Liquid-Vapor Critical Behavior of Helium in High Porosity Aerogels
Abstract
Confinement in small pores can dramatically change the liquid-vapor critical behavior of fluids. Previous measurements with helium and nitrogen confined in a 95% porosity aerogel showed suppressed critical temperatures, T_c, and very narrow coexistence curves. However, recent measurements further from Tc found long time constants and hysteresis between filling and emptying. We have made high resolution isotherm measurements near Tc for helium in a number of aerogels. By using in situ capacitive gauges to measure the pressure and the helium density both inside and outside the aerogel, we could directly check for equilibrium. This was important in the coexistence region where thermal relaxation was very slow. We found hysteretic but rate-independent isotherms, indicating the importance of surface tension and capillary condensation in this system. In a 95% porosity aerogel, the hysteresis disappeared about 43 mK below the bulk T_c, but the coexistence curve was not as narrow as in the earlier measurements. We discuss how the coexistence curves and critical behavior depend on the porosity of the aerogel used to confine the helium.
- Publication:
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APS March Meeting Abstracts
- Pub Date:
- March 2004
- Bibcode:
- 2004APS..MARH15007B