Topics in Particle and Dispersion Science

Effective optical particle size

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If a particle is a homogeneous sphere, the effective optical size of that particle is simply the diameter of the sphere. What if a particle is nonspherical and has structural features that span a range of "sizes", such as in an aggregate? The interaction of such a particle with an EM wave combines characteristics of interaction of the wave with the features of each size. If the particle has a low refractive index, one can assume these features to interact with the wave independently of each other. In the first approximation, each feature can be represented by a sphere with the diameter equal to the characteristic size of the feature. Hence, from the perspective of light scattering, the particle itself can be represented by a collection of size-distributed spheres, as it has been done for ice crystals by, for example, Grenfell TC and Warren 1999. They showed that such representation works well assuming that the size-distributed population of spheres has a volume-to-surface ratio equal to that of the particle. This was explained on theoretical grounds by Shepelevich NV et al 2001.

However, the manifestation of interaction of the particle features with an EM wave depends on the characteristics of that interaction, which are being measured or modelled. Several studies (for example, Volten H et al 2007, West RA 1991) imply that in the case of randomly oriented aggregates, the overall size of the aggregate determines the phase function at the small scattering angles, while the size of the component particles determines the phase function at the large scattering angles and the polarization properties (see Measures of polarization and Scattering matrix) of the scattered light. Kozasa T et al 1993 confirmed these findings and strengthened the conclusion that the linear polarization of light scattered by aggregates is governed by the constituent particles, independent of the aggregate size and chemical composition. Furthermore, studies of light scattering by randomly oriented plate- or needle-like spheroids with sizes comparable to the wavelength (Zakharova NT and Mishchenko 2000) imply that the asymmetry parameter and the phase function of such highly nonspherical particles are representative of the relative size of a sphere with the same projected area as an orientation-averaged spheroid; whereas polarization properties and the optical cross sections are representative of the relative particle size corresponding to the smallest dimension of the spheroids.

The effective optical particle size also depends on the characteristic size of a nonspherical particle. For example, the attenuation of light by particles that are very small as compared to the wavelength of an EM wave and also absorb light, is dominated by absorption. The absorption of light by such particles is proportional to the particle volume because the absorption efficiency of a particle is proportional to the relative particle size in that size range (see Mie theory: Small-particle limit equations). Hence, a volume-equivalent effective optical particle size is a better choice for such particles (for example, Min M et al 2003a), although particle shape-effects may be significant in evaluating the attenuation of light (for example, Mishchenko MI 1990). For a particle that is very large as compared to the wavelength of the EM wave, the attenuation efficiency tends to a constant (equal to 2, for example, Jonasz M and Fournier 2007, pp. 30-31). Hence, an area-equivalent effective optical particle size is more appropriate for the very large particles.

CITATION: Jonasz M. 2010. Effective optical particle size (www.tpdsci.com/Tpc/EfOptPtSz.php). In: Top. Part. Disp. Sci. (www.tpdsci.com).

HISTORY: Published: 04-Dec-2010 Modified: 14-Dec-2010 Peer-reviewed: 14-Dec-2010

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