Control of the wetting properties of ${}^4\text{He}$ crystals in superfluid

To investigate whether it is possible to control the wetting of ${}^4\text{He}$ crystals on a wall in superfluid, the contact angles of ${}^4\text{He}$ 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 ${}^4\text{He}$ crystals were increased by approximately ${10}^{\circ }$ on the rough wall coated with the agent. Therefore, the increase in the repellency of ${}^4\text{He}$ 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 ${}^4\text{He}$ 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.

Figure 1

(a) and (b) SEM images of a glass surface coated with a car mirror coating agent. The coating roughened the surface significantly on multiple scales. (c) SEM image of a control glass surface without the coating.

Figure 2

(a) Overall view of the experimental cell through observation windows. A glass plate was attached vertically on the upper part of the cell and its surface was aligned in the observation direction. Coated and uncoated sides are indicated. Growth of a ${}^4\text{He}$ crystal with the hcp structure from the cell bottom and its $c$ and $a$ facets are clearly observed. (b)–(f) Typical growth of crystals with different orientations used for measurement of the contact angles. Profiles of the crystals are drawn in the upper part of each frame.

Figure 3

(a) Horizontal crystal-superfluid interface touching the vertical glass plate. (b) Enlarged image of the contact region. The menisci of the crystal-superfluid interface appeared in the vicinity of the glass plate, where the contact line was lower on the coated left side than on the uncoated right side. The contact angle on the coated side ${\theta }^{*}$ was larger than that on the uncoated side, $\theta$.

Figure 4

Contact angles for ${}^4\text{He}$ crystals of different orientations as a function of temperature. Different symbols represent the different crystals. Closed symbols are the contact angles for the coated side ${\theta }^{*}$, and open symbols are those for the uncoated side, $\theta$. The solid and dashed lines represent the average for ${\theta }^{*}$ and $\theta$, respectively. ${\theta }^{*}$ was larger than $\theta$ in most cases, which indicates that the coated surface successfully repelled the ${}^4\text{He}$ crystals in superfluid at very low temperatures.

Figure 5

(a) SEM image of the rough surface of the glass plate coated with a coating agent. (b) Binarized image with an appropriate threshold. The white part fraction was $\varphi \approx 0.7$.