Tobias Herman

Deformation of Silica Aerogel During Fluid Adsorption

Aerogels are very compliant materials\char22{}even small stresses can lead to large deformations. In this paper we present measurements of the linear deformation of high porosity aerogels during adsorption of low surface tension fluids, performed using a linear variable differential transformer. We show that significant deformation can occur under conditions common in studies of helium in aerogel, and that the degree of deformation of the aerogel during capillary condensation scales with the surface tension. This scaling may then be used to suggest limits on safe temperatures for filling and emptying low density aerogels with helium.

Freezing and Pressure-Driven Flow of Solid Helium in Vycor

The recent torsional oscillator results of Kim and Chan suggest a supersolid phase transition in solid 4He confined in Vycor. We have used a capacitive technique to directly monitor density changes for helium confined in Vycor at low temperature and have used a piezoelectrically driven diaphragm to study the pressure-induced flow of solid helium into the Vycor pores. Our measurements showed no indication of a mass redistribution in the Vycor that could mimic supersolid decoupling and put an upper limit of about 0.003 microm/s on any pressure-induced supersolid flow in the pores of Vycor.

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

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 aerogels 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.

Adsorption and desorption of helium in aerogels

Abstract We have studied the adsorption and desorption of helium in a 95% porosity aerogel. The helium density in the aerogel was directly measured with a capacitive technique, while the pressure and bulk helium density were measured with in situ capacitive gauges. Thermal response is slow in aerogels, so we used a thin sample to minimize the time constant. The combination of high resolution and rapid equilibration allowed us to study the capillary condensation of helium, which in aerogels occurs very close to the bulk saturation pressure. We saw hysteresis between filling and emptying of the pores, even for very slow rates. The hysteresis becomes smaller as the critical temperature is approached. We discuss whether true two phase coexistence can be observed in the helium–aerogel system.

Acoustic study of the liquid–vapor critical point of neon and helium in aerogel

Abstract We used low-frequency acoustic resonators to study the liquid–vapor coexistence curve and critical behavior of simple fluids in low-density silica aerogels. Resonators provide direct measurements of sound speed at low frequency. Sound speeds exhibit sharp kinks at the liquid–vapor phase boundary that allow us to map out coexistence curves near the critical point. Our previous measurements showed that neon in 95% aerogel exhibits a narrowed coexistence curve shifted to higher fluid density, but were complicated by a significant quantity of bulk neon present in the cell. We discuss results from our current experiments on two different fluids (neon and helium). Results are compared to our earlier measurements and to studies by other groups.

Adsorption of Helium in Silica Aerogel Near the Critical Point

When fluids are adsorbed in small pores, capillary forces cause the pores to fill before bulk liquid appears, and isotherms usually show hysteresis between filling and emptying. The tenuous structure of aerogels provides a unique medium in which to study the effect of dilute impurities on liquid‐vapor properties, particularly near the critical point where it may be possible to observe equilibrium behavior. We have used a capacitive technique to study adsorption in silica aerogel of two different densities near the liquid‐vapor critical point of helium. We compare the disappearance of hysteresis in helium adsorption isotherms in different density aerogels to the behavior in vycor glass.

Pressure Induced Flow of Solid 4He

Recent torsional oscillator measurements on solid 4He in the pores of Vycor and in bulk have demonstrated non‐classical rotational inertia, interpreted in terms of a transition to a supersolid phase. In the interest of determining whether or not solid 4He exhibits any of the other unusual flow properties of a superfluid, we have used a piezoelectrically driven diaphragm to study the pressure‐induced flow of solid helium into the Vycor pores. We also present preliminary results of the pressure‐induced flow of bulk solid helium through an array of microchannels.

Deformation of low density silica aerogel by helium

Liquid‐vapor interfaces are present throughout any porous medium during adsorption and desorption below the liquid‐vapor critical point. Surface tension across these curved interfaces produces the pressure difference between the liquid and vapor phases which is responsible for capillary condensation; this pressure difference can also compress the medium. Aerogels have extremely small elastic moduli, so surface tension induced deformation can be significant and sometimes destructive. We present measurements of the dilation and compression of silica aerogel during adsorption and desorption of helium and discuss the implications for possible damage to aerogels.