Tracer particles for application to PIV studies of liquid helium

Material selection and methods for introduction of tracer particles into liquid helium are reviewed for application in particle image velocimetry experiments. The combination of low temperature environment and low-density fluid place unique requirements on particle selection. Options discussed include a variety of commercially available solid particles of different size and density as well as solid particles generated by freezing liquids or gases. Recommendations are presented based on the dynamic behavior of the particles. Also, methods for introducing the particles into liquid helium are discussed.