As seen in MVA Scientific Consultants “Small Things Considered”.
“What is the particle size of my material?” In most particle-driven
industries, this is a critical question, and it sounds like a fairly simple
one, doesn’t it? In reality, there is usually no one correct answer to
this question. While the most obvious parameter affecting the answer
is which of the many available methods for measurement is employed,
sometimes less obvious is the role that particle shape plays in the
apparent particle size of a material. Whether organic or inorganic,
pharmaceutical or mineral, the different particles of the world do
indeed come in all shapes and sizes. In fact, whether cubical, fibrous,
or infinitely complex, most particles are irregularly shaped in some
manner, and if we define particle size as the diameter of the particles
in question, how does one describe the particle size of an irregularly
shaped particle? Other than a perfect sphere, no shape has only one
specific diameter.
Since a sphere is the ideal shape for particle size characterization,
most particle sizing techniques, whether laser light scattering,
electrozone sensing, sedimentation, sieving, or other techniques,
express particle size in terms of “equivalent spherical diameter”.
That is, the question is asked: under the given set of test conditions,
the particles are behaving as spheres of what diameter? Using
this relation greatly simplifies the expression of particle size. For
example, if we take a simple cylinder, whose size cannot be expressed
with only one term, and transform it into a sphere of the same volume,
the question of how to express its diameter no longer exists since
the diameter is now constant no matter how the sphere is oriented.
The same would also hold true for the any other irregularly shaped
particles. Unfortunately, this simplification introduces a new set of
problems to the particle size analyst.
The further a given sample deviates from the ideal spherical shape,
the more difficult it becomes to accurately describe the “particle size”
of that material. For instance, when using a Micromeritics Sedigraph,
which measures the particle size of a material based on its settling
velocity in a given liquid, to analyze kaolin clay particles, one has to
consider how the plate-like structure of kaolin (Figure 1) impedes its
settling and causes it to fall more like a leaf falling to the ground than
a sphere falling through a liquid. This effect results in a finer distribution than may be expected (Figure 2). Also, when utilizing a laser
light scattering instrument, such as a Micromeritics Saturn Digisizer,
one has to consider how the orientation of irregularly shaped particles
may affect the way they scatter light. Typically, these orientations are
averaged out over the course of a measurement resulting in a broadened distribution (Figure 3).
Figure 1
Figure 2
While very powerful, no single particle size instrument can
completely and accurately characterize a material on its own. To
fully understand a given set of particle size data, it is necessary to
also understand the geometry of the sample in question, and how
the shape of the particles may affect the appearance of the data. To
this end, techniques such as light microscopy, scanning electron
microscopy or transmission electron microscopy are invaluable
in the understanding of particle shape and structure. Moreover,
techniques such as polarized light microscopy can aid in to determine
the refractive index of a material – a critical value when employing
laser light scattering particle size analysis. When armed with the
knowledge of how particle shape can affect particle size analysis,
one can utilize the various microscopy techniques offered by MVA as
well as the many particle sizing techniques offered by Micromeritics
Analytical Services as very powerful tools.
C. Mark Stephens
Laboratory Analyst
Micromeritics Analytical Services
http://www.particletesting.com/
Figure 3
Photo courtesy of MVA Scientific Consultants Inc.
All Shapes and Sizes...