Zemma, E., and J. Luzuriaga. "Anomalous Trajectories of H2 Solid Particles Observed Near a Sphere Oscillating in Superfluid Turbulent 4He." Journal of Low Temperature Physics 173, no. 1-2 (2013): 71–79.
Abstract: Using a relatively low cost apparatus, consisting of a glass dewar and a digital camera capable of taking images at 240 frames per second we have observed trajectories of frozen H2 particles which follow the flow of liquid helium below 2 K, around a sphere oscillating at 38 Hz. In some of the images the motion is compatible with laminar flow, while at high amplitudes, where we can reach Reynolds numbers of a few thousand in the normal component, the flow is clearly turbulent. In some of the videos taken we find particles being suddenly accelerated to several times the velocity of the oscillating sphere.
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Zemma, E., and J. Luzuriaga. "Turbulent Flow Around an Oscillating Body in Superfluid Helium: Dissipation Characteristics of the Nonlinear Regime." Journal of Low Temperature Physics 172, no. 3-4 (2013): 256–265.
Abstract: By examining the resonance curves of an oscillator submerged in superfluid liquid helium, it is found that their shape is affected by two distinct dissipation regimes when the amplitude is large enough to generate turbulence in the liquid. In a resonance curve, the central part close to resonance, may be in a turbulent regime, but the response is of much lower amplitude away from the resonance frequency, so that the oscillation can still be in the linear regime for frequencies not exactly at resonance. This introduces an ambiguity in estimating the inverse quality factor Q −1 of the oscillator. By analyzing experimental data we consider a way of matching the two ways of estimating Q −1 and use the information to evaluate the frictional force as a function of velocity in a silicon paddle oscillator generating turbulence in the superfluid.
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Zemma, E., M. Tsubota, and J. Luzuriaga. "Possible visualization of a superfluid vortex loop attached to an oscillating Beam." Journal of Low Temperature Physics 179, no. 5 (2015): 310–319.
Abstract: Visualization using tracer particles is a relatively new tool available for the study of superfluid turbulence and flow, which is applied here to oscillating objects submerged in the liquid. We report observations of a structure seen in videos taken from outside a cryostat filled with superfluid helium at 2 K, which is possibly a vortex loop attached to an oscillator. The feature, which has the shape of an incomplete arch, is visualized due to the presence of solid H2 tracer particles and is attached to a beam oscillating at 38 Hz in the liquid. It has been recorded in videos taken at 240 frames per second, fast enough to take ∼6 images per period. This makes it possible to follow the structure, and to see that it is not rigid. It moves with respect to the oscillator, and its displacement is in phase with the velocity of the moving beam. Analyzing the motion, we come to the conclusion that we may be observing a superfluid vortex attached to the beam and decorated by the hydrogen particles. An alternative model, considering a solid hydrogen filament, has also been analyzed, but the observed phase between the movement of the beam and the filamentary structure is better explained by the superfluid vortex hypothesis.
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