Directions for determining particle size of aerosols and fine sprays


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Directions for determining particle size of aerosols and fine sprays
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Yeomans, A. H ( Alfred Henry ), 1908-
United States -- Bureau of Entomology and Plant Quarantine
U.S. Department of Agriculture, Bureau of Entomology and Plant Quarantine (Washington, D.C )
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United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant quarantine


By A. H. Yeomans, Division of Control Investigations

The best method that has been found for determining the particle size
of insecticidal aerosols and fine sprays is to deposit a sample on a glass
slide and measure the particles under a high-power microscope. This method
shows the complete range of particle sizes involved. Goodhue et al. (3, 4)
used it for measuring particle sizes of aerosols deposited by settling.
This paper describes la detail the technique as it is now used.

Other, less satisfactory, methods have been devised. Gibbs (1) used
the rate of fall of the particles, and constructed a special instrument
for timing their fall. Goodhue et al. (2) used a dye in the solution and
by employing a photoelectric photometer determined the amount of deposit
per unit of time. Other workers determined by chemical analysis the rela-
tive deposition on wires of different sizes, but- their results did not
clearly show the range in sizes. Instruments that pass a light beam through
an aerosol cloud and measure the polarization of light scattering at right
angles, or the number of spectra formed in the scattered light, are suitable
only for measuring particles smaller than 2 microns in diameter.

Preparation of Slides

Particles of relatively nonvolatile materials can be measured before
they evaporate. To prevent excessive spreading, filming, or coalescence,
the slide must be coated with an oleophobic substance that will cause the
individual droplets to maintain their convexity to some degree. Two of the
most satisfactory materials for this purpose proved to be a 1-percent alco-
holic solution of mannitan monolaurate, and a silicon product marketed under
the trade name Dri-film 9987. The slides are first immersed in a cleaning
solution, dried, then Immersed in the oleophobic coating solution, and re-
dried. When dry the slides should be lightly polished with a soft cloth.
They may be stored in ordinary slide boxes for several days before they
are used.

Particles of volatile materials, which evaporate rapidly, cannot be
measured directly, but their size can be estimated by measuring the craters
they leave at the points of contact on slides coated with magnesium oxide
0or carbon soot. It is important to apply the right thickness of coating
for the range of particle sizes anticipated. The relation between the
actual particle diameter and the central circular spot (centrum) of the
crater is illustrated in the excerpt from the report of the University of
Chicago Toxicity Laboratory, which is appended.. A dye coating of polyvinyl
acetate suggested by workers in England for the same purpose proved less
satisfactory than the magnesium oxide or carbon soot.


Deposition of Particles on Slides

A sample of an aerosol or spray cloud can be deposited on a slide
by impingement or by settling.

Deposition by impingement may be accomplished by moving the slide
through an aerosol or spray cloud, or by moving the aerosol or spray cloud
past a slide in fixed position. The velocity of movement in either case
must be adjusted to the particle-size range expected. The velocity must
be increased as the average particle size decreases. Since the deposition
is in proportion to the particle size, compensation must be made for this
factor. The Cascade and Micro Impactors developed in England, wherein an
aerosol cloud is drawn through a series of orifices to vary the velocity,
and impinged on a different slide at each orifice, is useful only with
very small particles, mostly out of the range of insecticidal aerosols and

Deposition by settling should be limited to particle-size ranges below
20 microns in diameter. It may be accomplished by two means. An aerosol
or spray cloud is released in an enclosure and allowed to settle onto slides
placed on the floor or bottom. The cloud must be mixed to be uniform, and
the aerosol or spray released in such a way as to prevent impingement on
the sides or ceiling of the enclosure. The amount released should be small
enough to prevent coalescing in the air or too heavy a deposit on the slide,
Adequate time must be allowed for all the smaller particles to settle a
distance equal to the height of the enclosure. Convection currents should
be prevented as much as possible. A second and more rapid method is to
draw the aerosol or spray cloud through an electrical precipitator in which
slides have been placed. When the machine is turned on, all particles in
the field are precipitated in a matter of seconds. Deposition by settling
results in a slide representative of the entire range of particle sizes in
the sample, with each size present in true proportion so that no adjustment
or weighting is necessary.

Determination of Particle Size

After the sample of aerosol or spray has been deposited on a slide,
it is placed under a microscope and the individual particles are measured
with an eyepiece micrometer. A mechanical stage on the microscope Is
necessary. The diameter as measured on the slide is then corrected for
the amount of spread that has taken place, and the diameter of the original
sphere is determined.

At least 200 particles should be measured, according to DalaYalle ( ).
The more homogeneous the aerosol or spray, the fewer particles need be
counted. All particles should be counted as they are seen in the field.
An accurate method is to measure all particles from one edge of a slide to
the other that pass through the micrometer scale as the slide is moved by
the mechanical stage. Under some conditions of impingement, particles of
the smaller size groups are congregated along the margin of the slide.
Measurements in such areas should be avoided.


It is sometimes useful to photograph the particles or to project them
on a screen through a microscope. Better results have been obtained, how-
ever, by measuring the particles directly as seen in the microscope. It
is often more convenient to measure in terms of the divisions of an eyepiece
micrometer, and convert these divisions into microns after the median has
been determined.

Impinged Slides.-Samples may be collected by impingement on a coated
slide by waving the slide through an aerosol or spray cloud, or by drawing
the aerosol or spray past a slide in fixed position, such as in a wind
tunnel. The slide should be nearly perpendicular to the movement of the
aerosol or spray. In either case the rate of deposition has been demonstrated
to be in ratio to the square of the diameter. This rate of deposition was
suggested by the Central Aerosol Laboratory of Columbia University and was
based on Sell's law. To compensate for the decrease in the rate of deposition
as the particle decreases in size, each diameter is multiplied by the number
of particles of that size, I/ and expressed as the percent of the total of
such products. Representative data illustrating this method are given in
table 1.

Table 1.-Representative count of aerosol particles impinged on
microscope slides

Diameter :Number of :Diameter times: Percent o .Accumulative
(scale : particles number total of percentage
divisions) : aril "* nbe column 3 : pecnt .

0.5 2 1 0.2 0.2
1.0 26 26 6.3 6.
1.5 33 49.5 11.9 18.4
2.0 82 164 39.5 57.9
2.5 34 85 20.5 78.4
3.0 17 51 12.3 90.7
3.5 4 14 3. 94.1
.0 5 20 4.8 98.9
4.5 1 4.5 1.1 99.9
Total 20 415.0

11 The diameter is used in the first power only, since the particles impinge
in ratio to D', but the mass median diameter is computed on the basis of
their volume, which is in ratio to DT; therefore, the number of particles is
multiplied by D2/D3, or by D.


The accunmlative percentages from the last column are plotted on
t.e arithmetic probability ucale in figure 1. The 50-percent point of the
line so plotted is taken as the median of the particles as they appear on
the slide. In this example the 50-percent point has a value of 2 scale
divisions, or 30 microns, as each division was predetermined to equal 15

A correction factor must be determined for each slide. The original
spherical droplet as it is impinged on the slide becomes a convex lens,
erd the extent of its spread from its original shape can be calculated by
determining the focal length of the lens so formed. This method is des-
:ribed in Porton Report 2463 (May 6), a digest of which is appended. In
the example cited the correction factor is 0.40; therefore 30 microns X
0.140 gives a median particle diameter of 12.0 microns.

Settled Slides.--The median diameter of the particles collected on a
slide by gravitational settling or by electric precipitation is determined
by calculating the volume of each particle. The diameter of the particles
is measured in microns. The volume is determined by multiplying the cube
of the corrected diameter by 7T/6,or 0.5236. The volume of the particles
of each diameter is expressed as a percentage of the total volume. Repre-
sentative data illustrating this method are given in table 2. The accumu-
lative percentages from table 2 are also plotted on the arithmetic proba-
bility scale in figure 1, and the median particle diameter is determined
to be 40.O5 microns.

Table 2.-A representative count of aerosol particles ettled
on a uicroacope slide. (Volume =1/6 D3 = 0.5236w-')

Diameter Bumber of Volume Percent of Accumulative
(mAcrons) s particles micronss) total volume : percentage

1.4 1 1 0.01 0.01
2.1 55 267 3.3 3.31
2.8 101 116 14.5 17.81
35 50 1119 14.o 31.81
2 57 2211 27.7 59.51
4.9 11 677 8.5 68.01O
5.6 20 1838 23.0 91.01
6.3 524 6.6 97.61
7.0 1_ 180 2.3 99.91
Totals 300 7978


7 -- -7-- -"_ ^ --

6 71
G x^ 'c Settled simple
tW I (Microns)i
2 5 --- -___-- -- i i -

w 4

oC 3
9 Impinged sample
A^"-^ /(Divisions of Scale)

0.1 0.20.51 2 5 10 20304050607080 90 95 989999.5 99.9
Figure 1.--Percentage of the total volume of aerosol samples below each
stated particle diameter impinged and settled on microscope slides. The
mass median diameter is determined from the 50-percent point. The correc-
tion factor for spread has been applied to the data for the settled slide
(from table 2) but not the data for the impinged slide (table 1).

Literature Cited
(1) Gibbs, W. E.
1924. Clouds and smokes. 240 pp. Philadelphia.
(2) Goodhue, Lyle D., and Sullivan, W. N.
1943. Making and testing aerosols. Amer. Assoc. Adv. Sci.,
Pub. 20: 157-162.
(3) Diamond, P. T., and Riley, R. L.
1945. Determination of particle size of liquefied gas aerosols.
U. S. Dept. Comn., 0. T. S., PB 76015.
(4) ... and Riley, R. L.
1946. Particle-size distribution in liquefied-gas aerosols. Jour.
Econ. Ent. 39: 223-226.
(5) DalaValle, J. M.
1943. Micromeritics. 428 pp. New York.
(6) May, K. R.
1942. The Cascade Impactor-an apparatus for sampling solid and
liquid particulate clouds. Brit. Commonwealth Sci. Off.
Porton Rpt. 2463, 7 pp.



Thin-coated Carbon Slides

(Excerpt by W. R. Schmltz from University of Chicago Toxicity
Laboratory, Informal Progress Report N.S. 2, i.ay 15, 1945)

A method has been described for the use of carbon-coated slides in
estimation of drop size from sprays. In order to calibrate this procedure,
which uses a thin film of carbon instead of magnesium oxide, we coated one
half of the slide and left the other half plain.

The slides were washed carefully, rinsed in 1 aerosol in water,and
dried with a clean towel. A second slide was used to cover one half of
the slide to be coatedgf-nd the two were passed through a small sooty gas
flame. When the carbon deposit was thick enough to obscure rather heavy
black type it was considered suitable. (Later comparison of slides coated
by Lt. Wilson at MI8 indicate that our slides were somewhat thinner.)
Slides prepared in this fashion have a carbon coating on one half and one
half is clean. They were exposed to a graded series of clouds of the non-
volatile dibutyl phthalate. The true drop diameters were determined from
the lens diameters on the clean surface by determination of the focal length.
The sizes of the central circular spots produced on the coated surface were
also measured. The median diameters obtained on each type of surface were
determined ,and the results are given in Table XVII.

Table XVII

Slide No. Tre mass Ratio drop diameter -
median diameter diameter of cetnrun

1 4.48 0.71
2 4.63 0.89
9.09 0.81
10.5 0.95
5 17.9 0.70
6 18.8 0.74
7 20.8 0.67
8 49.0 0.67
9 51.5 0.54

There is ample evidence of marked variation in the ratio of drop
diameter to centrum diameter of different slides,and the relation of this
ratio to increasing diameter (lMMD) is not regular, We have found no simple
method, applicable to field operations, which will ensure uniformly thick
coatings on all slides. furthermore the best optical definition occurs
at a thickness of carbon coating which is a function of the size of the
drop. If the coating is very thin it is possible to detect drops 2 microns
ia diameter but larger drops are poorly defined; conversely, small drops
are invisible on thick coatings. The diameter and appearance of the annulus
surrounding the centrum appear to be very sensitive to variations in thick-
ness of coating.


Procedure for, Measuring Spread Factor of Oil Droplets
on Oleophobic Slides
(A digest of Porton Report No. 2463)

1. Use a compound high-power microscope.

2. Use a flat mirror.

3. Remove condenser.

4. Use outside light.

5. Focus on particle, and measure and record enact diameter.

6. Set reading on fine focus adjustment at zero.

7. IIanipulate coarse focus adjustment and mirror until some distant
object (window frame) is in as sharp a focus as possible, using
the drop as a lens.

8. Then focus downwa-.rd with fine focus adjustment until the drop
is in clear focus.

9. The difference between the No. 6 reading of the fine focus
adjustment (zero) and the No. 8 reading is the focal-length change.

10. Compute f' (focal-length change)
2A (diameter of particle)

Example: The diameter of a particle covering 4 divisions in
an eyepiece micrometer (1 division = 15.4 microns)
would be 4 X 15.4 microns, or 61.6 microns. With
a focal-length change of 206 microns, the spread
factor of the.particle vould be 206/61.6 or 3-3.

With this factor of 3.3, the correction factor ( .fX/2A ) for this
drop would be 0.40 (see below for spread-factor ratios).

Spread-Fact or Ratios
ft Correction ft Correction ft Correction
2A Factor 2A Factor 2A Factor

1.48 0.650 2.0 0.48 4.0 0.375
1.50 0.57 2.1 o.47 4.8 0.3
1.55 0.55 2.2 0.46 5.0 0.34
1.60 0.54 2.3 0.45 5.5 0.33
1.65 0.53 2.6 o.44 6.0 0.32
1.70 0.52 2.65 0.43 6.8 0.31
1.75 0.51 2.8 0.42 7.0 0.30
1.80 0.50 3.1 0.41 8.0 0.29
1.95 0.49 3.3 0.40 9.0 0.28
10.0 0.27


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