Prof Ryuji Nomura (Hokkaido University) gives a webinar on ‘Anomalous Oscillation and Dripping of Superfluid Droplets’ at 10AM UK time.

The differences between superfluids and classical fluids are particularly intriguing, especially in highly nonequilibrium conditions. We observed a superfluid 4He pendant droplet suspended from a horizontal surface using a high-speed video camera. This droplet formed on the bottom surface of a cup with various shapes, while superfluid 4He flowed into the droplet from the cup via a film flow. As the superfluid pendant droplet grew on the bottom surface, it eventually pinched off and fell. The oscillation of the droplet was triggered by the recoil from the pinch-off, causing the remaining pendant droplet to oscillate with high amplitude and negligible damping until the next pinch-off occurred. A key difference between superfluids and classical fluids is that the surface is covered by the superfluid film, which prevents any pinning at the edge of the droplet. In contrast, the microscopic roughness of a surface tends to pin the edge of droplets in classical fluids. In the case of the wide enough bottom surfaces, the entire droplet body, including its edge on the surface, oscillated. The oscillation period was anomalous and independent of the droplet size [1], contradicting the common belief in classical fluid dynamics that larger droplets should oscillate in longer periods. The dripping periods were observed to be quantized at discrete values, even when the input flow rates were gradually varying [2]. This was also different from the known behaviors of dripping of classical fluids in which dripping periods are irregular and show chaos, even if the flow rate remains constant. On the wide bottom, another anomalous mode was observed occasionally; the droplets horizontally translated on a flat surface and bounced off at the corner [1]. This is known as the Noether mode which can only be realized in a system with perfect translation symmetry. Although it has been discussed theoretically, it was previously thought impossible in realistic systems of classical fluids due to pinning. When the bottom of the container was narrower than the natural droplet size, the edge of the droplet became pinned at the corner and remained stationary. Similar to what is observed in classical fluids, these droplets oscillated with periods that depended on their size, and their dripping periods were widely distributed. Thus, the anomalous oscillations and drippings can be attributed to the free movement of the droplet’s edge across the surface, which is characteristic of superfluid behavior.

[1] K. Onodera, R. Nagatomo, K. Miyake, R. Yamane, S. Takamatsu, Y. Aoki, and R. Nomura, Phys. Rev. Lett. 133, 216001 (2024).

[2] R. Nagatomo, K. Onodera, S. Kashimoto, Y. Aoki, and R. Nomura, J. Phys. Soc. Jpn. 92, 124601 (2023).