(70ck) Particle Separation & Classification Using Superfluid Helium (He II)*

Liquid helium is an exceptional fluid that can exist in either of two phases, normal helium (He I) or superfluid helium (He II). These two phases are separated by the so-called lambda phase transition, which under saturation conditions is at about 2.2 K. While He I is essentially a Navier-Stokes fluid obeying classical thermal fluid dynamic equations, He II displays many unique transport properties, such as a superior thermal conductivity that is many orders of magnitude greater than that of ordinary fluids. It also has the lowest kinematic viscosity of all known fluids being about three orders of magnitude smaller than that of air. These unique transport properties are the reasons why He II is used today in a variety of technical applications from the cooling of superconducting magnets to space-based infrared telescopes.

The transport properties of He II are described by the two-fluid model in which the He II is comprised of two interpenetrating fluid components. The normal fluid component behaves like an ordinary Navier-Stokes fluid while the superfluid component has no viscosity or entropy. This model leads to a unique heat transport mechanism known as thermal counterflow. With a heat flux applied to He II, the entropy containing normal fluid component carries the heat away from the source. At the same time, the superfluid component moves in the opposite direction toward the source in order to conserve mass. This very efficient heat transport process combined with the essentially constant density of He II ensures that under most circumstances a He II bath will be isothermal with negligible ordinary bulk convection. Thus, a bath of He II boiling under saturated conditions appears quiescent with evaporation only occurring at the free surface.

The unique properties of He II make it an ideal medium in which to separate and/or classify ultra-fine particles. A micron scale solid particle placed in a bath of He II will rapidly achieve a terminal settling velocity based on a balance between the gravitational force and the Stokes viscous drag [1]. This process is the same as occurs for classical fluids except that in the He II case the viscous drag is only with the normal fluid component, which can be selected by adjusting the temperature of the He II. The absence of ordinary convection and the low temperature environment reduces the amount of random motion by the particles. Since larger, more-dense particles will settle more quickly in a He II bath than small, light particles, the settling process combined with a time-of-flight selection can be used for particle separation.

A particle separator based on a He II column would have several significant advantages over conventional separation techniques that use gas or liquid columns. He II is an extremely pure substance since all impurities are frozen out. Further, He II adheres to all substances so it will tend to coat the particles with an inert film that prevents agglomeration. Thus in the low temperature environment He II acts as an effective and chemically inert ?surfactant? that can be easily removed when the particles are returned to room temperature.

The properties of He II described above have been well established by numerous experimental and theoretical investigations. What is new in the present work is the exploitation of these properties to create a medium in which particles may be classified by size, density, defect etc. Towards this end we have launched an R&D program to establish the operating principles of a He II based particle separator. Issues to be addressed include:

1. Can particles be introduced into He II without undue agglomeration? 2. Do particles in He II sediment via the Stokes drag law? 3. Can particle dynamics in He II be controlled by application of a heat current in the He II? 4. How would we combine these technologies to produce a device that can separate micron and sub-micron size particles?

Preliminary results from the program are promising as we have been able to suspend micron scale particles in a He II column. Figure 1 shows 1.5 micron diameter silica particles suspended in a 20 mm wide He II column. In this paper, we will report on techniques for seeding particles in He II, characterizing their size based in Stokes flow and concepts for separation according to size using the unique transport properties of He II.

[1] T. Zhang and S. W. Van Sciver, The motion of micron-size particles in He II counterflow as observed by the PIV technique, J. Low Temp. Phys. Vol. 138, 865 (2005)

*Work supported in part by Oxford Instruments Inc. The National High Magnetic Field Laboratory is supported by the National Science Foundation in cooperation with the State of Florida.