dsorption in Silica A erogelnear the Liquid-Vapor C riticalPoint

W ehaveinvestigated theadsorption and desorption ofhelium nearitsliquid-vaporcriticalpointin silica aerogelswith porositiesbetween 95% and 98% .W e used a capacitive m easurem enttechnique which allowed usto probe the helium density inside the aerogeldirectly,even though the sam ples were surrounded by bulk helium . The aerogel’s very low therm alconductivity resulted in long equilibration tim esso wem onitored thepressureand thehelium density,both insidetheaerogeland in the surrounding bulk,and waited ateach pointuntilallhad stabilized.O urm easurem entswere m ade attem peraturesfarfrom thecriticalpoint,where a wellde ned liquid-vaporinterface exists, and at tem peratures up to the bulk criticalpoint. Hysteresis between adsorption and desorption isotherm s persisted to tem peratures close to the liquid-vapor criticalpoint and there was no sign ofan equilibrium liquid-vapor transition once the hysteresis disappeared. M any features ofour isotherm s can be described in term s ofcapillary condensation,although this picture becom es less applicable astheliquid-vaporcriticalpointisapproached and itisunclearhow itcan beapplied to aerogels,whose tenuousstructure includesa wide range oflength scales.