Particle size (general)

Particle size is a notion introduced for comparing dimension of various small material objects. Larger objects are usually called stone, rock, a piece of something, etc. Term particle indicates small size, usually less than centimeter. Particle might be either solid, or liquid, or even gas. Liquid particles are called droplets. Gaseous particles are bubbles. All of them can be characterised in terms of size.

• Particle size of spherical object equals to its diameter.

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However, a typical material object is likely to be irregular in shape. This causes ambiguity in definition of particle size. Existing definitions are based on replacent of a given particle with imaginary sphere that has one of the properties identical with the particle.

• Volume based particle size equals to the diameter of the sphere that has same volume as a given particle;

• Weight based particle size equals to the diameter of the sphere that has same weight as a given particle;

• Area based particle size equals to the diameter of the sphere that has the same surface area as a given particle;

• Hydrodynamic particle size equals to 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 micron. When particle becomes that small, thickness of interface layer 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^[1], ^[2], ^[3],^[4],^[5], ^[6].

Definition of the ensemble particle size presents another problem. Real systems are practically always polydisperse, whch means that particles in encemble have different sizes. The notion of particle size distribution reflcts this polydispersity. There are still often a need of a certain average size for encemble. There are several different way of introducing such size.

• There is Interanation Standard on presentaing various characteristic particle sizes ^[7]. 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.

References

1. ^ Lyklema, J. “Fundamentals of Interface and Colloid Science”, vol.2, page.3.208, 1995
2. ^ Hunter, R.J. "Foundations of Colloid Science", Oxford University Press, 1989
3. ^ Dukhin, S.S. & Derjaguin, B.V. "Electrokinetic Phenomena", J.Willey and Sons, 1974
4. ^ Russel, W.B., Saville, D.A. and Schowalter, W.R. “Colloidal Dispersions”, Cambridge University Press,1989
5. ^ Kruyt, H.R. “Colloid Science”, Elsevier: Volume 1, Irreversible systems, (1952)
6. ^ Dukhin, A.S. and Goetz, P.J. "Ultrasound for characterizing colloids", Elsevier, 2002
7. ^ ISO Standard 9276-5 "Representation of results of particle size analysis" (2004)