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

TL;DR

This study investigates helium adsorption and desorption in silica aerogels near the liquid-vapor critical point, revealing persistent hysteresis and complex capillary effects in a highly porous, low thermal conductivity medium.

Contribution

It provides direct measurements of helium density inside silica aerogels near the critical point, highlighting the persistence of hysteresis and the limitations of capillary condensation models.

Findings

Hysteresis persists close to the critical point.

No clear equilibrium transition observed after hysteresis disappears.

Capillary condensation explains many features, but its applicability diminishes near the critical point.

Abstract

We have investigated the adsorption and desorption of helium near its liquid-vapor critical point in silica aerogels with porosities between 95% and 98%. We used a capacitive measurement technique which allowed us to probe the helium density inside the aerogel directly, even though the samples were surrounded by bulk helium. The aerogel's very low thermal conductivity resulted in long equilibration times so we monitored the pressure and the helium density, both inside the aerogel and in the surrounding bulk, and waited at each point until all had stabilized. Our measurements were made at temperatures far from the critical point, where a well defined liquid-vapor interface exists, and at temperatures up to the bulk critical point. Hysteresis between adsorption and desorption isotherms persisted to temperatures close to the liquid-vapor critical point and there was no sign of an equilibrium liquid-vapor transition once the hysteresis disappeared. Many features of our isotherms can be described in terms of capillary condensation, although this picture becomes less applicable as the liquid-vapor critical point is approached and it is unclear how it can be applied to aerogels, whose tenuous structure includes a wide range of length scales.