Shapes of rotating normal fluid He3 versus superfluid He4 droplets in molecular beams

Abstract

Previous single-pulse extreme ultraviolet and x-ray coherent diffraction studies revealed that superfluid 4 He droplets obtained in a free jet expansion acquire sizable angular momentum, resulting in significant centrifugal distortion. Similar experiments with normal fluid 3 He droplets 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 isolated 3 He and 4 He droplets following expansion of the corresponding fluids in vacuum at temperatures as low as ∼2 K. Large 3 He and 4 He droplets with average radii of ∼160 and ∼350 nm, respectively, were produced. We find that the majority of the shapes of 3 He droplets in the beam correspond to rotating oblate spheroids, in agreement with previous observations for 4 He droplets. The aspect ratio of the droplets is related to the degree of their rotational excitation, which is discussed in terms of reduced angular momenta () and reduced angular velocities (), 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.

Aggregation of solutes in bosonic versus fermionic quantum fluids

“…Most outlines are ellipses, consistent with spheroidal, rotating droplets ( 8 , 15 , 17 , 21 , 22 ). In previous studies, it was found that a cryogenic fluid expansion into vacuum readily produces rotating 4 He and 3 He droplets ( 15 , 17 ).…”

“…Most outlines are ellipses, consistent with spheroidal, rotating droplets ( 8 , 15 , 17 , 21 , 22 ). In previous studies, it was found that a cryogenic fluid expansion into vacuum readily produces rotating 4 He and 3 He droplets ( 15 , 17 ). It was also found that droplets of different isotopes have very similar average aspect ratios of about 1.05 for their projections on the detector plane ( 17 ).…”

“…The lower boundaries for the droplet’s angular velocity, estimated from their aspect ratios ( 17 ), are ≈1.1 × 10 7 and ≈1.5 × 10 6 rad/s for the droplets in Fig. 2 (C and D, respectively) .…”

“…In previous studies, it was found that a cryogenic fluid expansion into vacuum readily produces rotating 4 He and 3 He droplets ( 15 , 17 ). It was also found that droplets of different isotopes have very similar average aspect ratios of about 1.05 for their projections on the detector plane ( 17 ). We hypothesized that during the passage of fluid helium through the nozzle, the fluid interacts with the nozzle channel walls and acquires vorticity, which is eventually transferred to the droplets.…”

“…This effect was assigned to inhomogeneities within the droplets due to the presence of quantum vortices, which attract dopant particles ( 11 – 14 ). The vortices were found to originate from an unavoidable rotational excitation of free helium droplets in the beam ( 8 , 15 – 17 ), implying that the superfluid nature of 4 He enhances the inhomogeneity of matrix-dopant interactions.…”