Structural Anisotropy of Fractal Aggregates and Its Exhibition in Electrooptical Effects

Abstract

The first experimental examination of orientational turbidimetric effect [differential orientational spectrum of optical density δD(λ) in suspensions of fractal clusters was presented. Experiments were performed for aqueous suspensions of powdered Aerosil OX-50 aggregates ordered by the alternating electric field oriented along or across the unpolarized light beam. Experimental differential spectra δD(λ) are essentially different from the known spectra for bacterial and yeast suspensions. In particular, the points of zero effect lie in the red (but not in the blue) region of spectrum, and the |δD(λ)| dependence is a decreasing function in the range 300–800 nm. To explain these results, a theory was developed based on the three-dimensional lattice model of cluster-cluster aggregation and the optical model of interacting dipoles. This theory confirms main conclusions obtained previously in the Rayleigh–Debye–Hans approximation (Khlebtsov, N.G. and Mel'nikov, A.G., Kolloidn. Zh., 1998, vol. 60, no. 6, p. 843) and agrees with experimental δD(λ) spectra. It was shown experimentally and theoretically that the structural anisotropy of aggregates exhibited scale invariance, and the differential spectra for the aggregates with various numbers of particles are described by a single universal curve δD(kRg), where k= 2π/λ and Rgis the mean radius of gyration of aggregates.