure, the aerogel is homogeneously filled with helium denser than the bulk liquid. The optical signal scattered at different angles reveals the existence of scattering domains correlated over distances larger than the gel correlation length. These domains appear and grow only on the hysteresis cycles, when the compressibility is high enough. Approximating these domains as spherical, we estimate their maximum size, just before the aerogel is completly filled, between 100 and 400~nm.
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Contributor : Pierre-Etienne Wolf <>
Submitted on : Wednesday, January 16, 2008 - 2:09:51 PM
Last modification on : Friday, December 18, 2020 - 1:34:03 PM
Long-term archiving on: : Tuesday, April 13, 2010 - 9:07:11 PM

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  • HAL Id : tel-00206043, version 1

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CNRS | UJF | UGA

Citation

Thierry Lambert. Etude optique de la transition liquide-gaz de l'hélium
confiné dans les aérogels de silice.. Analyse de données, Statistiques et Probabilités [physics.data-an]. Université Joseph-Fourier - Grenoble I, 2004. Français. ⟨tel-00206043⟩

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