M. P. Lilly

Hysteresis and Avalanche Behavior of Helium in Anopore and Nuclepore

Superfluid4He exhibits hysteretic behavior in the porous material Nuclepore due to capillary condensation, and one observes avalanches in the draining for a narrow range of chemical potential. We have demonstrated that the avalanche behavior involves pores distributed at a low density across the entire sample, and that avalanches are enabled by the superfluid film. We present results which compare the behavior of superfluid4He in Nuclepore to its behavior in Anopore, a porous membrane having no interconnected pores, and draw conclusions concerning the role of the superfluid film and the pore intersections.

Hysteresis and avalanche phenomena of helium in nuclepore

We report on the hysteresis and avalanche behavior of4He capillary condensing into the nearly cylindrical pores of the porous membrane Nuclepore. Following capillary condensation, withdrawal of the4He results in distinct jumps or avalanches where 107 pores can be involved in a single avalanche. By using Nuclepore with 2000Å and 300Å pore diameters, we find that the avalanche distribution depends on details of the pore structure.

Compressibility of 4He Liquid Confined in Nuclepore

Adding and removing4He from an experimental cell which contains the porous material Nuclepore results in capillary condensation, hysteresis and avalanches. The volume of liquid in the nearly cylindrical pores is monitored with a capacitance technique. For the situation when all of the pores are full, the initial removal of4He from the cell results in a slight non-hysteretic decrease in the capacitance, which we attribute to the isothermal compressibility of4He. As4He atoms are removed, the surface tension at the meniscus applies a negative pressure to the liquid in the filled pores, which results in a reduction of the density and a decrease in the capacitance. We present results for4He capillary condensed in 2000 Å pore diameter Nuclepore.

Hysteresis in a Capillary Condensation System

We report measurements of the hysteretic capillary condensation of liquid 4 He into the porous material Nuclepore. Results are presented for global and hysteretic behavior for 300 A diameter pores. These observations deviate from the predictions of the Preisach model of independent hysteresis domains. By modifying the Preisach model so as to restrict the ability of some of the pores to drain normally we are able to model many features of the experimental data.

Non-linear interaction between colliding third sound pulses

We report on time of flight measurements of third sound pulses in collision. The collision causes a shift, δτ, in the time of flight. For low amplitude unsaturated third sound pulses, we compare δτ with predictions based on film thickness and velocity changes induced by the colliding pulse.

Spatial extent of capillary condensation avalanches

Studies of hysteretic capillary condensation of4He in Nuclepore show that the initial draining of the pores occurs over a narrow range of decreasing chemical potential. We observe avalanches as groups of pores drain simultaneously. Measurements of the spatial extent of these avalanches show them to be distributed widely over the sample; they are not local events.