Figure 1

Figure 2

Figure 3

“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 distri-bution 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 broad-ened distribution (Figure 3).

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 StephensLaboratory AnalystMicromeritics Analytical Serviceshttp://www.particletesting.com/

As seen in MVA Scientific Consultants “Small Things Considered”.

All Shapes and Sizes...

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