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A new method for the measurement of aerosol electrification
A NEW METHOD FOR T H E M E A S U R E M E N T OF AEROSOL ELECTRIFICATION B. L. Hinkle,* Clyde Orr, Jr., and J. M. DallaValle The Georgia Institute of Technology, Engineering Experiment Station, Atlanta, Georgia Received June 8, 1953; revised September 29, 195S ABSTRACT Because of the importance of the electrical forces associated with aerosol particles, an electrification analysis apparatus permitting rapid evaluation of the amounts of negatively and positively charged particles in an aerosol by measurement of the lateral deflection of the aerosol stream in an electrical field has been developed. Results obtained thus far have been in excellent agreement with expectation and with the published data of other investigators. The principal advantages of the method presented are the rapidity and ease of obtaining data and the elimination of the need for numerous samples to give statistically accurate results. An equation is presented for the calculation of the average electron charge carried by particles. INTRODUCTIOST The electrical characteristics of certain aerosols have been examined by a mlmber of investigators. DallaValle (1) has summarized the work of the early investigators, notably KRoblauch (2), Rudge (3) and Whitman (4), who showed that the charge acquired by an aerosol was dependent on the composition and size of the particles, the nature of the ducts through which the aerosol was conveyed, the conveying velocity, and the relative humidity. These studies were concerned chiefly with the net charge acquired by a dust cloud, however. Whytlaw-Gray and co-workers (5) presented information on the relative amounts of negatively and positively charged particles in smokes of ammonium chloride, stearic acid, magnesium oxide, and tobacco. Kunkel (7) published similar information on the electrification of quartz dusts, and Daniel and Brackett (8) have applied a scheme which determines the charge:mass ratio to the analysis of quartz dusts. One point of agreement among the diverse studies is clear: the electrification of a freshly prepared aerosol is largely dependent on the method of formation. Violent reactions, such as the burning of magnesium ribbon, give rise to highly electrified particles, whereas volatilization at low temperatures results in only a small fraction of charged particles. Aerosols pro* Present address, E. I. DuPont de Nemours & Co. (Inc.), Richmond, Virginia. 70
MEASUREMENT OF AEROSOL ELECTRIFICATION
71
duced with a LaMer (9) generator, for example, are relatively uncharged. Wilson and LaMer (10) found that less than 25 % of the particles comprising a glycerol aerosol were charged, even though radioactive sodium produced the nuclei for the formation of the glycerol particles. None of the particles carried as many as four electronic charges. White and Hill (11) found that aerosols produced with a LaMer generator contained particles either uncharged or carrying single electronic charges. Recently, LaMer et al. (12) found that dioctyl phthalate aerosols produced with a LaMer generator were almost devoid of charged particles. At present the data on aerosol electrification are still scanty, and more information on a variety of aerosol systems is needed. The chief obstacles to detailed studies of aerosol electrification have been the difficulties encountered in experimental methods and techniques. Whytlaw-Gray and Patterson (5) used an ultramicroscope cell containing horizontal electrodes and noted the action of the particles in the field of vision. Kunkel and Hansen (13) used a modification of the apparatus described by Hopper and Laby (14), who employed a photographic technique to measure the deflection of oil drops falling under gravity in a horizontal electric field. LaMer et al. (12) used a modified Millikan oil drop apparatus. Gillespie (15) used an instrument in which the charged particles were deposited on microscope slides and the neutral particles were collected in a thermal precipitator. The technique reported here avoids many of the experimental difficulties which accompany photographing or observing discrete particles and permits rapid evaluation of the electrification of an aerosol, but it introduces certain new difficulties. These new limitations Can be considerably minimized, however, and will be discussed in detail later. APPARATUS AND TECHNIQUE
The apparatus developed for the present work is designed to permit evaluation from the deflection of an aerosol stream. Since the aerosol stream serves as the sample, the necessity for obtaining numerous photographs in order to establish a precise sample average is removed, and the electrification of an aerosol may be determined in a single, brief experiment. A schematic diagram of the main elements of the apparatus is given in Fig. 1. The aerosol stream passing through the electrification chamber was observed and photographed through a window which was located at an angle of 45 degrees with the forward path of the light beam. Since the intensity of scattered light is greatest in the near-forward direction, observation at an angle of 45 degrees with the direction of propagation of the incident beam was accomplished at some sacrifice in scattered light intensity. Background light was virtually eliminated by the 45-degree angle
72
HINKLEj ORR~ JR.~ AND DALLAYALLE AnalyzingChamber
~,A Light Source
Camer~r'
CopperChlorideSotutlon Light S t o P 7 + ~
Light
Cylindrical Electrodel
Air $hletd
Section
AA
FIG. 1. Schematic diagram of aerosol electrification apparatus.
arrangement, however, and photographs showed excellent contrast between the image of the aerosol stream and the remainder of the film. A copper chloride solution was used to reduce the irffrared intensity of the incident light, and a slit at the entrance to the electrification chamber controlled the size of the light beam. All surfaces within the chamber and ducts were painted with an optically black paint. Previous investigators in the field of aerosol electrification have invariably used systems involving parallel plate electrodes. This construction generally requires dark-field illumination or observation at very acute angles, because small separation between the plates cannot be effected without having the edges of the plates project into the light path. The restrictions on observation angle were overcome by constructing cylindrical electrodes, located horizontally beneath the path of the incident light, and with axes at an angle of 90 degrees with the observational axis. With these modifications of electrode shape and location, a high-voltage gradient between the electrodes may be obtained without interference with the light beam. The aerosol entered through a glass tube which tapered to, and terminated as, a slit 7.5 mm long and 0.2 ram. wide just below the plane of the electrodes. This central glass tube was, in turn, surrounded by another tube through which pure air could be drawn. In operation, the air velocity in the outer tube was synchronized with the aerosol velocity so that an aerosol stream having a width of very nearly 0.2 mm. passed through the
MEASUREMENT OF AEROSOL ELECTRIFICATION
73
A
F l a . 2A. A stream of tobacco smoke in the absence of an electric field. B. A s t r e a m of tobacco smoke in an electric field of 5,000 volts per centimeter.
observational zone in the absence of an electrical field. This aerosol inlet tube was rotated so that the narrowest dimension of the stream was presented for observation or photographing. B y this means an aerosol stream was obtained containing a sufficient number of particles to result in adequate exposure of the film negative even when the particles were dispersed in the electrical field. This flow system has an additional advantage over methods in which the settling of discrete particles under gravity is measured. In the present scheme, thermal forces on the movement of the particles are negligible in comparison to forces due to the velocity of the stream, whereas unequal radiative heating in some previous systems caused spurious measurements and necessitated excessive insulation. The high-voltage source was controlled with a variable transformer so that the optimum dispersion of an electrified aerosol could be readily established. Maximum output was 20,000 volts, although it was seldom necessary to exceed 8,000 volts. Photographic negatives of the aerosol stream, such as shown in Fig. 2, were taken with the camera located a suitable distance from the camera port to obtain proper magnification and focus. A desirable feature of the photographic elements was a copying attachment, which permitted replacement of the camera with a ground-glass plate for visual observation. During a test, the image of the aerosol stream was viewed through the ground-glass plate, and when a fine, steady stream was clearly visible, the camera was switched to replace the ground-glass plate, and photographs were taken, thus giving photographs showing the change in the flow pattern of the aerosol due to the electric field. Photographic negatives were analyzed for optical density with a modified
7~
HINKLE, ORR, JR., AND DALLAYALLE
Model B Beckman Spectrophotometer. Companion negatives showing the aerosol stream with and without the presence of an electric field were placed in a sample holder adapted for use with the spectrophotometer. The optical densities of the films were then obtained by traversing each negative. Results using different portions of the negatives were reproducible in all cases.
Three possible limitations of the apparatus construction and of the experimental procedure warrant discussion. First, there is a question regarding the possible electrification of aerosol particles because of their passage through the tubes from the aerosol reservoir to the electrification chamber and through the electric field within the chamber. The charging of particles through ducts appears to be a function of the velocity of the particles (t), and since this velocity was kept at an extremely low value (of the order of 1 cm. per second), it would be unlikely that the aerosols could become charged by passage through the tubes. The charging of particles by passage through an electric field takes place when corona discharge occurs, and calculations show that approximately double the voltage gradient used in this work would have been necessary to cause significant charging. These conclusions were justified by examining an aerosol formed by exposing titanium tetrachloride to a moist atmosphere. Measurements showed that this aerosol was uncharged both before and after passage through the tubes and the electric field. The second question involves the possible effect of secondary light scattering, where the total light scattered by the aerosol stream might be less than the sum of the light scattered by the individual particles. According to Sinclair (16), particles scatter light essentially independently of one another when the distance between the particles is 10, or preferably 100, times the radius of the particle. In aerosols having a particle radius of 1 ~ and a number concentration of 106 per cc., the volume available to each particle is ~0-6 cc., the distance of particle separation is approximately the cube root of 10-6 or 10-2 co., and thus the ratio of separation to radius is 10-2: 10-4 or 100. For a 10-~ particle aerosol of the same concentration, the ratio is reduced to 10. In such an aerosol, some interference between the scattering of neighboring particles would be expected, but such high concentrations are seldom found in practice. With the exception of an elutriated clay aerosol with a median diameter of 3 to 5 tL, the aerosols used in this study exhibited median diameters of less than 1 ~. Since the concentration of the clay aerosol was of the order of 105 per cc., and concentrations of the other aerosols never exceeded l06 per cc., the assumption of independent scattering may be justified. The third question concerns the photography. It has been shown that, to ensure an accurate analysis from film negatives, exposure conditions
~¥[EASUREMENT OF AEROSOL ELECTRIFICATION
75
(light intensity and time) should be chosen so that the optical density of the negative is a linear function of the incident light. Proper choice of exposure was determined by calibration of the film according to the procedure recommended in a publication of the Eastman Kodak Company (17), and particular care was exercised to avoid any overexposure of the negatives showing aerosol streams dispersed in the electric field. RESULTS
The behavior of a stream of tobacco smoke particles in the absence of an electric field is shown in Fig. 2A, and the flow pattern of the same aerosol in a horizontal field of 5,000 volts per centimeter is shown in Fig. 2B. The electrode and high-voltage source connections were such that the light area to the left of center in Fig. 2B represents positive particles, and that to the left, negative particles. The central line represents neutral particles. The relatively dark spaces between the central line and the light areas in Figure 2B suggests that the charged tobacco smoke particles at the outer boundaries of the dark spaces are carrying minimum charges. The charge will be shown later to correspond to a single electronic charge. No such dark spaces were evidenced by an aerosol containing clay particles when the range of particle size was great. Figure 3 shows the results of optical analysis of the negatives correspondlug to Figures 2A and 2B. In Fig. 3, the solid line presents the results of the optical scanning of Fig. 2A, i.e., of tobacco smoke in the absence of an electric field, and the dotted line represents the results of the scanning of Fig. 2B, i.e., of the same aerosol dispersed because of the presence of the field. Since it has been shown that secondary light scattering is negligible for the aerosols studied, and since care was exercised that the optical density of the negative for the aerosol in the presence of an electric field was a linear function of the number of particles causing the exposure, the areas under the broken curve of Fig: 3 may be taken as proportional to the number of particles scattering the light. If the total area under the dottedline curve is fixed at unity, the area under the central peak of the curve represents the fraction of neutral particles in the sample; the area under the left peak, the negative fraction; and the area under the right peak, the positive fraction. The solid-line curve is not of great importance other than to serve as a guide. However, it is well to note that if no parts of either negative were overexposed or underexposed, the area under the solid-line curve should be equal to the area under the dotted-line curve. Some of the results obtained with a number of aerosols are given in Table I. In addition to the agreement with Whytlaw-Gray and co-workers (5, 6) on the electrification of magnesium oxide, the results serve as verification of the work of others (10, l l , 12), who found that aerosols pro-
~
HINKLE~ ORR~ 5R.~ AND D A L L A V A L L E 2,00
LeO With - -
field
Without
field
1.60
1.40
§
i20
o°
I
I t
O0
i I
/ //
0,60
I /I
\
I I
t 0,60
,
\
I I ~ ;I /
0.4Q
\
/ 020
\
\
/ 0
l
// 0.40 DISTANCE
060 OSO FROM REFERENCE
\,, LO0 120POINT~ inches
1.40
FIG. 3. Optical density curves from film negatives corresponding to Figs. 2A ~nd 2B. duced with the LaMer generator were only slightly charged. The importance of the method of generation on the electrification of an aerosol has been amply demonstrated. As shown in Table I, an ammonium chloride aerosol produced with a spark discharge type of LaMer generator contained less than 5 % charged particles, whereas an aerosol made by atomizing an alcoholic solution of ammonium chloride contained over 70% charged particles. In the latter method, the high degree of electrification is not surprising, in view of the violent forces attending rupture of the liquid at the atomizer nozzle. The values of positive and negative fractions reported in Table I represent the average values for a number of samples. Deviation from the average value rarely exceed l0 %. Since the possible variation of electrification with time had not been determined, all measurements were made with freshly prepared aerosols.
MEASUREMENT OF AEROSOL ELECTRIFICATION
77
TABLEI EiectricaI Charge Datafor Various Aerosols Soon ~terGeneration
Aerosol
Method of formation
Approx. Chargedistribution average I d~am. Positive(%)/Nega-(%)tive Neutral (%) _
Tobacco . . . . . . . . . . . . . Burning Magnesium oxide ..... Burning magnesium ribbon Magnesium oxide ..... Burning magnesium ribbon Elutriation Clay . . . . . . . . . . . . . . . . . Stearie acid . . . . . . . . . . LaMer generator Stearic acid . . . . . . . . . . Low-temperature volatilization Ammonium chloride. LaMer generator Ammonium chloride. Low-temperature volatilization Ammonium chloride. Dispersed from alcoholic solution by atomizer Dispersed from alcoholic soSugar . . . . . . . . . . . . . . . . lution by atomizer
_
0.1-0.25 0.8-1.5 --
3-5 0.2 --
_
_
_
40 44 48~ 47 2
_
_
/
34 42 43~ 44 2
_
26 14 9~ 9 96 93a
3 ~
4
0.2
2
2
96 95~
0.8-1.5
40
40
20
0.8-1.5
40
39
21
Data of Whytlaw-Gray and associates (5, 6). I n general, electrified aerosols were found to contain approximately equal amounts of positive and negative particles, with occasionally a slight excess of positive particles. Th~s result is in agreement with the data of W h y t l a w - G r a y and Patterson (5), who ascribe the slight excess of positive particles to the greater mobility of negative ions. Complete analysis of the charges associated with aerosol particles entails consideration of several factors. The amount of lateral deflection of a particle, in the system described, is a function of the mass of the particle, the charge, the field strength, and the flow rate of the aerosol stream through the electrification chamber. An analysis of the electric field between cylindrical electrodes has been summarized by Harnwell (18). If the boundary conditions imposed b y the design of the apparatus and b y the experimentM technique are applied, an expression m a y be derived which relates the deflection of the particle passing through the field to the charge of the particle. The electric forces on a particle in the field surrounding cylindrical electrodes arranged parallel to one another, as in this investigation, have also been discussed b y Harnwell. Electrodes of infinite length were assumed in the theoretical analysis, however. The following derivation assumes t h a t the finite length of the electrodes will not greatly affect the analysis and t h a t the velocity of the particle will not be materially altered b y the acceleration and deceleration effects of the electric field.
78
HINKLE~ ORR, JR., AND DALLAVALLE
The force causing horizontal deflection of the particle is given by F= where qL q d ~rKo 0
q~%-q-, cose ~K~ d
[1]
= = = = =
charge per unit length of electrode, charge on the particle, distance from electrode to particle, conversion factor, and angle between the plate of the electrodes and the force vector between either electrode and the particle. From the analysis of Harnwell, qL •-Ko
-
V cos h -1 c/2b
[2]
where V = potential difference, c = separation between electrode centers, and b = electrode radius. From geometric considerations; Eq. [1] m a y be modified to give F~ = V c q . 2w
where w t V0 y0
= = = =
(Vot -
1 yo) 2 -4- @/2) 2
[3]
]n(c/2b + 1/~ V / (c/b)~. _ 4),
time vertical velocity of the particle, and vertical distance from the aerosol inlet to the plane of the electrodes.
If we assume that the viscous resistance
6~r~r~/-i force is much greater
than the inertial resistance (ma) force and equate the viscous resistance to F~, dx 67r~r dt -
Vcq [(2w)[(Vot - yo) 2 + (c/2) 2]
[4]
where ~ = viscosity of the medium, r = radius of the particle, and x = horizontal deflection of the particle. Integration of Eq. [4], together with application of the appropriate conversion factors, yields (1.6 X 10-m)Vq x =
6m~rVow
tan_l 2y~ + tan_l
[5]
-cI
where yl = vertical distance from electrodes to point of observation. The
MEASUREMENT
OF AEROSOL
79
ELECTRIFICATION
potential is measured in volts, the charge oil the particle in electrons, and the other quantities in cgs units. When the inertial resistance was considered, substitution of absolute d2x
numerical values showed that the term associated with the m d-~ force dx
was negligible in comparison to the term associated with the 6m~r ~ force. In the foregoing derivation, it should be noted that the amount of lateral deflection is also a function of the horizontal position of the particle in the aerosol stream, and that the equation is rigorous only when the width of the stream approaches the width of a single particle. Therefore, the aerosol stream's width was maintained as small as practicable (about 0.2 ram.). The minimum deflection, determined from the width of the relatively dark space between the central stream and the charged particle area in Fig. 3, corresponds to tobacco smoke particles with a maximum diameter of the order of 0.6 ~, carrying charges of one electron. According to Eq. [5], optical scanning of a negative showing the electrification of an aerosol perfectly homogeneous with respect to particle size would give optical density curves exhibiting a series of maxima and minima in the charged particle area, each peak corresponding to a different multiple charge on the particle. Therefore, the departure from this predicted peaked curve to the relatively smooth curve of Fig. 3 indicated nonhomogeneity of particle size. Calculations by Eq. [5] may be based on a median particle size, thus permitting estimations of the fractious of particles carrying multiple charges, but the accuracy of the estimation depends upon the deviation of the distribution of particle sizes from the median value. For the tobacco smoke aerosols used in the present study, it was found that the deflection from the central stream to the peak of the curve in the charged particle area corresponded to singly charged particles from 0.1, t~ to 0.25 t~ in diameter. This result, in excellent agreement with the observations of Gibbs (19) and Sinclair (16) and with other observations on the size of tobacco TABLE II Electron Charge on Charged Aerosol Particles Soon after Generation Aerosol
Method of formation
Tobacco smoke ....... Magnesium oxide ..... Clay . . . . . . . . . . . . . . . . . Stearic acid . . . . . . . . . . Ammonium chloride.. Ammonium chloride..
Burning Burning Elutriation LaMer generator LaMer generator Atomization of an alcoholic solution
Average particle diameter (microns)
Average charge
0.1-0.25 0.8-1.5 2-4 0.2 0.2 0.8-1.5
1-2 8-12 20-40 1 1 12-t5
(electrons)
80
HINKLE~ ORR~ JR.~ AND DALLAVALLE
smoke particles, is evidence that the relatively clear zone between neutral and charged particle areas on a film negative arises because the charged particles must carry at least a single electronic charge. Equation [5] has been used to calculate the average charge carried by charged particles in several different aerosols. The results are summarized in Table II. The multiple charges carried by magnesium oxide particles may be attributed to the violent reactions which occur during the combustion of magnesium ribbon. It is interesting to note that the average charge of clay particles is essentially the same value reported by Kunkel (7) for a quartz dust of corresponding particle size. The results showing that a single electronic charge is carried by the charged particles in aerosols formed with the LaMer generator confirms the work of others (11, 12). The investigation is being continued to obtain electrification data on a number of aerosol systems and to clarify the effects of electrification on aerosol behavior. I:~EFERENCES 1. DALLAVALLE~J. M., Micormeritics. Pitman Publishing Co., New York and London, 1948. 2. KNOBLAUCH,0., Z. physik. Chem. 39,225 (1901). 3. RUDGE, W. A. D., Phil. Mag. 26, 481 (1913). 4. WHITMAN,V. E., Phys. Rev. 28, 1287 (1926). 5. WHYTLAW-GRAY,R., AND PATTERSON, H. S., Smoke. Edward Arnold and Co., London, 1932. 6. PATTERSON, H. S., Wg:eTLAw-GRAv, R., AND CAWOOD,W., Proc. Roy. Soc. (London) 124A, 523 (1929). 7. KUN•EL, W. B., J. Appl. Phys. 21,820 (1950). 8. DANIEL, J. H., AND BRACKETT, F. S., Arch. Ind. Hyff. and Occupational Med. 3, 505 [1951). 9. LAMER, V. K., Proc. 1st Natl. Air Pollution Symposium. Stanford Research Institute, Stanford, California, 1949. 10. WILSON, I. B., XND LAMER, V. K., J. Ind. Hyg. Toxicol. 30,265 (1948). 11. WHITE, L., AND HILL, D. G., J. Colloid Sci. 3,251 (1948). 12. LAMER, V. K., GOYER, G., GRUEN, RUTH, AND KRUGER, JOAN, United States Atomic Energy Commission NYO-514. Technical Information Service, Oak Ridge, Tennessee, June 30, 1952. 13. KUNKEL, W. B., AND HANSEN, J. W., Rev. Sci. Instr. 21,308 (1950). 14. HOPPER, V. D., AND LABr, T. H., Proc. Roy. Soc. (London) 178A, 243 (1941). 15. GILLESPIE, W., Porton Technical Paper No. 289. Chemical Reference Experimental Establishment, Porton, Wiltshire, England, June, 1952. 16. SINCLAIR,D., Handbook on Aerosols, Chapters 7, 8. Summary Technical Report of Division 10, National Defense Research Committee. U. S. A. E. C., Washington, D. C., 1950. 17. Kodak Materials for Spectrum Analysis. Eastman Kodak Company, Rochester, New York. 18. HARNWELL,G. P., Principles of Electricity and Electromagnetism. McGraw-Hill Book Co., New York, 1938. 19. GIBBS, W. E., Clouds and Smokes. Blakiston Company, Philadelphia, 1924.
MEASUREMENT OF AEROSOL ELECTRIFICATION
71
duced with a LaMer (9) generator, for example, are relatively uncharged. Wilson and LaMer (10) found that less than 25 % of the particles comprising a glycerol aerosol were charged, even though radioactive sodium produced the nuclei for the formation of the glycerol particles. None of the particles carried as many as four electronic charges. White and Hill (11) found that aerosols produced with a LaMer generator contained particles either uncharged or carrying single electronic charges. Recently, LaMer et al. (12) found that dioctyl phthalate aerosols produced with a LaMer generator were almost devoid of charged particles. At present the data on aerosol electrification are still scanty, and more information on a variety of aerosol systems is needed. The chief obstacles to detailed studies of aerosol electrification have been the difficulties encountered in experimental methods and techniques. Whytlaw-Gray and Patterson (5) used an ultramicroscope cell containing horizontal electrodes and noted the action of the particles in the field of vision. Kunkel and Hansen (13) used a modification of the apparatus described by Hopper and Laby (14), who employed a photographic technique to measure the deflection of oil drops falling under gravity in a horizontal electric field. LaMer et al. (12) used a modified Millikan oil drop apparatus. Gillespie (15) used an instrument in which the charged particles were deposited on microscope slides and the neutral particles were collected in a thermal precipitator. The technique reported here avoids many of the experimental difficulties which accompany photographing or observing discrete particles and permits rapid evaluation of the electrification of an aerosol, but it introduces certain new difficulties. These new limitations Can be considerably minimized, however, and will be discussed in detail later. APPARATUS AND TECHNIQUE
The apparatus developed for the present work is designed to permit evaluation from the deflection of an aerosol stream. Since the aerosol stream serves as the sample, the necessity for obtaining numerous photographs in order to establish a precise sample average is removed, and the electrification of an aerosol may be determined in a single, brief experiment. A schematic diagram of the main elements of the apparatus is given in Fig. 1. The aerosol stream passing through the electrification chamber was observed and photographed through a window which was located at an angle of 45 degrees with the forward path of the light beam. Since the intensity of scattered light is greatest in the near-forward direction, observation at an angle of 45 degrees with the direction of propagation of the incident beam was accomplished at some sacrifice in scattered light intensity. Background light was virtually eliminated by the 45-degree angle
72
HINKLEj ORR~ JR.~ AND DALLAYALLE AnalyzingChamber
~,A Light Source
Camer~r'
CopperChlorideSotutlon Light S t o P 7 + ~
Light
Cylindrical Electrodel
Air $hletd
Section
AA
FIG. 1. Schematic diagram of aerosol electrification apparatus.
arrangement, however, and photographs showed excellent contrast between the image of the aerosol stream and the remainder of the film. A copper chloride solution was used to reduce the irffrared intensity of the incident light, and a slit at the entrance to the electrification chamber controlled the size of the light beam. All surfaces within the chamber and ducts were painted with an optically black paint. Previous investigators in the field of aerosol electrification have invariably used systems involving parallel plate electrodes. This construction generally requires dark-field illumination or observation at very acute angles, because small separation between the plates cannot be effected without having the edges of the plates project into the light path. The restrictions on observation angle were overcome by constructing cylindrical electrodes, located horizontally beneath the path of the incident light, and with axes at an angle of 90 degrees with the observational axis. With these modifications of electrode shape and location, a high-voltage gradient between the electrodes may be obtained without interference with the light beam. The aerosol entered through a glass tube which tapered to, and terminated as, a slit 7.5 mm long and 0.2 ram. wide just below the plane of the electrodes. This central glass tube was, in turn, surrounded by another tube through which pure air could be drawn. In operation, the air velocity in the outer tube was synchronized with the aerosol velocity so that an aerosol stream having a width of very nearly 0.2 mm. passed through the
MEASUREMENT OF AEROSOL ELECTRIFICATION
73
A
F l a . 2A. A stream of tobacco smoke in the absence of an electric field. B. A s t r e a m of tobacco smoke in an electric field of 5,000 volts per centimeter.
observational zone in the absence of an electrical field. This aerosol inlet tube was rotated so that the narrowest dimension of the stream was presented for observation or photographing. B y this means an aerosol stream was obtained containing a sufficient number of particles to result in adequate exposure of the film negative even when the particles were dispersed in the electrical field. This flow system has an additional advantage over methods in which the settling of discrete particles under gravity is measured. In the present scheme, thermal forces on the movement of the particles are negligible in comparison to forces due to the velocity of the stream, whereas unequal radiative heating in some previous systems caused spurious measurements and necessitated excessive insulation. The high-voltage source was controlled with a variable transformer so that the optimum dispersion of an electrified aerosol could be readily established. Maximum output was 20,000 volts, although it was seldom necessary to exceed 8,000 volts. Photographic negatives of the aerosol stream, such as shown in Fig. 2, were taken with the camera located a suitable distance from the camera port to obtain proper magnification and focus. A desirable feature of the photographic elements was a copying attachment, which permitted replacement of the camera with a ground-glass plate for visual observation. During a test, the image of the aerosol stream was viewed through the ground-glass plate, and when a fine, steady stream was clearly visible, the camera was switched to replace the ground-glass plate, and photographs were taken, thus giving photographs showing the change in the flow pattern of the aerosol due to the electric field. Photographic negatives were analyzed for optical density with a modified
7~
HINKLE, ORR, JR., AND DALLAYALLE
Model B Beckman Spectrophotometer. Companion negatives showing the aerosol stream with and without the presence of an electric field were placed in a sample holder adapted for use with the spectrophotometer. The optical densities of the films were then obtained by traversing each negative. Results using different portions of the negatives were reproducible in all cases.
Three possible limitations of the apparatus construction and of the experimental procedure warrant discussion. First, there is a question regarding the possible electrification of aerosol particles because of their passage through the tubes from the aerosol reservoir to the electrification chamber and through the electric field within the chamber. The charging of particles through ducts appears to be a function of the velocity of the particles (t), and since this velocity was kept at an extremely low value (of the order of 1 cm. per second), it would be unlikely that the aerosols could become charged by passage through the tubes. The charging of particles by passage through an electric field takes place when corona discharge occurs, and calculations show that approximately double the voltage gradient used in this work would have been necessary to cause significant charging. These conclusions were justified by examining an aerosol formed by exposing titanium tetrachloride to a moist atmosphere. Measurements showed that this aerosol was uncharged both before and after passage through the tubes and the electric field. The second question involves the possible effect of secondary light scattering, where the total light scattered by the aerosol stream might be less than the sum of the light scattered by the individual particles. According to Sinclair (16), particles scatter light essentially independently of one another when the distance between the particles is 10, or preferably 100, times the radius of the particle. In aerosols having a particle radius of 1 ~ and a number concentration of 106 per cc., the volume available to each particle is ~0-6 cc., the distance of particle separation is approximately the cube root of 10-6 or 10-2 co., and thus the ratio of separation to radius is 10-2: 10-4 or 100. For a 10-~ particle aerosol of the same concentration, the ratio is reduced to 10. In such an aerosol, some interference between the scattering of neighboring particles would be expected, but such high concentrations are seldom found in practice. With the exception of an elutriated clay aerosol with a median diameter of 3 to 5 tL, the aerosols used in this study exhibited median diameters of less than 1 ~. Since the concentration of the clay aerosol was of the order of 105 per cc., and concentrations of the other aerosols never exceeded l06 per cc., the assumption of independent scattering may be justified. The third question concerns the photography. It has been shown that, to ensure an accurate analysis from film negatives, exposure conditions
~¥[EASUREMENT OF AEROSOL ELECTRIFICATION
75
(light intensity and time) should be chosen so that the optical density of the negative is a linear function of the incident light. Proper choice of exposure was determined by calibration of the film according to the procedure recommended in a publication of the Eastman Kodak Company (17), and particular care was exercised to avoid any overexposure of the negatives showing aerosol streams dispersed in the electric field. RESULTS
The behavior of a stream of tobacco smoke particles in the absence of an electric field is shown in Fig. 2A, and the flow pattern of the same aerosol in a horizontal field of 5,000 volts per centimeter is shown in Fig. 2B. The electrode and high-voltage source connections were such that the light area to the left of center in Fig. 2B represents positive particles, and that to the left, negative particles. The central line represents neutral particles. The relatively dark spaces between the central line and the light areas in Figure 2B suggests that the charged tobacco smoke particles at the outer boundaries of the dark spaces are carrying minimum charges. The charge will be shown later to correspond to a single electronic charge. No such dark spaces were evidenced by an aerosol containing clay particles when the range of particle size was great. Figure 3 shows the results of optical analysis of the negatives correspondlug to Figures 2A and 2B. In Fig. 3, the solid line presents the results of the optical scanning of Fig. 2A, i.e., of tobacco smoke in the absence of an electric field, and the dotted line represents the results of the scanning of Fig. 2B, i.e., of the same aerosol dispersed because of the presence of the field. Since it has been shown that secondary light scattering is negligible for the aerosols studied, and since care was exercised that the optical density of the negative for the aerosol in the presence of an electric field was a linear function of the number of particles causing the exposure, the areas under the broken curve of Fig: 3 may be taken as proportional to the number of particles scattering the light. If the total area under the dottedline curve is fixed at unity, the area under the central peak of the curve represents the fraction of neutral particles in the sample; the area under the left peak, the negative fraction; and the area under the right peak, the positive fraction. The solid-line curve is not of great importance other than to serve as a guide. However, it is well to note that if no parts of either negative were overexposed or underexposed, the area under the solid-line curve should be equal to the area under the dotted-line curve. Some of the results obtained with a number of aerosols are given in Table I. In addition to the agreement with Whytlaw-Gray and co-workers (5, 6) on the electrification of magnesium oxide, the results serve as verification of the work of others (10, l l , 12), who found that aerosols pro-
~
HINKLE~ ORR~ 5R.~ AND D A L L A V A L L E 2,00
LeO With - -
field
Without
field
1.60
1.40
§
i20
o°
I
I t
O0
i I
/ //
0,60
I /I
\
I I
t 0,60
,
\
I I ~ ;I /
0.4Q
\
/ 020
\
\
/ 0
l
// 0.40 DISTANCE
060 OSO FROM REFERENCE
\,, LO0 120POINT~ inches
1.40
FIG. 3. Optical density curves from film negatives corresponding to Figs. 2A ~nd 2B. duced with the LaMer generator were only slightly charged. The importance of the method of generation on the electrification of an aerosol has been amply demonstrated. As shown in Table I, an ammonium chloride aerosol produced with a spark discharge type of LaMer generator contained less than 5 % charged particles, whereas an aerosol made by atomizing an alcoholic solution of ammonium chloride contained over 70% charged particles. In the latter method, the high degree of electrification is not surprising, in view of the violent forces attending rupture of the liquid at the atomizer nozzle. The values of positive and negative fractions reported in Table I represent the average values for a number of samples. Deviation from the average value rarely exceed l0 %. Since the possible variation of electrification with time had not been determined, all measurements were made with freshly prepared aerosols.
MEASUREMENT OF AEROSOL ELECTRIFICATION
77
TABLEI EiectricaI Charge Datafor Various Aerosols Soon ~terGeneration
Aerosol
Method of formation
Approx. Chargedistribution average I d~am. Positive(%)/Nega-(%)tive Neutral (%) _
Tobacco . . . . . . . . . . . . . Burning Magnesium oxide ..... Burning magnesium ribbon Magnesium oxide ..... Burning magnesium ribbon Elutriation Clay . . . . . . . . . . . . . . . . . Stearie acid . . . . . . . . . . LaMer generator Stearic acid . . . . . . . . . . Low-temperature volatilization Ammonium chloride. LaMer generator Ammonium chloride. Low-temperature volatilization Ammonium chloride. Dispersed from alcoholic solution by atomizer Dispersed from alcoholic soSugar . . . . . . . . . . . . . . . . lution by atomizer
_
0.1-0.25 0.8-1.5 --
3-5 0.2 --
_
_
_
40 44 48~ 47 2
_
_
/
34 42 43~ 44 2
_
26 14 9~ 9 96 93a
3 ~
4
0.2
2
2
96 95~
0.8-1.5
40
40
20
0.8-1.5
40
39
21
Data of Whytlaw-Gray and associates (5, 6). I n general, electrified aerosols were found to contain approximately equal amounts of positive and negative particles, with occasionally a slight excess of positive particles. Th~s result is in agreement with the data of W h y t l a w - G r a y and Patterson (5), who ascribe the slight excess of positive particles to the greater mobility of negative ions. Complete analysis of the charges associated with aerosol particles entails consideration of several factors. The amount of lateral deflection of a particle, in the system described, is a function of the mass of the particle, the charge, the field strength, and the flow rate of the aerosol stream through the electrification chamber. An analysis of the electric field between cylindrical electrodes has been summarized by Harnwell (18). If the boundary conditions imposed b y the design of the apparatus and b y the experimentM technique are applied, an expression m a y be derived which relates the deflection of the particle passing through the field to the charge of the particle. The electric forces on a particle in the field surrounding cylindrical electrodes arranged parallel to one another, as in this investigation, have also been discussed b y Harnwell. Electrodes of infinite length were assumed in the theoretical analysis, however. The following derivation assumes t h a t the finite length of the electrodes will not greatly affect the analysis and t h a t the velocity of the particle will not be materially altered b y the acceleration and deceleration effects of the electric field.
78
HINKLE~ ORR, JR., AND DALLAVALLE
The force causing horizontal deflection of the particle is given by F= where qL q d ~rKo 0
q~%-q-, cose ~K~ d
[1]
= = = = =
charge per unit length of electrode, charge on the particle, distance from electrode to particle, conversion factor, and angle between the plate of the electrodes and the force vector between either electrode and the particle. From the analysis of Harnwell, qL •-Ko
-
V cos h -1 c/2b
[2]
where V = potential difference, c = separation between electrode centers, and b = electrode radius. From geometric considerations; Eq. [1] m a y be modified to give F~ = V c q . 2w
where w t V0 y0
= = = =
(Vot -
1 yo) 2 -4- @/2) 2
[3]
]n(c/2b + 1/~ V / (c/b)~. _ 4),
time vertical velocity of the particle, and vertical distance from the aerosol inlet to the plane of the electrodes.
If we assume that the viscous resistance
6~r~r~/-i force is much greater
than the inertial resistance (ma) force and equate the viscous resistance to F~, dx 67r~r dt -
Vcq [(2w)[(Vot - yo) 2 + (c/2) 2]
[4]
where ~ = viscosity of the medium, r = radius of the particle, and x = horizontal deflection of the particle. Integration of Eq. [4], together with application of the appropriate conversion factors, yields (1.6 X 10-m)Vq x =
6m~rVow
tan_l 2y~ + tan_l
[5]
-cI
where yl = vertical distance from electrodes to point of observation. The
MEASUREMENT
OF AEROSOL
79
ELECTRIFICATION
potential is measured in volts, the charge oil the particle in electrons, and the other quantities in cgs units. When the inertial resistance was considered, substitution of absolute d2x
numerical values showed that the term associated with the m d-~ force dx
was negligible in comparison to the term associated with the 6m~r ~ force. In the foregoing derivation, it should be noted that the amount of lateral deflection is also a function of the horizontal position of the particle in the aerosol stream, and that the equation is rigorous only when the width of the stream approaches the width of a single particle. Therefore, the aerosol stream's width was maintained as small as practicable (about 0.2 ram.). The minimum deflection, determined from the width of the relatively dark space between the central stream and the charged particle area in Fig. 3, corresponds to tobacco smoke particles with a maximum diameter of the order of 0.6 ~, carrying charges of one electron. According to Eq. [5], optical scanning of a negative showing the electrification of an aerosol perfectly homogeneous with respect to particle size would give optical density curves exhibiting a series of maxima and minima in the charged particle area, each peak corresponding to a different multiple charge on the particle. Therefore, the departure from this predicted peaked curve to the relatively smooth curve of Fig. 3 indicated nonhomogeneity of particle size. Calculations by Eq. [5] may be based on a median particle size, thus permitting estimations of the fractious of particles carrying multiple charges, but the accuracy of the estimation depends upon the deviation of the distribution of particle sizes from the median value. For the tobacco smoke aerosols used in the present study, it was found that the deflection from the central stream to the peak of the curve in the charged particle area corresponded to singly charged particles from 0.1, t~ to 0.25 t~ in diameter. This result, in excellent agreement with the observations of Gibbs (19) and Sinclair (16) and with other observations on the size of tobacco TABLE II Electron Charge on Charged Aerosol Particles Soon after Generation Aerosol
Method of formation
Tobacco smoke ....... Magnesium oxide ..... Clay . . . . . . . . . . . . . . . . . Stearic acid . . . . . . . . . . Ammonium chloride.. Ammonium chloride..
Burning Burning Elutriation LaMer generator LaMer generator Atomization of an alcoholic solution
Average particle diameter (microns)
Average charge
0.1-0.25 0.8-1.5 2-4 0.2 0.2 0.8-1.5
1-2 8-12 20-40 1 1 12-t5
(electrons)
80
HINKLE~ ORR~ JR.~ AND DALLAVALLE
smoke particles, is evidence that the relatively clear zone between neutral and charged particle areas on a film negative arises because the charged particles must carry at least a single electronic charge. Equation [5] has been used to calculate the average charge carried by charged particles in several different aerosols. The results are summarized in Table II. The multiple charges carried by magnesium oxide particles may be attributed to the violent reactions which occur during the combustion of magnesium ribbon. It is interesting to note that the average charge of clay particles is essentially the same value reported by Kunkel (7) for a quartz dust of corresponding particle size. The results showing that a single electronic charge is carried by the charged particles in aerosols formed with the LaMer generator confirms the work of others (11, 12). The investigation is being continued to obtain electrification data on a number of aerosol systems and to clarify the effects of electrification on aerosol behavior. I:~EFERENCES 1. DALLAVALLE~J. M., Micormeritics. Pitman Publishing Co., New York and London, 1948. 2. KNOBLAUCH,0., Z. physik. Chem. 39,225 (1901). 3. RUDGE, W. A. D., Phil. Mag. 26, 481 (1913). 4. WHITMAN,V. E., Phys. Rev. 28, 1287 (1926). 5. WHYTLAW-GRAY,R., AND PATTERSON, H. S., Smoke. Edward Arnold and Co., London, 1932. 6. PATTERSON, H. S., Wg:eTLAw-GRAv, R., AND CAWOOD,W., Proc. Roy. Soc. (London) 124A, 523 (1929). 7. KUN•EL, W. B., J. Appl. Phys. 21,820 (1950). 8. DANIEL, J. H., AND BRACKETT, F. S., Arch. Ind. Hyff. and Occupational Med. 3, 505 [1951). 9. LAMER, V. K., Proc. 1st Natl. Air Pollution Symposium. Stanford Research Institute, Stanford, California, 1949. 10. WILSON, I. B., XND LAMER, V. K., J. Ind. Hyg. Toxicol. 30,265 (1948). 11. WHITE, L., AND HILL, D. G., J. Colloid Sci. 3,251 (1948). 12. LAMER, V. K., GOYER, G., GRUEN, RUTH, AND KRUGER, JOAN, United States Atomic Energy Commission NYO-514. Technical Information Service, Oak Ridge, Tennessee, June 30, 1952. 13. KUNKEL, W. B., AND HANSEN, J. W., Rev. Sci. Instr. 21,308 (1950). 14. HOPPER, V. D., AND LABr, T. H., Proc. Roy. Soc. (London) 178A, 243 (1941). 15. GILLESPIE, W., Porton Technical Paper No. 289. Chemical Reference Experimental Establishment, Porton, Wiltshire, England, June, 1952. 16. SINCLAIR,D., Handbook on Aerosols, Chapters 7, 8. Summary Technical Report of Division 10, National Defense Research Committee. U. S. A. E. C., Washington, D. C., 1950. 17. Kodak Materials for Spectrum Analysis. Eastman Kodak Company, Rochester, New York. 18. HARNWELL,G. P., Principles of Electricity and Electromagnetism. McGraw-Hill Book Co., New York, 1938. 19. GIBBS, W. E., Clouds and Smokes. Blakiston Company, Philadelphia, 1924.