Separation of Micron-Scale Particles by Size Using Helium II

Here we report on a new technique we are developing to classify micron scale particles, i.e., separate them by size, using superfluid 4 He (He II). The technique is based on the unique property of He II, that is its two fluid nature which includes the viscous normal fluid component that flows in the direction of a heat current. The gravitational settling of small particles of a particular size can be balanced by the viscous drag from the normal fluid in a bath of He II. Further a non-uniform heat flux can allow a range of particle sizes to be suspended and collected at different locations within a He II counterflow channel. To demonstrate this principle, we have built a prototype particle separator and tested the process with particle diameters in the range 1 μm to 10 μm. The motion of the particles in this separator was measured using the Particle Image Velocimetry technique. Separation of a mixture of particles into two different size categories was achieved demonstrating the validity of this concept to classify particles. Particle agglomeration was also observed and is an issue particularly for particles with a diameter smaller than a few microns.