comparing two different methods on the same sample, one should not be
surprised when the results differ by a factor of two, and even larger errors
may occur. Limitations exist in the particle size range accessible, in the
maximum particle concentration, in the method of preparing the sample and
thereby in the probability of producing artifacts, etc.
What Is Measured. The methods employed vary widely in
underlying principle. Some methods count and sizeindividual particles.
Best known are the various types of microscopy applied. Figure 9.5 shows
the useful size ranges. In principle, there are few other limitations, but the
method of making preparations can introduce errors, especially in electron
microscopy. Some other methods sense individual particles in a very dilute
dispersion flowing through an opening. Particles can be sensed by scattered
light, by the change in electrical conductivity when passing through a
narrow hole (as in the Coulter counter), etc. If the relation between particle
size and signal intensity is known, a size distribution can be determined.
Other methods directly split the sample, or the particles in a sample,
into somesize classes. The paramount example is sieving, but that is only
useful for particles that are (a) large, (b) smooth, (c) fairly isometric, and (d)
fairly hard (undeformable). Something similar can be achieved by
determining sedimentation rate. By application of gravity and centrifugal
fields of various intensity, a wide range of particle sizes is accessible.
A wide variety of methods determines amacroscopic property, that is
subsequently related to particle size. The prime example is scattering of light
or other radiation, either static or dynamic. In static scattering (see also
Section 9.2), several different methods can be used, but in all of them the
time-averaged intensity of the light scattered by the particles is measured in
some way. Static scattering can be applied for a wide range of sizes, from
polymer molecules to millimeter particles, but each separate method allows
a narrower range. Dynamic scattering measures the magnitude of the
Doppler shift in wavelength due to the Brownian motion of the particles,
and the shift is thus a measure of the diffusion coefficient, and hence of
particle size. It is useful for particles up to a size of 1mm diameter.
Various methods give rise to varioustypes of size or average.In
Section 9.3.3 some types of equivalent sphere diameter were mentioned.ds
would result from a method where total surface area is determined, e.g., by
an adsorption method.dfis the result of sedimentation analysis anddeof
sieving. Some particle sensing methods, e.g., change in conductivity, yield
dv. For polydisperse spheres, the average diameter obtained would bed 32 for
a surface area–related method, and d 53 for sedimentation analysis.
Scattering methods can yield a wide variety of averages, up to order 9
(d 63 ), according to the method and the particle size range.