Abstract:
Advice for sorting separating and sizing very small particles is disclosed and claimed. The device comprises a cryogenic chamber within which particle movement, travel and separation can occur; a particle loading chamber for loading particles into the cryogenic chamber; and a particle collector. Also disclosed and claimed is helium, and more specifically helium in its superfluid state, for separating the particles.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Very small particles are used in a wide variety of manufactured products. In many of these products, the particles must be of extremely small size, but the small particles must be as uniform in size as possible. Examples of products containing extremely small particles of uniform size include pharmaceuticals, abrasives, inks, high-performance liquid chromatography columns, foodstuffs, and many others.  
         [0002]     Separation of particles by size is a critical step in the production of such particles. Particles having an average size of about 40 microns or larger can be collected by using micromachined filters. To collect, or harvest, particles having an average size smaller than about 40 microns is often accomplished by air classification. Air classification uses Stokes drag to separate particles by size. A particle falling under the influence of gravity (either the earth&#39;s gravity or an artificially induced gravitational field such as that provided by a centrifuge) in a viscous fluid medium such as air will have a terminal velocity that is strongly dependent upon the diameter of the falling particle. Differently sized particles fall through air or other viscous fluid media at different rates, thus separating in space and enabling them to be easily harvested by size.  
         [0003]     Air classification processes works tolerably well for the separation and harvesting of particle sizes down to about two microns. However, the efficiency of the classification process depends crucially on the properties of the viscous fluid medium through which the particles are sedimenting.  
         [0004]     But when the particles to be separated, sized, and harvested have a geometric average diameter of less than about two microns, air classification and other systems are difficult to use. Reasons for this can be inferred from  FIG. 1 : 
        1. Terminal velocities for these extremely small particles become very low.     2. Brownian motion begins to dominate particle dynamics.     3. Particle agglomeration due to Van der Waal&#39;s attraction between particles begins to retard particle separation.        
 
       OBJECTS OF THE INVENTION  
       [0008]     It is accordingly the general object of this invention to provide a device for separating extremely small particles according to their average diameter or size. A related object is to provide such a device which will operate in a relatively rapid and reliable matter.  
         [0009]     Another related object of the invention is to utilize a sedimentation medium which will encourage and permit particles of extremely small size to separate and sediment relatively rapidly and in a reliably predictable manner.  
         [0010]     Yet another object of the invention is to provide a device for separating extremely small particles which can be operated relatively easily and at relatively small expense.  
         [0011]     Although the preferred embodiment described below provides a device for separating particles by size, it is clear that in general the process could be extended to separate particles by shape, mass, density mechanical defect, or any other characteristic which causes some of the particles to fall through a medium faster than other particles.  
         [0012]     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings. Throughout the drawings, like reference numerals refer to like parts. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a graph showing the relationship of the geometric average diameter of particles falling through a fluid medium of known viscosity, the terminal velocity of those particles, and the Brownian displacement to which those particles are susceptible.  
         [0014]      FIG. 2  is a phase diagram of helium.  
         [0015]      FIG. 3   a  is a sketch showing an extremely small particle and the wetting action imposed by a fluid surrounding the particle.  
         [0016]      FIG. 3   b  is a sketch similar to  FIG. 3   a  showing two extremely small particles insulated from one another and deterred from agglomeration by layers of adhered atoms of the medium in which they are immersed.  
         [0017]      FIG. 4  is a schematic drawing showing, in sectional aspect, the top of a device for separating, sizing and classifying extremely small particles.  
         [0018]      FIG. 5  is a schematic drawing showing, in sectional aspect, the bottom of the device shown in  FIG. 4  for separating, sizing and classifying extremely small particles.  
         [0019]      FIG. 6  is an image derived from a scanning electronic microscope showing the top layer of a mixture of 7 micron and 2 micron diameter particles prior to sedimentation in superfluid helium.  
         [0020]      FIG. 7  is an image derived from a scanning electron microscope showing the top layer of a mixture of 7 micron and 2 micron diameter particles after sedimentation in superfluid helium. 
     
    
     DETAILED DESCRIPTION  
       [0021]     While the invention will be described in connection with certain preferred embodiments and procedures, it will be understood that it is not intended to limit the invention to these embodiments or procedures. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.  
         [0022]     To accomplish the above objects, the invention comprises a quantity of low viscosity high wetting parameter fluid; means for injecting the particles to be sorted, separated and sized into that fluid; and means for harvesting at least some of those separated particles from the fluid.  
         [0023]     The properties of superfluid helium make it an excellent medium in which to separate small particles. Ordinary liquid helium at 4.2 degrees Kelvin has a viscosity 5.5 times less than that of air at 20 degrees centigrade. A low viscosity medium suggests a relatively high terminal velocity for particles passing through the medium. In addition, if a low temperature can be maintained in the medium, the effect of Brownian diffusion on particle dynamics will be minimized. Furthermore, liquid helium has a very high wetting parameter; that is, helium atoms have a greater affinity for foreign objects than they do for other helium atoms. As a result, and as suggested in  FIGS. 3   a  and  3   b,  solid particles  10  immersed in liquid helium  12  quickly become insulated from one another in layers  14  of adhered helium atoms that have only a very weak Van der Waals attraction to one another. The layers of helium atoms thus acts as a surfactant to deter particle agglomeration while the particles are immersed in the cold liquid. Accordingly, liquid helium is a good candidate for a better particle sedimentation protocol than protocols currently achievable using air classifying equipment or other currently available techniques.  
         [0024]     However, because liquid helium will boil under ordinary conditions, superfluid helium is an even better sedimentation medium. Superfluid helium can be produced relatively simply by lowering the pressure above a container filled with ordinary liquid helium. The physical properties of superfluid helium are so different from ordinary helium liquid helium, or Liquid Helium I, that superfluid helium is considered to be a unique state of matter; it is neither a solid nor liquid nor gas. Either 3He or 4He or a combination of 3He and 4He can be used.  
         [0025]     The pressure and temperature constraints for superfluid helium or Liquid Helium II are shown in the phase diagram of  FIG. 2 . To generalize somewhat, superfluid helium can be produced and maintained at pressures less than 2.5 atmospheres and temperatures below 2 degrees Kelvin. In accordance with the invention, superfluid helium can be used to efficiently, effectively and inexpensively separate and sort extremely small particles.  
         [0026]     A device for sorting, separating and sizing extremely small particles is suggested schematically in  FIGS. 4 and 5 . To separate extremely small particles according to their average diameter or size, and to do so in a relatively rapid, reliable yet inexpensive manner in accordance with the invention, the illustrated device comprises, a cryogenic chamber  40  within which particle movement and separation can occur; a loading chamber  20  connected to the cryogenic chamber  40  for loading particles into the cryogenic chamber, and a collector device  80  connected to the cryogenic chamber  40  for collecting at least some of the particles after they have been separated by size.  
         [0027]     The closed, gas-tight loading chamber  20  includes a receiver  22  for receiving particles  10 ,  11  and  13  to be separated and for loading the particles  10 ,  11  and  13  into the cryogenic chamber  40 . A gate valve  23  is interposed between the receiver  22  and the cryogenic chamber  40  for controlling the flow of particles from the loading chamber  20  to the cryogenic chamber  40 .  
         [0028]     Above or upstream of the receiver  22 , a vacuum conduit  25  is connected via appropriate valving  27 ,  28  to a vacuum or exhaust pump (not shown) for drawing air from the receiver  22 . A delivery conduit  29  delivers helium to the receiver  20  when appropriate valving  30 ,  27  is opened. The particles  10 ,  11 ,  14  to be classified, sorted and sized can be delivered from a remote source (not shown) through a conduit  32  and inlet valve  34  to the receiver  22 . At appropriate time, the gate valve  23  is opened and the particles flow from the receiver  22  through a delivery conduit  36  extending into the interior of the cryogenic chamber  40  by a sufficient distance so that the particles are deposited within superfluid in the cryogenic chamber  40 .  
         [0029]     In accordance with one aspect of the invention, the cryogenic chamber is adapted to produce and maintain a column of very low viscosity, high wetting fluid such as superfluid helium 4He. An OptistatSXM Helium bath cryostat can be adapted and used for this purpose. This device is available from Oxford Instruments Superconductivity USA of 130A Baker Avenue Extension, Concord, Mass.  
         [0030]     As indicated above, particles falling through the superfluid medium in the cryogenic chamber tend to separate according to their size; larger particles tend to fall faster and arrive at the bottom of the column before the slower-falling smaller particles. To distinguish between these differently sized particles in accordance with another aspect of the invention, differentiation or size recognition equipment  90  can be provided, as suggested in  FIG. 5 . In the illustrated embodiment, this particle size indicating and recognition equipment  90  takes the form of a laser  91  mounted to direct a beam of light  92  through windows  93  and  94  in the cryogenic chamber. Light which illuminates the particles falls on a target screen  95 . The laser should provide light at a frequency far from that absorbed by the superfluid so that the heat load on the superfluid helium is minimized. For example, a Nd:YAG laser operating on a low duty cycle at the 532 nm line may be effective. As the particles fall through the laser light beam, diffraction patterns are created on the receiving screen  95 . Differently sized particles create differing diffraction patterns. Differences in the diffraction patterns can be detected and sensed by a computer  96  connected to the target screen  95 , and information about the particles sizes can be delivered to the system operator by any suitable means.  
         [0031]     This information about particles sizes can be used to harvest particles of a desired size or sizes and to discard particles which are excessively large or excessively small. This particle harvesting can be accomplished in any of a number of ways. For example, particles  11  which are too large will reach the bottom of the chamber apparatus first, before any particles of the desired size arrive. Under the circumstances, the superfluid helium in a discard conduit  42  can be pumped out, drawing off the oversized particles  11  with the fluid. Thereafter, when particles of the desired size begin to reach the bottom of the column, discard column pumping is halted and the superfluid helium and right-sized particles can be drawn-off from the column  40  by a harvest conduit  44  and pump (not shown). When particles  13  which are too small to meet requirements begin to arrive at the bottom of the column  40 , pumping and particle draw off or removal through the harvest column  44  can be halted and particle withdrawal through the discard column  42  can be resumed.  
         [0032]     Alternatively, a diverter baffle  47  can be located at the column bottom as illustrated in  FIG. 5 , and the diverter baffle  47  can be connected by a shaft or any other suitable means  48  to a baffle control  49  as illustrated in  FIGS. 4 and 5 . The diverter baffle is oriented, sized and located to direct particles falling upon it to either a discard portion  48  of the column bottom or to a harvest or collection portion  49  of the column bottom. The operation of this diverter baffle can be controlled by the particle size sensing computer  97 .