Control of the wetting properties ofHe4crystals in superfluid

What is it about?

In order to investigate whether controlling the contact angle of a 4He crystal in superfluid is possible or not, contact angles of 4He crystals were measured on rough and smooth walls at very low temperatures. A rough surface was made by coating a surface of a glass plate with a commercially available coating agent for a glass to be superhydrophobic. Contact angles of 4He crystals were found to increase by about 10 degrees on the rough wall coated with the agent. Therefore, the increase of repellency of 4He crystals in superfluid was demonstrated to be possible on the very rough surface.

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Why is it important?

From the comparison of the contact angles and SEM image of the rough surface, the Cassie-Baxter state of solid was realized for the first time on the rough surface; the 4He crystal had a contact with the protruding parts of the rough surface but superfluid was entrapped between the crystal and the hollow parts of the wall. Realization of the Cassie-Baxter state on a rough wall will provide some insights into other wall-related phenomena in 4He such as “supersolidity” and mass flow in solid 4He.