Des travaux récents ont mis en évidence la très forte solubilité de l’hélium dans la silice vitreuse à haute pression : jusqu’à 1 molécule d’hélium par molécule de SiO2 à la pression de 5 GPa. Ces concentrations élevées inattendues remettent en question les mécanismes, acceptés jusqu’à maintenant, d’adsorption physique des gaz dans le volume libre du réseau vitreux. Je présenterai ici les résultats d’une étude, par diffusion Brillouin de la lumière, de l’élasticité de la silice sous pression hydrostatique des gaz : hélium, néon et argon. Nous montrons que le module de compression de v-SiO2 dépend du fluide de pressurisation et qu’il diffère de celui obtenu par des méthodes statiques [1]. Dans le cas de l’hélium et du néon, nous mettons en évidence la déformation volumique macroscopique associée à l’adsorption du fluide au sein de la structure vitreuse. Nous montrerons que ce comportement peut être décrit par un modèle de poroélasticité généralisée

Below 10 GPa, hydrostatic pressures P are usually obtained in diamond-anvil cell with soft pressure transmitting medium such as methanol-ethanol mixture. When such experiments are performed in this P range for v-SiO2, the volumetric strain of the sample agrees with that expected from its bulk modulus, . It was revealed recently that v-SiO2 submitted to high pressures under He atmosphere exhibits surprisingly small relative volume changes leading to an apparent B greater than hundred GPa at ambient P, much higher than the well-known modulus GPa of silica. Using Brillouin light scattering we evidence that the elastic moduli are only weakly affected by the presence of He. The difference between the static and the acoustic measurements reveals that silica pressurized under He behaves as an open system allowing He to penetrate into the interstitial free volume of the network. In that case, the static measurement underestimates the compressibility by a term due to the gas charging the sample. The large difference found here shows that He continues penetrating v-SiO2 up to our highest investigated P, i.e. 6 GPa. Such a strong effect of the pressurizing medium on the static compressibility is usually observed in materials with open porosity, such as zeolites. It results from pore filling by molecules of the pressurizing fluid. An estimate of the amount of He entering v-SiO2 is derived from the measured refractive index, leading to the unexpected value of more than 1 mole of He per mole of SiO2 at 6 GPa [1]. Finally we present molecular simulations relating the amount of fluid adsorbed to the volumetric strain of the network. We show that a generalized poromechanical approach can be applied to silica glass in which the free volume exists only at the sub-nanometric length scale. In that picture, the small apparent compressibility of silica under high He or Ne pressure results from an adsorption-induced expansion [2].[1] C. Weigel, A. Polian, M. Kint, B. Rufflé, M. Foret, R. Vacher,, “Vitreous Silica Distends in Helium Gas: Acoustic Versus Static Compressibilies” Phys. Rev. Lett., 109, 245504 (2012).[2] B. Coasne, C. Weigel, A. Polian, M. Kint, J. Rouquette, J. Haines, M. Foret, R. Vacher, B. Rufflé, “The Startling Poroelastic Behavior of Vitreous Silica” submitted (2014).

Nonequilibrium glass transitionsin active matter

Thinning or thickening? Complexrheology of dense suspensions

Thermodynamic fluctuations inmodel glasses

Dense assemblies of active andliving particles