Abstract
The function of particles in many paints, plastics, and paper applications is to interact with light—either through absorption or scattering. In this chapter, we focus on light scattering. There are three ways by which individual particles scatter light—reflection, refraction, and diffraction. The relative importance of the three mechanisms of light scattering is determined by particle size, and the overall efficiency of light scattering is determined by four factors: the refractive indices of the particle and the surrounding matrix, the size of the particle and the wavelength of light to be scattered. While scattering from an isolated particle is well understood based on theoretical principles, the scattering strengths of particles within groups are more complex. This is because particles close to one another interfere with one another’s ability to scatter light, a phenomenon known as “dependent light scattering”. This reduces the scattering strength of particles in many real-life applications.
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Notes
- 1.
Magnetic monopoles were originally predicted mathematically by Dirac, but as yet have not been detected.
- 2.
This equation applies only to light that is incident normal to the surface. The relationship between reflected intensity and angle of incidence is complicated, but this does not alter our analysis.
- 3.
There are few examples of light diffraction in our everyday lives because diffraction occurs at roughly the same scale as the wavelength of light being diffracted. Visible light spans the wavelength range of roughly 390 nm–700 nm, and features at this length scale are nearly impossible to see with the naked eye.
- 4.
By contrast, the paper industry relies heavily on extender particles to provide opacity. The key difference between this and paints or plastics is that the particles in a sheet of paper are surrounded by air rather than polymer.
- 5.
While not obvious, the TiO2 particle size for optimal light scattering is also affected by the concentration of the TiO2 particles. Specifically, as concentration increases, the optimal size increases. This is discussed in more detail in Chap. 4.
- 6.
Surface-to-surface distances are of greater importance than center to center distances because dependent light scattering is controlled by overlap of the portion of the scattering volume that is outside the particle volume—that is, in the region between the surfaces. For this reason, the extent of this overlap is best correlated to the distance between the particle surfaces.
- 7.
In three dimensions, there are two packing arrangements that do this—hexagonal closest packing and cubic closest packing.
- 8.
The use of PVC to measure particle concentration is discussed in detail in Chap. 4.
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Diebold, M., Backer, S.D., Niedenzu, P.M., Hester, B.R., Vanhecke, F. (2022). Light Scattering 1—The Physics of Light Scattering. In: Pigments, Extenders, and Particles in Surface Coatings and Plastics. Springer, Cham. https://doi.org/10.1007/978-3-030-99083-1_3
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