Sizing of Aerosol Particles by Centrifugation Dietrich Hochrainer and Paul M. Brown National Center for Atmospheric Research, Boulder, Colo. 80302
Two centrifuge-type aerosol spectrometers, one cylindrical and one conical in design, employing the rotating ring slit principle of Berner and Reichelt have been built and tested. These spectrometers precipitate aerosol particles on removable foils according to their aerodynamic diameter in centrifugal force fields up t o 5000 G . The foils, having simple geometries, are easily analyzed for size distribution of deposited aerosols. The instruments are designed to provide the clean air required in their operation without the use of external apparatus, eliminating the need for auxiliary equipment normally used and, especially, eliminating the need for rotating seals. Although both centrifuges will precipitate particles in the aerodynamic diameter range of 0.1 to 5 pm with good size resolution, the conical model will do so in a single operation. A second foil niay be placed in the conical model upon which relatively large amounts of aerosol particles are precipitated in a cumulative size distribution.
A
erosol particles may be classified according to their aerodynamic diameter, i.e., the diameter of a sphere of unit density having the same terminal velocity as the aerosol particle in a given gravitational or centrifugal force field and gas of same viscosity. For many applications, like deposition in the lung and other areas of the respiratory tract, or scavenging by water droplets in the atmosphere, the aerodynamic diameter is more significant than optically measured diameters. Instruments which subject aerosol particles to gravitational or centrifugal force fields may be divided into two general classifications: one classifies the particles in a cumulative size deposition; the other, in a discrete size deposition and can justifiably be called a spectrometer. For classifying particles using the gravitational field, a horizontal channel can be used for cumulative or discrete size separation. In crude form, the instrument, having a rectangular cross section, is situated perpendicular to the gravitational field. Aerosol is then admitted in a laminar flow over the complete cross section, and a cumulative deposition is formed along the bottom of the channel. By admitting aerosol particles in a narrow region at the upper part of the flow and clean air over the remainder of the cross section, a discrete particle size spectrum will be formed along the bottom of the channel. One limitation to this type of instrument is that the channel must be relatively long for smaller sized particles to precipitate on the bottom of the channel. Walkenhorst and Bruckmann (1966) designed such an instrument where aerosol particles are collected on a filter placed perpendicular to the gas flow. This allows the collection of particles of small sizes in a much shorter distance. The principal limitation of this type of instrument is, for extremely small particles, that it is possible to have displacement due to Brownian motion on the same order of magnitude as the precipitation displacement. To shift this limitation t o a smaller particle size, the particles can be subjected to a centrifugal force field many times that of gravity. Instruments which subject airborne particles to centrifugal 830 Environmental Science & Technology
force fields are called aerosol ccntrifuges, and literature describes instruments of this type which deposit areosol particles either cumulatively or in discrete size separation. For cuniulative size deposition, instruments have been described by Wells (1933), Goetz (1957). Goetz. Stevenson, et al. (1960), and Kast (1961). Instruments capable of discrete size separation have been described by Sawyer and Walton (1950), Keith and Derrick (1960), Hauck and Schedling (1968), and Stober and Zessack (1966). Each of the instruments incorporates the use of an inlet system where a portion of the system is nonrotating. This requires the use of rotating seals, which can be cunibersome, difficult to support, and can cause problems with heating at high rotational speeds. Instrument resolution can also be affected by the shear generated between rotating and nonrotating components. Instruments used for discrete size separation use an external system to filter the clean air supply and to control accurately the gas flow through the spectrometer. This flow must be well controlled, as precipitation and deposition of a particle of a given size depends upon the flow through the spectrometer as well as upon the centrifugal field. Berner and Reichelt (1968) introduced a new concept in their cylindrical aerosol spectrometer, which greatly simplifies the construction and operation of this type of instrument. They allowed the complete inlet system to rotate with the spectrometer, avoiding the need for rotating seals. Air or gas containing particles enter the interior of the Spectrometer through a ring slit located near the center of rotation. The air or gas exhausts through nozzles located on an outer diameter of the instrument. When the spectrometer is rotated, a pressure gradient is established between the inlet and outlet nozzles; and air or gas will flow through the spectrometer. the rate being controlled by nozzle sizes and speed of rotation. Their instrument demonstrated the ability to deposit a monodisperse polystyrene aerosol in a narrow line and to separate particles having very small differences in their Stokes’ diameters. For their “clean air,” however, they have used only room air. which places a severe limitation on using the instrument to measure the size distribution of atmospheric aerosols. To overcome this restriction, the authors have designed two centrifuge spectrometers. one cylindrical and one conical in design, having an inlet system which incorporate5 a ring slit similar to that of Berner and Reichelt. as well as a method for furnishing the clean air req~iiredin their operation. Description o j ’Instriuiwtzt
Cylindrical Centrifuge. The cylindrical centrifuge is shown schematically in Figure 1 . Gas or air containing aerosol particles enters the instrument through inlet ( I ) and through the flow-limiting nozzle (2) and the ring slit between the two center cylinders, into the centrifuge chamber (8). Gas containing aerosol particles or room air enters the cleaning head through six inlet holes (3) (only two of which are shown in Figure 1) and flows through region (4) around the body ( 5 ) . Most of the aerosol particlesare deposited on the inner surface of the outer cylinder. A portion of those particles not deposited will form a uniformly deposited background in the centrifuge chamber. The size of largest particle in the background de-
Figure 2. Cross section of conical centrifuge
Figure 1. Cross section of cylindrical centrifuge
3
5
10
SCALE (crnl 5
0
SCALE (crn)
posit is at most one-half that of the smallest size precipitated from the aerosol flow. In the centrifuge chamber, the aerosol particles entering from the ring slit move by centrifugal force into the clean air stream, and are carried downward with the gas flow. For larger particles, the movement caused by centrifugal force is dominant and they reach the cylinder wall almost at the same height as the inlet ring slit; smaller particles are carried downward with the gas flow before they reach the cylinder wall (9). Particles smaller than those collected at the lowest point of the cylinder wall (9) will leave the centrifuge with the gas flow through a ring slit formed by the conical body (10) and the outside cylinder wall. The gas leaves the centrifuge, finally, through a channel connected to six outlet nozzles (1 1). The cylinder (9) is pressed with a shrink fit into the bottom plate (12), which is mounted directly on the shaft (13) of a high-speed grinding motor. In operation, the head, held in place by a split nut, is removed to gain access to the interior of the centrifuge. Inside cylinder (9) a foil is inserted which covers the entire inner surface of the cylinder. The head is then replaced, the spectrometer is rotated at some appropriate speed, and aerosol is allowed to flow onto the top of the head. Due to the position of the outlet nozzles with respect to the inlet holes, a “self-pumping” action occurs and gas or air will flow through the centrifuge. The percentage of aerosol to clean air flow is controlled by placing a restriction (2) (Figure 1) in the center intake hole. When sampling is complete, the spectrometer is stopped and the head and foil removed. The foil can then be analyzed for particle size distribution by counting the particles at various positions using a microscope equipped with dark field illumination and a calibrated mechanical stage. The cylindrical centrifuge subjects particles of all sizes to the same centrifugal force field and flow velocities; and, for this reason, the deposition lengths get to be relatively large for the smaller sized particles. It is possible to decrease the deposition lengths for smaller sized particles by changing the centrifuge geometry from cylindrical to conical. In a conically shaped chamber formed between concentric conical surfaces with a constant volume flow, the flow velocity decreases as it moves down the chamber. Particles in the gas are subjected to a centrifugal force, which increases as they move down with the gas flow due to the increasing distance from the center of rotation. Conical Centrifuge. The conical centrifuge is shown schematically in Figure 2. Aerosol enters the instrument through intake (1) and is admitted through a flow-limiting nozzle (2)
and the ring slit between the center cylinder and the truncated cone into the centrifuge chamber. Aerosol or room air enters the cleaning head through four holes (not shown in Figure 2) between the screws on top of the instrument and flows through channel (3), around the cqlinder (S), and finally through the clean air inlet ring slit (4). Aerosol particles contained in the air flow are deposited on the inner surface of cylinder (7) as the air flows through region (3). As in the cylindrical centrifuge, the large particles are deposited on cone (6) close to the height of the inlet slit while smaller particles are carried downward for some distance before they are deposited. Particles smaller than a certain size leave the instrument with the gas flow through the outlet ring slit (10) and, finally, through the flow-limiting nozzles (12). The centrifuge is mounted on the shaft (13) of a high speed grinding motor. Before operation of the instrument, the head, mainly consisting of the parts (9,(6), and (7), is removed by loosening screws (9). A properly cut foil is put into the cone (6), and the head is fastened to the base plate by tightening the screws (9). Once in place, the foil rests on top of the lip on the outside of the ring slit (10). This slit also gives a fixed index from which particle deposition positions can be measured. Tightening the screws (9) also compresses the O-ring (11) which forms a gas seal between the aerosol flow channel, the clean air channel, and the outer air. The centrifuge is rotated at an appropriate speed. and an aerosol flow is directed onto the top of the instrument. Since the outlet nozzles are at a radius larger than the intake holes, a “self-pumping’’ action occurs and the gas or air will flow through the spectrometer. The percentage of aerosol to clean air flow through the region (3) is controlled by the restriction (2) in the intake. When sampling is completed, the foil is removed and analyzed in the same manner as the foils from the cylindrical centrifuge. Theory of Centrifugal Deposition
Under the assumption of solid body rotation of gas within the cylindrical centrifuge, the particle trajectories and the distance travelled before deposition can be exactly calculated. This has been done by Stober and Zessach (1966) and the present authors have included the Cunninghmi correction:
where 2 = distance in axial direction from slit-ring to point of
deposition (cm.). \’olume 3, Number 9, September 1969 831
F a
b f p
D X
A
viscosity of suspending gas (gm.cm.? sec.-l). total flow per unit of time (cm.' set.-'). inner radius of outer cylinder of centrifuge (cm.). radius of inner cylinder of centrifuge (cm.). = number of revolutions per unit of time of centrifuge (sec.-l). = density ofaerosol particle(gm. ern.-?. = diameter of aerosol particle (cm.). = mean free path of gas molecule (cm.). = constant in Cunningham correction. = = = =
The values of a and 6 for the cylindrical centrifuge described in this paper are 0.5 cm. and 1.0 cm., respectively. Trajectories of particles in the conical centrifuge are more difficult to predict than those in the cylindrical centrifuge. Stober and Zessack (1966) have derived a solution which has been modified by Hauck and Shedling (1968) to include the coordinant h, s a and the Cunningham correction: Dp'h
=
where D
geometrical diameter of particle (cm.). density of particle (gm. r m . 3 = viscosity of suspending gas (gm. an-'). = total gas flow through centrifuge ( ~ mset.).-' .~ = horizontal dimension of flow channel (cm.). = number of revolutions per unit time of centrifuge =
p =
7 Q a
f
(sec-1).
ho, I , = initial coordinates of particle in centrifuge chamber
(cm.).
Figure 3. Sample of 0.557-pm.polystyrene spheres taken with cylindrical centrifuge
832 Envimnmental Seienee &Technology
distance from entrance to point of particle deposition (cm.). ip = half angle of cone (deg.). A = constant in Cunningham correction. X = near free path of gas molecule (cm.). 1
=
The values of a , ho, lo, and qo for the conical centrifuge described in this paper are 0.57 cm., 0.57 cm., 2.25 cm., and 30", respectively. Performance of the Centrifuges
Calibrations. The centrifuges were calibrated by nebulizing dispersions of monodisperse, polystyrene latex particles (supplied by the Dow Chemical Co., Midland, Mich.) of known sizes. Figure 3 shows a typical deposit of 0.557-pm. diameter spheres (30 minutes sampling time) collected on a foil of black glossy paper using the cylindrical centrifuge. The band structure is a result of aggregation of the spheres prior to entering the centrifuge chamber. The lowermost line is the deposit of the single particles; the next line is the deposit of doublet spheres; the third line is the deposit of triplets, etc.; up to the eleventh line which is the deposit of cluster of eleven spheres. For the calibration (Figure 4), we measured the length from the line opposite the entrance slit to the center of the line of single particles. The upper group of calibration lines were taken with a 2.54 X cm. diameter inlet nozzle and 7.62 X lo-%cm. diameter outlet nozzles. The lower single calibration line was taken with a 1.52 X IO-* cm. diameter inlet nozzle and 2.54 X lo-* cm. diameter outlet nozzles. From the three calibrations taken using the ssme set of nozzles, it was determined through a change in the rotational speed from 3600 to 10,000 r.p.m. that the ratio of the largest to the smallest particle size collected at a given distance from the entrance slit is a constant value of about 1.3. This demonstrates that the size calibration of the centrifiler is relatively insensitive to variations it From the calibration curves of Figu
....
5.0I
j
lOpOOrpm
c
01
1
2
DISTANCE
Figure 4.
1
5 IO 20 50 FROM ENTRANCE SLIT imm)
Calibration
curves for
cylindrical
centrifuge
the distance a given particle travels before being depositecI is inversely proportional to the square of the particle diamet er, at least, in first approximation. The slight curvature of Ithe lines could possibly be predicted by applying Cunningharn's correction to Stokes' law and allowing for nonzero relat ive angular velocity between the gas and the centrifuge walls. Calibration of the conical centrifuge was carried out by 1the same technique as the cylindrical spectrometer. Figure 5 shaIWS a typical deposit of 0.557-pm. polystyrene spheres (30 minu tes sampling time). The band structure is similar to that shown in Figure 3 for the cylindrical centrifuge, the lowest line being Ithe deposit of single spheres. The principal lines are deposits of particles assembled into closepacked configuration, as WOI*Id he expected. The less dense lines, or "ghost lines," between 1the second and third principal lines and between the third a nd fourth principal lines are deposits of chain-like assemblies of three and four particles, respectively. This is in contrast to 1the triangular and tetrahedral assemblies, as might be expected.
.
Any errors in alignment of the instriiment wm lead to variation in the particle deposits as can tK seen in Figure 3 where the deposits are not straight lines arid in Figure 5 where the particle density varies along the deposit line. Figure 6 shows photographs taken of the particle traces similar to those of Figure 5 using a scann ing electron micro"...ti,.lro Cnn. scope. Referring again to Figure 5 , higher nrrlnr yu..Lc.~o sisting of more than 11 spheres are classified into discrete deposits, as can be seen in the fine structure beyond the eleventh line. In one similar foil, we have been able to distinguish 20 discrete lines. This is strong evidence of the resolution caoabilities of the centrifuge and the abse air flow within the chamber. Figure 7 shows the calibration curvr lor ~ ncomca~ r L~LIIIIfuge. Although not shown in the calibration, tests were conducted which proved that the calibration is relatively insensitive to variations in the rotational speed, as in the case of the cylindrical centrifuge. The calibration curve also agrees approximately with the curve predicted by Equation 6. T o test the efficiency of the cleaning head (Figure 21, an additional foil was placed inside cylinder (7) by disassembling the cleaning head. (This was not done for the cylindrical centrifuge since the head cannot be disassembled.) Aerosols of polystyrene spheres with different sizes down to 0.126-pm. diameter were directed onto the top of the head while the centrifuge was in operation. Then the maximum distance the aerosol travelled down the foil before deposition was measured. A summary of the results are shown in Figure 8 by the solid line. To determine the minimum-sued aerosol particle collected in the cleaning head, the curve in Figure 8 was extended (dashed line) to intersect the coordinate representing the lowest point of cylinder (7) (Figure 2) where an aerosol particle can be collected. This distance (7.5 cm.) is the height of the cylinder (S), Figure 2. The minimum size as determined by this extrapolation is about 0.035 prn. The centrifuge head may he effective at removing particles of sizes _y_
Figure 5. Sample of 0.557-pm. polystyrene spheres taken with conical centrifuge
Volume 3, Number 9, September 1969 833
Figure 7. Calibration curve for conical cent& hrge
0
20
50
SURFACE OF 0AFf
Figure 6. Photographs taken with scanning electron microscope
rn)
".
Fig,, YrPUr...UII various sized particle
a) first trace-single particles b) second trace-doublets c) third trae-triplets d) fourth tracetriplets e) fifth tracequadruplets f) sixth trae-uadruplets g) seventh trae-uintuplets h) eighth tra-xtuplets i, j, k, I)randomly selected dmter containing more than seven spheres
834 Enviroomental Science & Technology
.
1
in cleaning he
__ __ -.__
smaller than 0.035 pm. since the particle drag predicted by the Cunningham correction to Stcikes' law becomes appreciably small for these smaller sized pa rticles. The flow through the conical :entrifuge was adjusted so that approximately 99% passes t hrough the cleaning head. Thus, relatively large concentrat]ions of aerosols could he collected on a foil placed inside the head. This material is collected in a cumulative size distr ibution in the cleaning head while the discrete size distribution1 is obtained directly in the conical section (Figure 2). Flow Rate Determinations. For' both the cylindrical and conical centrifuges, the total flow im d aerosol flow rates were measured by the same technique. The aerosol flow rate was .. .. measured by inserting a press-fitten, cytmartcai plug, macnmen out of teflon with a hole drilled along its longitudinal axis, into the inlet (1) (Figures 1 and 2). A hypodermic needle was selected which tightly fit the hole drilled in the teflon plug and the flow rate versus pressure differential measured; the hypodermic needle was then inserted in the teflon plug. With the centrifuge rotating at operating wee(l(10,OOO r.p.m.), the flow through the needle was adjusted unt il the pressure was equal to atmospheric in the region betweenI the teflon plug and inlet nozzle. The flow was then directly nneasured. The total flow
..
.b..S1..~
.
was measured in a similar manner by sealing a met". . all the inlet holes. The rotating seal was between conical surface of metal and teflon or nylon. This wiU produce an error in the total flow of 4% for the cylindrical centrifuge and 10% in the conical centrifuge due to the position of the clean air intake holes. Under normal operations the pressure above these holes is atmospheric hut slightly higher during the flow measurements yielding a higher flow. The cylindrical spectrometer rotating at 10,000 r.p.m., an inlet nozzle of 2.54 X IO-% cm. diameter and outlet nozzles of 7.62 X cm. diameter produced an actual total flow of 576 cm.$ per minute with an aerosol flow of 12 c m a per minute, The conical spectrometer rotating at 10,000 r.p.m., an inlet nozzle of 2.54 X 1 0 P cm. diameter and outlet nozzles of 5.08 x cm. diameter produced an actual total flow of 1.18 liters per minute with an aerosol flow of 25.8 cm.l per minute. The calibration curves (Figures 4 and 7) along with Equations l and 2 were used to calculate the total Bow through the centrifuges as a comparison with the directly measured values. Using the data of the larger particles the calculated flow was greater than the flow calculated using the data of the smaller particles. In general, the calculated flow is greater than the directly measured values hut does tend toward the measured ~. .. values as the particle sizes decrease. This discrepancy can be quantitatively explained by the fact that solid body rotation is better approximated in the region of deposition ofthe smaller sized particles.
The aerosol flow rates for the cylindrical and conical centrifuges may appear to be rather low compared to other instruments used to precipitate airborne particles by centrifugal force. In comparing different instruments used to precipitate airborne particles within a given size range by centrifugal force, a parameter of great interest is the number density of particles precipitated per unit of time. Instruments which precipitate particles in a cumulative distribution have a larger total aerosol flow than spectrometers such as ours which precipitate particles in a discrete size distribution, provided the total gas flow is comparable through both types. In instruments of similar dimensions and comparable flow rates, the number density of a monodisperse aerosol precipitated per unit time is the same for both types of instruments; for the instrument precipitating particles in the discrete size distribution, however, the area of deposition is smaller by the ratio of the aerosol flow rates than the deposit in the instrument precipitating particles in a cumulative distribution. For the instruments described in this paper, a 0.557-pm. polystyrene aerosol is deposited over an area of 63 mm. * in the cylindrical centrifuge and over an area of 69 mm. * in the conical centrifuge at the volume flow rates and rotational speeds given above. In contrast, the instrument described by Goetz, Stevenson, et al. (1960) operated under similar conditions but with a total aerosol flow of 2.5 liters per minute, will precipitate the same sized aerosol over an area of 1500 mm.2. In the case of a polydisperse aerosol, an instrument yielding a cumulative size distribution is a disadvantage if the deposition is to be analyzed for particle size distribution because of the difficulty of analyzing large particles in the presence of a large number of smaller particles. Application to Measurement of Aerodynamic Diameter
Any instrument, such as the ones described in this paper, which precipitates particles in a centrifugal force field can be used to measure the aerodynamic diameter of aerosol particles. As can be seen in the photographs (Figures 3 and 5 ) , the aerodynamic diameter of aggregates of various numbers of single particles can be determined. The ratio of the aerodynamic diameter of an aggregate of spheres of the same diameter to the diameter of a single sphere in the aggregate is shown in Table I. These values agree with those of Stober, Berner, et al. (1969) which were compiled from the experimental results of several investigators. Greater significance is placed on the values for the ratio of the aerodynamic diameters of doublets and triplets to single particles because these are the averages of many measurements taken with different particle sizes and rotational speeds. No significant dependence in the aero-
Table I. Ratio of Aerodynamic Diameter of an Aggregate of Spheres of the Same Diameter to the Diameter of a Single Sphere in the Aggregate 1
2 3 4 5 6 7 8 9 10 11
1.185 1.336 1.45 1.53 1.65 1.73 1.80 1.87 1.92 1.97
1.190 1.335 1.44 1.57 1.65 1.71 1.80 1.85 1.93 2.00
dynamic diameter ratios upon the centrifugal force field or particle size was found. Acknowledgment The authors are indebted to H. Paul Geisert for his skilled machining of the spectrometers and the preliminary scanning electron microscope work done by Jack Hutchinson, International Business Machines, Boulder, Colo. Literature Cited Berner, A., Reichelt, H., Staub28, 158 (1968). Goetz, A., Geojis. Pura Appl. 36,49-69 (1957). Goetz, A., Stevenson, H. J. R., Preining, O., J. Air Pollution Control Assoc. 10, 378-414 (1960). Hauck, H., Schedling, J. A., Staub28,18-21 (1968). Kast, W., Staub 21,215-23 (1961). Keith, C. H., Derrick, J. C., J . Coll. Sci. 15, 340-56 (1960). Sawyer, K. F., Walton, W. H., J . Sci. Instr. 27,272-6 (1950). Stober. W.. Berner., A.., Blaschke. R., J . Coll. Interf. Sci. 29. 710-19 (1969). Stober, W., Zessack, V., Zentralblatt fiir biol. Aerosolforschung, 13,263-81 (1966). Walkenhorst, W., Bruckmann, E., Staub 26,221-5 (1966). Wells, W. F., Am. J . Public Health 23,58-9 (1933). Receiced for reciew December 16, 1968. Accepted June 9, 1969. This study was supported, in part, by the National Science Foundation in connection with its contract with the National Center for Atmospheric Research. D. H. was supported by a Public Health Sercice International Postdoctoral Research Fellowdiip (No. I F05-TW-1090-01). Presented in part at the Dicision of Colloid and Surface Chemistry, 156th meeting, A C S , Atlantic City, N . J., September 1968.
END OF SYMPOSIUM
Correction
CHEMISTRY OF MANGANESE IN LAKE MENDOTA, WISCONSIN In this article by J. J. Delfino and G. F. Lee [ENVIRON. SCI. TECHNOL. 2, 1094 (1968)], in Table 11, footnote c should read 1.35 x 10-7.
Volume 3, Number 9, September 1969 835
