On the Visualization of Thermal Counterflow of He II Past a Circular Cylinder

Thermal counterflow of superfluid He-4 is investigated experimentally in the proximity of a 3 mm diameter cylinder by analyzing the motions of micrometer-sized solid particles, focusing especially on the occurrence of macroscopic vortices. The influence of heating an opaque brass cylinder by the light source is studied, as thermal counterflow is generated from its surface, and compared with the case of a transparent plexiglass cylinder of the same size. Additionally, we report our preliminary investigation of vertical counterflow around the transparent cylinder. We find that care should be taken when applying conventional visualization techniques-particle image velocimetry and particle tracking velocimetry-as spurious vortical structures might be identified in quantum flows displaying two-fluid behavior.