Herbert A. Pohl

Some Effects of Nonuniform Fields on Dielectrics

Some of the more interesting effects of nonuniform electric fields are described in this report. Experimental and theoretical studies show the effects to be rather striking for particles larger than molecular size. The results show that the effect can be used to produce a fairly efficient pumping action of nonconducting liquids, to cause continuous and easily measureable separations in coarse suspensions, to cause selective precipitation, and to produce mixing.By this means, liquids may be thrown several feet into the air with an electromechanical efficiency of about 25%. A separation factor of at least 2.5 in continuous separatory operation may be produced in a suspension of polyvinyl chloride in carbon tetrachloride‐benzene mixture. Suspensions of polar materials in less polar liquids may be either dispersed or precipitated. In one interesting ``demonstration type experiment, drops were ``hung in mid‐air.

Dielectrophoresis of Cells

Dielectrophoresis, the motion produced by the action of nonuniform electric field upon a neutral object, is shown to be a simple and useful technique for the study of cellular organisms. In the present study of yeast (Saccharomyces cerevisiae) using a simple pin-pin electrode system of platinum and high-frequency alternating fields, one observes that the collectability of cells at the electrode tip, i.e. at the region of highest field strength, depends upon physical parameters such as field strength, field uniformity, frequency, cell concentration, suspension conductivity, and time of collection. The yield of cells collected is also observed to depend upon biological factors such as colony age, thermal treatment of the cells, and chemical poisons, but not upon irradiation with ultraviolet light. Several interesting side effect phenomena coincident with nonuniform electric field conditions were observed, including stirring (related to jet effects at localized electrode sites), discontinuous repulsions, and cellular rotation which was found to be frequency dependent.

A Study of Living and Dead Yeast Cells Using Dielectrophoresis

Using a simple new physical technique, dielectrophoresis, living cells can rapidly be distinguished from dead ones, and collected separately. A detailed study using yeast cells is reported here. A comparative study of the dielectrophoresis of aqueous suspensions of living and dead yeast (Saccharomyces cerevisiae) was made over a range of frequencies, aqueous conductance, field strength, and time. The amount collected in a given period was observed to increase with field strength and aqueous resistivity except in the highest ranges where a reverse trend occurs. At high field strengths reversal of collection occurs (i.e., dispersion). The frequency responses of living and dead cells are different, enabling remarkably selective collection of living cells from a mixture of living and dead cells. Cells collected in this manner survive.

Factors Affecting Separations of Suspensions in Nonuniform Electric Fields

A study of the more quantitative aspects of the behavior of suspended polymer particles in a real dielectric is presented. While it is recognized that nonuniform fields exert an attractive force of appreciable magnitude upon particles of high dielectric constant suspended in a liquid medium of lower dielectric constant, the limits of such behavior are not precisely known. This report presents some quantitative data on the subject. The results show that in real dielectrics there is an appreciable effect due to the very small currents, about 1013 amperes at applied voltages in the kilovolt range [R. Coelho, Progress Report No. 22, pp. 13–17, Laboratory for Insulation Research, Massachusetts Institute of Technology (December, 1957)] which even a good liquid insulator supports.It is shown that the initial attraction to the central electrode due to the nonuniform field and its polarizing induction is gradually overcome by the repulsive effects of charge accumulated on the particles resulting from ionic conduct...

Some effects of non-uniform fields on dielectrics

Some of the more interesting effects of non-uniform electric fields are described in this report. Experimental and theoretical studies show the effects to be rather striking for particles larger than molecular size. The results show that the effect can be used to produce a fairly efficient pumping action of non-conducting liquids, to cause continuous and easily measureable separations in coarse suspensions, to cause selective precipitation, and to produce mixing.

Reaction Spinning of Fibers

A method of forming fibers which is relatively new, yet of potentially wide applica tion, is described herein. The process may be termed reaction spinning. In reaction apinning, the usual viscosity increase required between spinneret and windup is ac complished by chemical reaction. Incompletely polymerized or incompletely solubilized polymer is forced through an extrusion die or spinneret under such conditions of rate, temperature. concentration, and activation that the polymerization proceeds rapidly within the fiber after it leaves the spinneret. This causes the soft extruded fiber to set to a solid or suitably semisolid state in fiber form on the windup package. where further reaction will complete the transformation to the desired fiber properties. Principles and experimental examptes of the process are described.

The isomotive cell, a new particle separator using dielectric polarization

The behavior of particles in a combined gravitational and nonuniform electric field is used to produce separations of materials having different dielectric constants. This paper describes recent improvements to the apparatus previously used for the continuous separations of physical mixtures. The separatory operation consists of passing the finely divided material through a pair of diverging electrodes while the particles fall down a tilted tray. The apparatus is in effect an electrical siphon, and requires extremely little electrical energy other than that to produce mechanical vibratory motion and falling of the particles through the cell.