Revisiting independent versus dependent scattering regimes in suspensions or aggregates of spherical particles

Independent scattering refers to situations when particles are sufficiently distant that some of the radiation characteristics of particle systems can be determined by adding the contributions of each particle. When particles are in close proximity, however, dependent scattering prevails and is affected by near-field interactions and far-field interferences among scattered waves from nearby particles. The dimensionless parameters governing the scattering cross-section and asymmetry factor of non-absorbing bispheres, disordered and ordered suspensions and aggregates with up to 8 spherical particles were found to be the particle size parameter chi(s), the relative index of refraction m, the interparticle distance-to-wavelength ratio d*, and the number of particles. Here, chi(s) ranged from 0.031 to 8.05, m varied from 0.667 to 2.6, and d* reached up to 30. Dependent effects were observed in aggregates with particles of all sizes and were strongly affected by the relative index of refraction in particle systems with chi(s) >= 2 due to large phase shifts across the particles. Moreover, new criteria for the transition between the dependent and independent scattering regimes for the scattering cross-section and the asymmetry factor were derived. For the scattering cross-section of structures with a narrow interparticle distance distribution, the independent scattering regime prevailed when the average interparticle distance-to-wavelength ratio d* exceeded (i) 2 for particles with chi(s) <= 2 and (ii) 5 for particles with chi(s) >= 2. For the asymmetry factor, the transition from the dependent to the independent regimes for particles with chi(s) <= 2 was achieved for d* as high as 25. These transition criteria could be extended to particle systems with a broad interparticle distance distribution when based on the minimum interparticle distance-to-wavelength ratio. Finally, the relative index of refraction m did not affect these transition criteria. (C) 2020 Elsevier Ltd. All rights reserved.