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We have investigated the adsorption and desorption of helium near itsliquid-vapor critical point in silica aerogels with porosities between 95% and98%. We used a capacitive measurement technique which allowed us to probe thehelium density inside the aerogel directly, even though the samples weresurrounded by bulk helium. The aerogel's very low thermal conductivity resultedin long equilibration times so we monitored the pressure and the heliumdensity, both inside the aerogel and in the surrounding bulk, and waited ateach point until all had stabilized. Our measurements were made at temperaturesfar from the critical point, where a well defined liquid-vapor interfaceexists, and at temperatures up to the bulk critical point. Hysteresis betweenadsorption and desorption isotherms persisted to temperatures close to theliquid-vapor critical point and there was no sign of an equilibriumliquid-vapor transition once the hysteresis disappeared. Many features of ourisotherms can be described in terms of capillary condensation, although thispicture becomes less applicable as the liquid-vapor critical point isapproached and it is unclear how it can be applied to aerogels, whose tenuousstructure includes a wide range of length scales.Comment: 15 Pages, 17 Figure

Helium adsorption in silica aerogel near the liquid-vapor critical point