We have used high-speed video and interferometry to investigate the impact, spreading, and eventual contraction of superfluid ${}^4\mathrm{He}$ drops on a sapphire substrate in a saturated atmosphere of helium vapor. We find that the short-term kinetic spreading of superfluid drops (time $t<10$ ms) is qualitatively similar to both normal helium and conventional fluids at room temperature. In contrast, the contraction phase of the superfluid drops is highly unusual. Superfluid drops survive for only a few seconds on the substrate due to superflow out of the drop into the surrounding helium film. The drop lifetime is strongly dependent on temperature and diverges at the superfluid transition temperature $T_{\lambda }\sim 2.17$ K. The contracting drops undergo a geometry-dependent two-phase contraction, which includes a toroidal phase where the radius decreases linearly in time and subsequently a spherical cap phase where the radius decreases with the square root of time. The receding contact angle is temperature dependent and becomes small near $T_{\lambda }$. We also observe that the superfluid outflow causes surprising edge effects, including the emergence of satellite droplets on the perimeter of the expanding drop, as well as ragged and frayed drop edges at lower temperatures.

中文翻译：

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超流体氦滴在固体表面上的冲击和生命周期

我们已经使用高速视频和干涉仪来研究超流体的影响，扩散和最终收缩 ${}^4他$在饱和的氦气气氛中，水滴落在蓝宝石衬底上。我们发现超流体液滴的短期动力学扩散（时间$Ť<10$ms）在质量上与室温下的普通氦气和常规流体相似。相反，超流体液滴的收缩阶段非常不寻常。由于超微滴从微滴中流到周围的氦膜中，因此超微滴在基板上仅存活几秒钟。液滴的寿命主要取决于温度，并且在超流体转变温度时会发散${Ť}_{\lambda }〜2.17$ K.收缩液滴经历几何形状相关的两相收缩，其中包括一个环形相，在该环形中半径随时间线性减小；随后是一个球形帽盖相，该半径随时间的平方根减小。后退接触角取决于温度，并且在附近变小${Ť}_{\lambda }$。我们还观察到，超流体流出会引起令人惊讶的边缘效应，包括在扩展液滴周长上出现卫星液滴，以及在较低温度下参差不齐和磨损的液滴边缘。