Structural anisotropy of fractal clusters and orientational optical effects in transmitted light

Spatial orientational ordering contributes significantly to the optical eff ects in transmitted light caused by the structural anisotropy of fractal ag gregates. Dependence of linear dichroism and the turbidimetric effect of or ientationally ordered fractal clusters on the diffraction parameter kaM(1/D ) approximate to kR(g) (k is the wave number; M and a are the number and ra dius of primary particles, respectively; R-g is the radius of gyration; and D is the fractal dimension) was studied theoretically. For the generation of three-dimensional aggregates, the lattice ballistic model of cluster-clu ster aggregation was used. According to this model, the exponent of the rad ius of gyration D-beta = 1.97 (50 less than or equal to M less than or equa l to 1000) and the exponent of structural scattering factor D-s approximate to 2.04 (2 less than or equal to qR(g) less than or equal to 10). It was s hown that both orientational effects are only determined by the diffraction parameter and are virtually independent of M thus exhibiting scale invaria nce of structural anisotropy. Effective anisotropy of random ballistic aggr egates estimated in terms of the turbidimetric effect and dichroism corresp onds to anisotropy of the uniform spheroidal model with axis ratio e = 2 fo r small clusters (kR(g) less than or equal to 1-2) and decreases to e = 1.5 with an increase in diffraction parameter (1 < kR(g) < 5). The turbidimetr ic effect is characterized by the local maximum at kR(g) similar to 1, wher eas, at kR(g) similar to 2, the ordered and unordered suspensions are virtu ally indistinguishable in the transparency so that the turbidimetric effect changes the sign and is close to zero. Linear dichroism is maximal near th e point of zero turbidimetric effect kR(g) similar to 2.