- Particle size (general)
The particle size of a
sphericalobject can be unambiguously and quantitatively defined by its diameter.
However, a typical material object is likely to be irregular in
shapeand non-spherical. The above quantitative definition of "particle size" cannot be applied to non-spherical particles. There are several ways of extending the above quantitative definition, so that a definition is obtained that also applies to non-spherical particles. Existing definitions are based on replacing a given particle with an imaginary spherethat has one of the properties identical with the particle.
*Volume based particle size equals the diameter of the sphere that has same volume as a given particle.
*Weight based particle size equals the diameter of the sphere that has same weight as a given particle.
*Area based particle size equals the diameter of the sphere that has the same
surfacearea as a given particle.
Hydrodynamicor aerodynamicparticle size equals the diameter of the sphere that has the same drag coefficient as a given particle.
Another complexity in defining "particle size" appears for particles with sizes below a
micrometre. When particle becomes that small, thickness of interfacelayer becomes comparable with the particle size. As a result, position of the particle surface becomes uncertain. There is convention for placing this imaginary surface at certain position suggested by Gibbs and presented in many books on Interface and Colloid Science[ Lyklema, J. “Fundamentals of Interface and Colloid Science”, vol.2, page.3.208, 1995] , [Hunter, R.J. "Foundations of Colloid Science", Oxford University Press, 1989] , [Dukhin, S.S. & Derjaguin, B.V. "Electrokinetic Phenomena", J.Willey and Sons, 1974] , [Russel, W.B., Saville, D.A. and Schowalter, W.R. “Colloidal Dispersions”, Cambridge University Press,1989] , [Kruyt, H.R. “Colloid Science”, Elsevier: Volume 1, Irreversible systems, (1952)] , [Dukhin, A.S. and Goetz, P.J. "Ultrasound for characterizing colloids", Elsevier, 2002] .
Definition of the particle size for an ensemble (collection) of particles presents another problem. Real systems are practically always
polydisperse, whch means that the particles in an ensemble have different sizes. The notion of particle size distributionreflects this polydispersity. There is often a need of a certain average particle size for the ensemble of particles. There are several different ways of defining such a particle size.
*There is an International Standard on presenting various characteristic particle sizes [ISO Standard 9276-5 "Representation of results of particle size analysis" (2004)] . This set of various average sizes includes median size, geometric mean size, average size.
There are several methods for measuring particle size. Some of them are based on
light, other on ultrasound, or electric field, or gravity, or centrifugation. They are briefly described in the section particle size distribution.
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