Shapes of rotating normal fluid He-3 versus superfluid He-4 droplets in molecular beams

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