Particle Image Velocimetry Studies of Counterflow Heat Transport in Superfluid Helium II

Heat is transported in helium II by a mechanism known as thermal counterflow where the liquid behaves as if it consists of two fluid components: the viscous normal fluid that carries the entropy and the inviscid superfluid that flows in opposition to conserve mass and momentum. Although the two fluid model has been successful at interpreting a number of unique transport processes in helium II, only recently has there been a significant effort to actually observe the associated fluid component motion. We have previously shown that Particle Image Velocimetry (PIV) can be used to visualize the motion of micron scale particles in helium II in response to thermal counterflow. These studies have led to some exceptional observations. The present paper summarizes our recent PIV experimental results on counterflow in helium II including the observed flow patterns around a cylinder and through a rectangular channel with backward facing step change in cross section, configurations which have been studied extensively in classical fluid mechanics. In addition to contributing to the fundamental understanding of helium II turbulence, as is discussed in the last section of the article, we show how this work also has a potential application in micron-scale particle classification and separation.