Mie scattering

When the particle diameter is of the order of the wavelength of incident light, the dominant effect is seen to arise from the wave nature of radiation. Quantities affected include wavelength of the scattered light (or frequency shift), changes in intensity with direction, phase, and polarization. An example of the first variety is seen in laser Doppler velocimetry, module 3. Large particles glowing in a sheet of light are utilized in particle image velocimetry (module 3), but will show strong directionality in light scattering as the particle size is reduced. Changes in the attribute of scattered light can be used for the measurement of particle size or particle velocity. An example considered in article 3 studies dispersion of liquid water into droplets, smaller droplets scattering less light when compared to larger ones. Depending on the extent of dispersion of the liquid phase, droplets may coalesce and produce larger ones that scatter strongly. Thus, the intensity pattern is now jointly developed by

• particle sizes created by atomization of the liquid and
• coalescence of droplets in the jet as determined by jet dispersion and hence the inter-drop spacing.

Scattered light under the circumstances is highly directional as sketched in Figure 7.4 below for a single drop of water. In mathematical terms, Mie scattering is entirely predictable from the electromagnetic field equations.

Figure 7.4: Mie scattering from a water droplet indicating a strong dependence of scattered light intensity on angle (from Mayinger, 1994). The polar intensity distribution depends on the shape and texture of the particle and also the ratio of refractive index of the particle with respect to its surrounding medium.