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Hysteretic capillary condensation and avalanche phenomena of $\sp4$He in Nuclepore
We report capacitive measurements of the hysteresis and avalanche properties of $\sp4$He in Nuclepore. Hysteresis was measured on global loops and subloops for Nuclepore with 30 nm and 200 nm diameter pores. Subloops exhibit the property of return point memory, but subloops between the same chemical potential end points were not congruent. The failure of congruence showed that the Preisach model, in which the pores do not interact, does not describe this system. Modification of the Preisach model to allow for pore intersection within the membrane resulted in qualitative agreement with the data. Using a model for hystersis by Mason which included pore-pore interactions, we calculated the pore size distribution for Nuclepore.^ Close examination of the primary desorption curve resulted in the observation of large groups of pores draining together. This was the first observation of avalanches in the draining of fluid from a porous material. The properties of the avalanches were documented for 200 nm and 30 nm pore diameter Nuclepore. Using two capacitors on the same Nuclepore substrate, we found that avalanche events were spatially extended involving a low density of pores. With a composite porous/nonporous sample we showed that pore coupling occurred via the superfluid $\sp4$He film. ^
Physics, Fluid and Plasma
Michael Patrick Lilly,
"Hysteretic capillary condensation and avalanche phenomena of $\sp4$He in Nuclepore"
(January 1, 1997).
Doctoral Dissertations Available from Proquest.