PARTICLE CHARGING AT LOW PRESSURES
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puter calculations’ have been met only with limited success because
of the complexity of the charging equations involved. It is hoped that
the present study, in which the mean free path and the electric mobility
of the ions are varied over a wide range of values because of the variation in pressure, will provide data that can be used as rather severe
tests of any new charging theories.
EXPERIMENTAL APPROACH AND APPARATUS
Any experimental study of the physics of aerosols can be made, in
principle at least, on a single particle or on a cloud of particles. The
latter approach is generally preferred since the statistical nature of
the processes generally involved would render the results of many
single-particle measurements quite difficult, if not impossible, to
interpret. Furthermore, for submicron particles of interest in the
present study, single-particle measurements are quite difficult to make.
Figure 1 is a diagram of the experimental system used, More
detailed diagrams of the particle charger, small ion-mobility analyzer,
and particle-charge spectrometer are shown in Figs, 2 and 3. It should
be noted that the charging and precipitation of the particles are carried
out in two separate devices. This approach of separating the particle
charger and the precipitator (the particle-charge spectrometer) has the
advantage of permitting a more accurate determination of the particle
charge than is possible when the charger and precipitator are combined as in the conventional electrostatic precipitator, It also permits
the variables affecting the particle charge tobe more precisely defined.
AEROSOL GENERATION AND NEUTRALIZATION
The aerosols used in these experiments were solid, spherical
particles of dye obtained by atomizing solutions of the dye and drying
the liquid droplets formed. Commercial-grade methylene blue dye was
used in all the tests with the exception of two in which uranine was
used. Uranine is somewhat hygroscopic and cannot be dried as readily
as methylene blue. However, the fact that very minute quantities of
uranine can be measured accurately by means of fluorescence tech-
niques? has made it a desirable material to use when the mass of
aerosol collected must be determined.
Aerosols produced by atomization processes have been found to
carry a very substantial electrical charge. Since it is desirable that
the aerosols be neutral initially, provision must be made to remove
these initial electrical charges. The usual procedure of neutralizing
an aerosol is to mix the aerosol stream with a mixture of ions of both
plus and minus signs. A sonic-jet bipolar ion generator (described in