Memory, congruence, and avalanche events in hysteretic capillary condensation

Abstract

We report measurements of the hysteretic capillary condensation of helium in the porous material Nuclepore which display a hierarchy of return point memory effects and near congruence of closed internal hysteresis loops. Deviations from predictions based on the Preisach model of independent elementary hysteresis domains, originally developed to understand magnetic phenomena, are observed and are attributed to the appearance of additional domains on draining induced by the internal structure of the porous material. Pore intersections result in weak interactions among the pores which lead to avalanches.PACS numbers: 67.70.+n, 47.55.Mh, 67.40.Hf, 68.45.Da Hysteresis is a ubiquitous phenomenon, encountered in a wide variety of physical systems including magnetic materials, fluid invasion of porous beds, microfracturing of rock, martensitic transformations, charge density waves, adsorption of gases, etc. As an example of hysteretic behavior, capillary condensation has been widely studied, but a thorough understanding has been limited by the often complex geometry of the pore structure of the material. In an effort to study hysteresis, and, in particular, capillary condensation, in a simpler geometry, we have carried out experiments on the evolution of the hysteretic capillary condensation of superfluid helium into the nearly cylindrical pores of the filter material Nuclepore as the chemical potential is varied. We have chosen superfluid helium as the working fluid for these measurements due to the absence of viscosity and the existence of a mobile superfluid film resulting in rapid equilibration times.Nuclepore [1], used in biomedical and other applications, is a thin polycarbonate sheet perforated by a high density of nearly cylindrical holes. The Nuclepore used in this study has a thickness of 10 fim and is threaded by (3x 10 8 ) ± 15% pores/cm 2 oriented at random within 34° of the normal to the plane of the Nuclepore. Each pore has a nominal diameter of 2000 A-%%. Previous studies [2,3] have indicated that the pores have minimum diameter at the surface of the Nuclepore. Our simulations of the material indicate that the pores intersect; the most probable number of intersections per pore is about five and few pores suffer no intersections with others. The intersection region is apparently well defined with minimal extra volume present. Cluster simulations indicate that relatively few isolated clusters of disconnected pores exist in the material. Most pores participate in a macroscopic interconnected network and are apparently internally connected beyond the percolation threshold.To conduct our experiments we measure the amount of helium adsorbed and capillary condensed in the Nuclepore as a function of the chemical potential of the helium (number of helium atoms present) at fixed temperature in a chamber located at the end of a standard cryostat insert. The technique [3] involves the measurement of the capacitance of the helium-Nuclepore sample as a function of the growing thickness of the helium film on the expo...

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