JOURNAL OF COLLOID SCIENCE 20, 585-601 (1965)
CHARGING AND DECAY OF MONODISPERSED AEROSOLS IN
THE PRESENCE OF UNIPOLAR ION SOURCES
Kenneth T. WWtby, Benjamin Y. H. Liu, and Carl M. Peterson
Mechanical Engineering Department, University of Minnesota,
Minneapolis 14, Minnesota
Received February 17, 1,965
INTRODUCTION
The effects of airborne electric charge carried either on small ions or on aerosol particles has
been of interest to meteorologists for a long time. More recently this subject has interested the medical profession because of the possible health effects of airborne charge. Aerosol technologists also
have an interest in aerosol charge because of its effect on aerosol deposition, on air cleaner efficiency, and on the general behavior of aerosols.
Natural concentrations of ions and charged aerosol particles are on the order of a few hundred to
a few thousand per cubic centimeter. At these low concentrations the effects of the electrical charge
are usually negligible, but at the higher concentrations encountered when generating artificial aerosols or downstream of electrical air-cleaning systems, the net space charge may cause high charge
concentration gradients, strong electric fields, and significant precipitation of ions and aerosols.
The physical behavior of uniform clouds of charged ions or particles was originally studied by
Townsend (1), and later by Fuks and Petryanov (2), Wilson (3), Foster (4), Dunskii and Kitaev (5),
and Pich (6). These workers concerned themselves principally with the case where the charge concentration was uniform throughout a space. However, the assumption of uniform charge concentration is not valid when ions are produced continuously in a space by concentrated ion sources-a case
potentially of more technological importance.
To provide a background for the presentation of the results obtained in the present study the previous work is briefly reviewed below.
SUMMARY AND CONCLUSIONS
Results of experimental and theoretical studies were presented for the decay of aerosols in the presence of a continuous source of ions. The test aerosols used in the experimental studies were natural
atmospheric contamination and generated aerosols. The generated aerosols were solid, spherical,
monodispersed, and initially electrically neutral. These aerosols were generated and tested in sizes
varying from a mass median diameter of 0.028 to 3.6 m having a geometrical standard deviation of
1.1 to 1.67.
The aerosol decay experiments were conducted in a 2000 cubic foot rectangular room with a point
source of unipolar ions located at the center of the room. Particle charge and concentration measurements were made and data were presented for three types of unipolar ion sources: a sonic-jet ion
generator, a free-needle, and a commercial ionizer. Ions of both polarity were studied while the free
ion current varied from 0.1 to 3.6A. The electrical charge measured on the particles was somewhat
greater for the particles exposed to negative ions than for those exposed to positive ions. The charge
measured on the particles with Dp < 1m and exposed to an ion output of at least 2.7A showed good
agreement with White's diffusion charging equation for Nt = 107. Charge on particles where Dp >
1m was slightly less than that predicted by White's equation.
The experimental half-lives of the decaying aerosols ranged from a minimum of 5.5 minutes for the
free-needle and a particle size of 0.26m mmd to a maximum of 58 minutes for a commercial ionizer
and a particle Size of 0.26m mmd. The performance of the sonic jet ionizer was intermediate between these two extremes. These results indicate that a continuous source of ions located in a space
can precipitate a significant quantity of aerosol, and in some conditions it may be an effective means
of air cleaning.
The experimental half-lives of the decaying aerosols were compared with values predicted by theory.
Generally good agreements between theory and data were obtained for the sonic jet ionizer. The differences between theory and data for the free-needle and the commercial ionizer were due to the fact
that the experimental conditions did not approximate those assumed in the theory.
NOMENCLATURE
C = Cunningham correction, dimensionless.
Dp= particle diameter, cm.
E = electric field, statvolt/cm
E,,= electric field at surface of wall enclosing space, statvolt/eni.
n = ion concentration, number/cm.'.
n0= initial ion concentration, number/cm.3.
np= particle concentration, number/cm
Np0= initial particle concentration, number/cm.'.
q = electron charge, 4.80 X 10-'o stat coul.
Qp= charge on particle, stat coul.
Q = ion generation rate, statamperes
r = radius, cm.
rw = room radius, cm.
t = time, seconds.
t1/2= half-life, seconds.
v = ion velocity, cm./see.
V = potential, statvolts.
Z = ion mobility, cm2 /statvolt-sec.
Zp = particle mobility, cm2/statvolt-sec.
g = fluid viscosity, poise.
REFERENCES
1. TOWNSEND, J., "Application of Diffusion to Conducting Gases," Phil. Mag. 45, 471 (1898).
2. FUKS, N., AND PETRYANOV, L, ZHFKH 7, 312-319 (1936).
3. WILSON, 1. B., "The Deposition of Charged Particles in Tubes", with Reference to Retention
of Therapeutic Aerosols in the Human Lung," J. Colloid Sci. 2, 771-776 (1947).
4. FOSTER, W. W, "Deposition of Unipolar Charged Aerosol Particles by Mutual Repulsion Bril. J.
Appl. Phys. 11 ZW13 Q959).
5. DUNSKII, V. F., AND KITAEV, A. V., "Precipitation of a Unipolarly Charged Aerosol in an Enclosed Space," Colloid J. (U.S.S.R.) 22, 167-175 (1960).
6. PICH, J., "Zur Theorie der Elektrostatischen Zerstreuung Monodisperser Aerosole," Staub 22,
15-17 (1962).
7. FuNs~ N. A., "The Mechanics of Aerosols," English translation by E. Lachowiez. CWL Special
Publ. 4-12 (1955).
8. WHITBY, K. T., and McFARLAND, A. R., "Decay of Unipolar Small Ions and Homogeneous Aerosols in Closed Spaces and Flow Systems," Proc. Intern. Conf. Air, pp. VII-1-30, Franklin Inst., Oct.
16~17 (1961).
9. WHITBY, E. T., McFARLAND, A. R., and LUNDGREN, D. A., "Generator for Producing High
Concentrations of Small lons. Technical Report No. 12 by Mech. Eng. Dept., Univ. of Minnesota to U.
S. Public Health Service, July 1960.
10. HICKS, W. W., and BECKETT, J. C., "The Control of Air Ionization and its Biological Effects,"
Trans. Am. Inst. Elec. Engrs . 30, Part 1, 108-111 (1957).
11. Bulletin on Philco Model ICF-6 Ion Counter, Philco Corp., 4700 Wessahickson Avenue, Philadelphia 44, Pennsylvania.
12. HURI), F. K.,and MULLINS, J. C., J. Colloid Sci. 17, 91 (1962).
13. WHITBY, K. T., and PETERSON, C. Xl., "Electrical "Neutralization and Particle Size Measurement of Dye Aerosols," Ind. Eng. Chem. Fundamentals 4, 66-72 (Feb., 1965).
14. WHITE, H. J., "Particle Charging in Electrostatic Precipitation," Trans. Am. Inst. Elec. Engrs.
70, 1186-1191 (1951).
DISCUSSION
T. GILLESPIE ( Dow Chemical Company, Midland, Michigan): Did you observe the
large scale movements observed by Whytlaw Gray in his pioneer work on unipolar smokes?
K. T. WHITBY No. This may be due to the fact that we worked with spherical particles and that we had forced circulation in the aerosol chamber. The measurements of
aerosol concentration along a room radius showed no variation with radius within
the accuracy of the measurements.