Microgravity and vacuum, singly or combined, are uniquely advantageous media for the use of dielectrophoresis as a mmaterial s separation technology. In order to assess these advantages, a free-fall vacuum dielectrophoretic separator was designed and constructed for use at the earth's surface

Rose, R. M.

English

NASA Technical Reports Server

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Dielectrophoresis in the Microgravity Environment 
Principal Investigator: Professor R.M. Rose f“ cR tJ 
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Abstract 
Microgravity and vacuum, singly or combined, are uniquely advantageous media for the use of 
dielectrophoresis as a materials separation technology. In order to assess these advantages a free-fall 
vacuum dielectrophoretic separator was designed and constructed for use a t  the earth’s surface. 
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Introduction 
“Dielectrophoresis” is the motion induced by the presence of nonuniform electric field. The field 
equations describing this phenomena are 
1 
where F is the resulting force on the particle, P the induced dipole, E the applied field, and K1 and K2 
the relative dielectric constants of the medium and the particle, respectively. Because dielectro- 
phoresis is so often confused with electrophoresis, Table I is presented for the reader. (This is a some- 
what more elaborate presentation of an earlier comparison by Pohl.1) 
As Equation 1-3 and Table I together indicate, the dielectrophoretic effect has several remark- 
able features, at least in principle. The effect depends principally on the complex electrical permit- 
tivity and the acceleration is independent of particle size and shape; thus it can (at least potentially) 
be used for separations based on materials properties only, as in the classical mineral separation tech- 
nologies based on density which are used a t  the earth’s surface. The principal disadvantage is that it 
is a second-order effect and the resulting forces are weak unless high fields (and gradients) are used. 
At the earth’s surface, particularly when a suspending medium is used, the fields will be limited by 
the properties of the medium and convection will usually wipe out the effect or at least render the 
separation irreproducible, difficult or both. These problems are completely eliminated by use of the 
microgravity/vacuum environment. The remaining problem is the presence of relatively strong, first- 
order forces due to residual static electrical charge on the particles to be separated. This problem is 
effectively dealt with by utilization of the effect implied by Eq. (31, that is, that the effect is indepen- 
dent of the sign of the applied field; the dielectrophoretic force always has the sense of the more 
intense field, regardless of field direction. Thus, alternating fields, in which the forces due to direct 
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P R B C 6 l N G  PAGE BZANP N W  FILMED I 
https://ntrs.nasa.gov/search.jsp?R=19890004301 2020-03-20T04:20:48+00:00Z
Table I. A ComDarison of ElectroDhoresis with Dielectronhoresis 
Electrophoresis Dielectrop horesis 
Induced Force and 
Velocity: Field reversal reverses motion. 
Depends on sign ofcharge and sign of field. Independent of sign of applied field. 
Particle moves to more intense field. 
Force is proportional to volume of particle. Force depends on Charge; docs not depend 
directly on volume. 
Induced Acceleration a llrz 
Acceleration: Excellent at  molecular level. 
Acceleration independent of size or shape. 
Feeble at molecular level unless large 
gradients are used. 
Homogeneous or inhomogeneous fields. Inhomogeneous fields only. Usually requires 
strong field gradients to overcome convection, 
Field 
Requirements: diffusion. 
High (eg.. 1 kVlm) fields, typically. 
Steady (DC) fields only. 
High to very high, typically 1-10 kV/m. 
Steady f DC) or alternating fields. 
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effects are oscillatory, do not affect the direction of the dielectrophoretic force at all. In an operating 
’ separation system the direct forces would be dealt with by the use of AC fields, and the primary 
technical problem in this connection would be to insure that the amplitude of oscillation induced by 
direct forces would be reasonable compared to the dimensions of the system. For some classes of 
materials the effect may be optimized by using the frequency dependence of the dielectric constants. 
Additionally, because recent advances have occurred in the understanding of gravitationally- 
driven and capillarity-driven (i.e., Marangoni-Levich) convection, the prospect for substantial 
progress in separation technology as  the earth’s surface may exist as well. To begin with a test was 
desired of the feasibility of vacuudmicrogravity separations by dielectrophoresis. The instru- 
mentality elected was free-fall separation in vacuum. 
Design Considerations 
In order to achieve maximum separation with simple electrode geometry, coaxial cylinders were 
decided on, a t  the expense of uniformity of acceleration. (Later modification to isomotive electrode 
geometry was allowed for.) A preliminary assessment of lateral oscillatory amplitude at audio fre- 
quencies indicated that a system width in the range 1 - 10 cm would probably sf l ice;  in any case, the 
amplitude could be decreased by increasing particle size and thereby decreasing the charge/mass 
ratio, assuming static charging. The amplitude would of course depend on the frequency, which was 
limited by practical considerations to the audio range, i.e., 20 Hz - 20 KHz. At such frequencies 
voltages on the order of 10 KV may be attained by pushing the state of the art of transformer design, 
which we did. For concentric cylindrical electrodes with 150 mm and 3 mm diameters, lateral 
accelerations were calculated to be in the 0.001 - 0.01 g range, depending on dielectric properties. A 
six-foot free-fall (dictated mainly by ceiling height in our laboratory) would then result in lateral 
displacements ofat  least a few mm, easily observable. Even small differences in dielectric properties 
should then result in particulate segregations which are observable at low magnification and 
analyzable by, e.g., the EDXA method. 
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()E ?O(X QBALITY Construction of the Free-Fall Separator 
The final form of the device is shown in Figures 1 and 2. The outer electrode is an evaporated 
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Figure 1. Free-fall Separator. 
gold film deposited on the interior of the Pyrex tube; the inner electrode is polished stainless steel. At 
the top is a concentric mechanical iris, motor-driven for the sake of safety and to avoid vibration of the 
column. At the bottom is a platform supported by a removable ao:ess plate. Mixed powders will be 
dropped through the iris and collected on adhesive-coated slides or even small containers for 
examination. Below the platform and access plate is the vacuum plenum. The system is pumped 
through this plenum by a water-cooled, two-inch diffusion pump backed by a 5 cfm mechanical pump. 
Vacuum instrumentation consists of a Bayard-Alpert ionization gauge and a thermocouple gauge. 
The system has been pumped out and is usually kept under vacuum. The ultimate vacuum is slightly 
above 10-8 torr. 
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ORIGINAL PAGE IS 
0% ]rOOR QUAJJTY 
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Figure 2. Detail of the Free-fall Separator. 
The system is designed to be driven at  20 Hz - 20KHz a t  voltages up to 10 K V  (RMS) by a 2 5 0  
watt audio amplifier coupled to the electrodes by a transformer. Performance at  the high-frequency 
end ofthis range will be limited by the capacitance of the separator (about 26 pF); the characteristics 
of the transformer, including the capacitance and inductance of the secondary; and, related to these 
concerns, by nearby resonances. Considerable difficulty was encountered obtaining adequate designs 
and hardware for this purpose, which we now have from two sources. At this time we do not expect to 
be able to drive this system to 10 K V  at the highest frequencies, but in general, the performance 
should be more than adequate for our experimental objectives. 
Utilization 
The system is now ready for use. The general scheme is to evaluate the separation of "model" 
powder mixtures (ceramics, polymerics, metallics) and develop quantitative models applicable to 
microgravity conditions. Some analysis of dynamic effects, particularly those resulting from 
relaxations, will be attempted. 
Reference List 
1. H.A. Pohl, "Dielectrophoresis," (Cambridge University Press, 1978). 
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Dielectrophoresis in the microgravity environment

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