Shapes of rotating normal fluid He3 versus superfluid He4 droplets in molecular beams
Shapes of rotating normal fluid He3 versus superfluid He4 droplets in molecular beams
复制标题
分子束中旋转正常流体 He3 与超流体 He4 液滴的形状
DOI：
10.1103/physrevb.102.014504
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发表时间：
2020
影响因子：
3.7
通讯作者：
Borgwardt, Mario
中科院分区：
文献类型：
--
作者：
Verma, Deepak;O’Connell, Sean M.;Feinberg, Alexandra J.;Erukala, Swetha;Tanyag, Rico Mayro;Bernando, Charles;Pang, Weiwu;Saladrigas, Catherine A.;Toulson, Benjamin W.;Borgwardt, Mario
先前的单脉冲极紫外和X射线相干衍射研究表明，在自由射流膨胀中获得的超流体液滴获得相当大的角动量，从而导致显着的离心畸变。对正常液滴进行的类似实验可能有助于阐明大程度旋转激发的起源，并突出正常液滴和超流体液滴动力学的相似性和差异。在这里，我们对相应流体在真空中、温度低至 ∼2 K 时膨胀后的孤立液滴和液滴的形状进行了比较。产生了平均半径分别为 ∼160 和 ∼350 nm 的大液滴。我们发现光束中液滴的大部分形状对应于旋转的扁球体，这与之前对液滴的观察结果一致。液滴的长宽比与其旋转激发的程度有关，这可以根据约化角动量 (Λ) 和约化角速度 (Ω) 进行讨论，发现两种同位素的平均值相似。这种相似性表明，无论同位素如何，类似的机制都会引起旋转。我们假设观察到的液滴尺寸和角动量的分布源于靠近喷嘴的致密区域的过程，其中显着的速度扩散和液滴之间的频繁碰撞导致过度旋转，然后导致液滴裂变。
Previous single-pulse extreme ultraviolet and x-ray coherent diffraction studies revealed that superfluiddroplets obtained in a free jet expansion acquire sizable angular momentum, resulting in significant centrifugal distortion. Similar experiments with normal fluiddroplets may help elucidate the origin of the large degree of rotational excitation and highlight similarities and differences of dynamics in normal and superfluid droplets. Here, we present a comparison of the shapes of isolatedanddroplets following expansion of the corresponding fluids in vacuum at temperatures as low as ∼2 K. Largeanddroplets with average radii of ∼160 and ∼350 nm, respectively, were produced. We find that the majority of the shapes ofdroplets in the beam correspond to rotating oblate spheroids, in agreement with previous observations fordroplets. The aspect ratio of the droplets is related to the degree of their rotational excitation, which is discussed in terms of reduced angular momenta (Λ) and reduced angular velocities (Ω), the average values of which are found to be similar in both isotopes. This similarity suggests that comparable mechanisms induce rotation regardless of the isotope. We hypothesize that the observed distribution of droplet sizes and angular momenta originates from processes in the dense region close to the nozzle, where a significant velocity spread and frequent collisions between droplets induces excessive rotation followed by droplet fission.