Topics in Particle and Dispersion Science

Particle size

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Particle size, also referred to as particle diameter, is an effective or equivalent size of a small particle (see also Effective optical particle size). It is unambiguously defined with a single value only for spherical and cubical particles. For other particle shapes, a number of definitions of the particle size is in use, most resulting from various methods of measuring the particle size. Key particle size measures include:

• equivalent spherical diameter (ESD)
  diameter of a sphere with volume equal to that of the particle, used in the electrical resistance measurement (ERM) method, for example, Kachel (1982)
• equivalent circular diameter (ECD)
  diameter of a circle with area equal to that of the projection of the particle at a plane; relevant for imaging methods of particle characterization used in microscopy. The ECD is also referred to as the Heywood diameter (for example, Xu and Di Guida 2003).
• equivalent hydrodynamic diameter (EHD)
  diameter of a sphere having hydrodynamic (aerodynamic) drag equal to that of the particle (Stokes' diameter), relevant to sedimentation methods and dynamic light scattering

The various definitions of the particle size can be formally related (for example, ESD and EHD: Hudson PK et al 2007, Hand JL and Kreidenweiss 2002, see also Hinds WC 1999). Such relationships, however, involve particle shape factors and may be difficult to implement for irregularly shaped particles.

In particle sizing methods that are based on light scattering or attenuation, the ECD is relevant for particles much larger than the wavelength of light in the surrounding medium, while the ESD is relevant for the smaller particles.

Other measures of the particle size are also in use, as discussed by Bowen (2002) and Jillavenkatesa et al (2001). These measures include variations of the spherical and circular diameters defined above, as well as, the Feret and Martin diameters, used widely in imaging methods of particle characterization, and the Sauter diameter, which has the meaning of an effective particle diameter in light scattering-based methods of particle sizing:

• Feret diameter
  mean value of the distance between pairs of parallel lines tangent to the two-dimensional outline of a particle shape; note that the maximum value of the Feret diameter is frequently quoted as the Feret diameter without qualification.
• Martin diameter
  mean chord length of the two-dimensional outline of a particle shape
• Sauter diameter
  also referred to as D₃₂ (for example, Nefedov AP et al 1997) and volume-surface diameter (Bowen P 2002), is a ratio of the third to the second moment of the frequency particle size distribution, n(D), as shown in Eq. 1 of Effective optical particle size

Hentschel ML and Page (2003) and Kindratenko V (1997), among others, discuss some of these and derived particle size parameters, such as the various measures of nonsphericity. Kindratenko V (1997) also reviews methods of approximating the particle shape with applications in the image analysis methods of particle size analysis.

CITATION: Jonasz M. 2006. Particle size (www.tpdsci.com/Tpc/PtSz.php). In: Top. Part. Disp. Sci. (www.tpdsci.com).

HISTORY: Published: 16-Jan-2006 Modified: 20-Jun-2008 Reviewed: PENDING

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