Visualization of counterflow dynamics using frozen nanoparticles

We study the dynamics of quantized vortices and quantum turbulence utilizing a particle tracking visualization technique. This is accomplished by using sub-micron and micron-sized hydrogen or atmospheric ice particles injected into He$^{4}$ flows that get trapped on the vortices. This technique has been used to observe and characterize reconnection of quantized vortices and thermal counterflow. We present the latest results using nano-sized ice particles. These sub-micron particles are superior to larger particles in a number of ways. In particular, being less affected by Stokes drag, they stay trapped on faster moving vortices and remain trapped closer to the lambda transition. Using these particles, we have made additional observations of counterflows at higher heat fluxes to shed light on the particle-vortex interaction mechanism. The technique has also been extended for visualization for fluid dynamics experiments using liquid nitrogen.