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The dynamics of quantized vortices in superfluids has received increased attention recently because of novel techniques developed to visualize them directly. One of these techniques [G. P. Bewley et al., Nature 441, 588 (2006)] visualized quantized vortices and their reconnections in superfluid flows of 4He by using solid hydrogen tracers of micron-size or larger. The present work improves upon the previous technique by using substantially smaller particles created by injecting atmospheric air diluted in helium gas. These smaller particles are detectable thanks to the higher index of refraction of nitrogen compared to hydrogen and thanks to an improved visualization setup. The optical counting estimate, which agrees with terminal velocity estimates, suggests that the tracer diameter is typically 400 ± 200 nm and could be as small as 200 nm; being smaller, but not so small as to be influenced by thermal motion, the particles get trapped on the vortices faster, perturb the vortices less, possess smaller Stokes drag, and stay trapped on fast-moving vortices, as also on vortices generated closer to the superfluid transition temperature. Unlike the past, the ability to create particles in the superfluid state directly (instead of creating them above the λ-point and cooling the fluid subsequently), ensures greater temperature stability for longer periods, and enables the tracking of long and isolated vortices. These advantages have also led to the direct visualization of Kelvin waves. The use of other seed gases could lead to the visualization of even smaller tracers for quantized vortices. We discuss the visualization setup and provide suggestions for further improvement. [ABSTRACT FROM AUTHOR]

Cryogenic fluid flows including liquid nitrogen and superfluid helium are a rich environment for novel scientific discovery. Flows can be measured optically and dynamically when faithful tracer particles are dispersed in the liquid. We present a reliable technique for dispersing commercially available fluorescent nanoparticles into cryogenic fluids using ultrasound. Five types of fluorescent nanoparticles ranging in size from 5 nm to 1 μm were imaged in liquid nitrogen and superfluid helium, and were tracked at frame rates up to 100 Hz. [ABSTRACT FROM AUTHOR]

Using liquid helium, liquid nitrogen, and water as test fluids, we attempt to generate homogeneous turbulence in a steadily rotating system. We create turbulence by pulling a grid in rotating channels with both square and round cross sections, and observe large-scale inertial waves in the flow. These inertial waves quickly sense the boundaries, and resonate at frequencies characteristic of the container. We describe some of their properties and argue that the resultant inhomogeneity is a feature of any real system. [ABSTRACT FROM AUTHOR]

When liquid helium is cooled to below its phase transition at 2.172 K, vortices appear with cores that are only ångströms in diameter, about which the fluid circulates with quantized angular momentum. Here we generate small particles of solid hydrogen that can be used to image the cores of quantized vortices in their three-dimensional environment of liquid helium. This technique enables the geometry and interactions of these vortices to be observed directly. [ABSTRACT FROM AUTHOR]