PROXIMITY EFFECTS IN CLUSTERS OF PARTICLES

When two spheres are brought together, there is an enhancement in the orientation-averaged extinction per unit volume compared to isolated s pheres; ''proximity effects'' such as this are expected to be most imp ortant at close separations. Using finite element methods for a pair o f nearby Rayleigh spheres, this is both verified and quantified for th e first time. The results are also expressed in terms of a normal mode analysis. As more spheres are added to the system there is a further growth in the extinction enhancement, which, however, depends on the t opology. Arrangements of touching spheres examined included stacks, cl ose packings, radiating structures, rings, chains, and clusters. Effec tive medium theory models are often used to describe composite or poro us particles. It is shown that with these models the extinction predic ted is significantly higher than with finite element treatment of roug hly equivalent particles. Furthermore, the effects of topology, not ac counted for explicitly in effective medium theory models, are found to be important. The modified prescription of effective medium theory pr oposed by Ossenkopf does not provide better agreement with finite elem ent methods unless the filling factor is high, higher than typical of even compact clusters. Comparison between finite element methods and e ffective medium theory models in the case of a laboratory soot sample with high extinction yields a paradox that is discussed briefly.