Capillary condensation and hysteresis for 4He in aerogel

When fluids are adsorbed in a porous medium, confinement and disorder can dramatically affect their critical behavior. Silica aerogels, with their high porosities and tenuous structures, provide an opportunity to study the effects of random impurities on the liquid-vapor transition. Previous measurements with helium and nitrogen confined in a 95% porosity aerogel showed no hysteresis between cooling and warming behavior near the critical temperature, T-c, allowing equilibrium behavior to be observed. T-c, was suppressed and coexistence curves were much narrower than for bulk fluids. However, recent measurements further from T-c, found long time constants and hysteresis between filling and emptying. We have made high resolution isotherm measurements near the critical point of helium in a 95% porosity aerogel, using a capacitive technique to measure the density directly. We also measured the pressure and bulk density in situ and used a ballast volume to control pressure. Thermal relaxation was very slow in the coexistence region, but our technique allowed us to directly check for equilibrium at each point. T-c, was suppressed, but the coexistence curve was not as narrow as in the earlier measurements. However, we observed hysteresis and finite compressibilities at all temperatures below T-c, indicating the importance of surface tension and capillary condensation.