Formation of superfluid liquid pocket in aerogel and its solidification by cooling

The phase boundary between the quantum solid and liquid phases of 4 He is strongly modified in a porous material. However the phase diagram at very low temperatures remains unexplored. We have used a variable-volume experimental cell with optical access to visualize the crystallization of 4 He in silica aerogels with independent control of the pressure and temperature. The onset of crystallization was investigated in two aerogel samples with differing porosity both by pressurization at constant temperature and by cooling at constant pressure. With isothermal pressurization we have established a low temperature phase diagram for each aerogel and we find that the equilibrium crystallization pressure is reduced with increasing aerogel porosity. Crystals also began to grow in the aerogel on cooling at fixed pressure below an onset temperature, Tgrow. We found that below this temperature the crystallization rate increased with decreasing temperature. The aerogel in our cell was surrounded by bulk crystals of 4 He and surprisingly Tgrow was found to be widely distributed when the surrounding bulk crystals were repressurized. In this experimental arrangement, crystallization within the aerogel on cooling requires mass flow from these exterior bulk crystals and is strongly influenced by the disordered structure at the interface between the bulk solid and the helium within the aerogel.

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Crystallization ofHe4in aerogel via mass flow from surrounding solidHe4

Matsuda

Ochi

Isozaki

et al. 2016

Phys. Rev. B

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Control of the wetting properties ofHe4crystals in superfluid

et al. 2016

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To investigate whether it is possible to control the wetting of ^{4}He crystals on a wall in superfluid, the contact angles of ^{4}He crystals were measured on rough and smooth walls at very low temperatures. A rough wall was prepared in a simple manner in which a commercially available coating agent for car mirrors, which makes the glass surface superhydrophobic, was used to coat a glass plate. The contact angles of ^{4}He crystals were increased by approximately 10^{∘} on the rough wall coated with the agent. Therefore, the increase in the repellency of ^{4}He crystals in superfluid was demonstrated to be possible on a very rough surface. The enhancement of the contact angles and a scanning electron microscopy image of the coated surface both suggest that a Cassie-Baxter state of ^{4}He crystals was realized on the surface; the crystals did not have full contact with the wall, but entrapped superfluid was present beneath the crystals in the hollow parts of the rough wall.

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Crystallization Onset of Liquid Pockets via Mass Flow through Solid ⁴He in Aerogel

2019

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Formation of superfluid liquid pocket in aerogel and its solidification by cooling

Cited by 3 publications

References 35 publications

Crystallization ofHe4in aerogel via mass flow from surrounding solidHe4

Crystallization ofHe4in aerogel via mass flow from surrounding solidHe4

Control of the wetting properties ofHe4crystals in superfluid

Crystallization Onset of Liquid Pockets via Mass Flow through Solid ⁴He in Aerogel

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Formation of superfluid liquid pocket in aerogel and its solidification by cooling

Cited by 3 publications

References 35 publications

Crystallization ofHe4in aerogel via mass flow from surrounding solidHe4

Crystallization ofHe4in aerogel via mass flow from surrounding solidHe4

Control of the wetting properties ofHe4crystals in superfluid

Crystallization Onset of Liquid Pockets via Mass Flow through Solid 4He in Aerogel

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Product

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Crystallization Onset of Liquid Pockets via Mass Flow through Solid ⁴He in Aerogel