The dynamics of relatively small particles in steady-state thermal counterflow of superfluid ${}^4\mathrm{He}$ (He II) is experimentally investigated by using the particle tracking velocimetry technique. We find that, close to the heat source, the mean distance between quantized vortices, representing the quantum length scale of the flow, is apparently about one order of magnitude smaller than that expected in the bulk, at the same temperature and heat flux. Possible physical mechanisms leading to this significant vorticity enhancement in the heater proximity are discussed and strongly support the view that the geometry of the channel where thermal counterflow occurs has a relevant influence on the observed flow features. Boundary and entrance effects, which have received little attention to date, should therefore be included in a comprehensive description of He II turbulent flows.

• Received 19 December 2017

DOI:https://doi.org/10.1103/PhysRevB.97.064512