Cryogenic flow visualization techniques have been proved in recent years to be a very powerful experimental method to study superfluid turbulence. Micron-sized solid particles and metastable helium molecules are specifically being used to investigate in detail the dynamics of quantum flows. These studies belong to a well-established, interdisciplinary line of inquiry that focuses on the deeper understanding of turbulence, one of the open problem of modern physics, relevant to many research fields, ranging from fluid mechanics to cosmology. Progress made to date is discussed, to highlight its relevance to a wider scientific community, and future directions are outlined. The latter include, e.g., detailed studies of normal-fluid turbulence, dissipative mechanisms, and unsteady/oscillatory flows. He is used as a coolant for superconducting magnets and infrared detectors (2), to astrophysics, where superfluidity is invoked to explain glitches in the rotation of neutron stars (3, 4) and the formation of cosmic strings (5, 6). More recently, superfluidity has been used to describe the collective behavior of birds (7) and a cosmological model has been used to obtain results relevant to superfluid turbulence (8). The latter form of turbulence, occurring in quantum fluids, is indeed an especially interesting topic because of its quantum peculiarities and its similarity to classical turbulence. Superfluids, in which turbulence can be directly visualized and studied, include superfluid 4 He and atomic Bose-Einstein condensates (9). Due to the limit of small sample volumes, the experimental study of turbulence in Bose-Einstein condensates has hardly begun. The development of visualization techniques applicable to superfluid 4 He is thus essential, if our understanding of quantum turbulence is to make significant progress in the near future.Superfluid 4 He is viewed as consisting of two interpenetrating fluids. The gas of thermal excitations forms the normal component, which can be considered as a viscous fluid. The superfluid component is inviscid and its rotational motion is possible only in the presence of topological defects, in the form of quantized vortex filaments. Turbulence in the superfluid component therefore takes the form of a tangle of quantized vortex lines. Turbulence in the normal fluid is more conventional, although the interaction between the normal fluid and the vortices leads to the nonclassical force of mutual friction between the two fluids. Turbulence in such a system can exhibit a behavior that is similar to that found in a classical fluid; but it may take forms that are unknown in classical fluid mechanics: for example, forms relevant to a fluid in which there is no viscous dissipation, and those that depend on the coexistence of the two fluids. Study of quantum turbulence can therefore enrich our knowledge of turbulence in general, as well as being interesting in its own right. Visualization TechniquesFlow visualization techniques have been developed to a high degree of precision and speed for clas...

Quantum turbulence in thermal counterflow of superfluid ${\text{} }^{4} \mathrm{He} $ is studied at length scales comparable to the mean distance $\ell $ between quantized vortices. The Lagrangian dynamics of solid deuterium particles, of radius ${R}_{p} $ about one order of magnitude smaller than $\ell $, is analysed in a planar section of the experimental volume by using the particle tracking velocimetry technique. We show that the average amplitude of the acceleration of the particles seems to increase as the temperature decreases and applied heat flux increases and this can be explained by exploiting the two-fluid model of superfluid ${\text{} }^{4} \mathrm{He} $. We also report that, at the probed length scales, the normalized distribution of the acceleration of the particles appears to follow an unexpected classical-like behaviour.

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