Measurements of helium density in aerogel near the liquid-vapor critical point

We investigate the behavior of He-4 confined in silica aerogels near the bulk liquid-vapor critical point. Using a new mechanical technique to measure the density of the confined He-4 along isotherms, we find that the density continuously increases from a low, density phase up to a dense phase as the pressure is increased up to slightly below the bulk saturated pressure. An hysteretic behavior is observed between emptying and filling, which is not uniquely due to thermal problems. We argue that our observations are more in favor of some kind of capillary condensation than of a genuine first order phase transilion.