Chain aggregate structure and magnetic birefringence in polydisperse ferrofluids.

The theory of particle association in chains in dilute ferrofluids and dipole fluids is generalized to the case of polydisperse systems. The chains could be formed by ferroparticles of different sizes, so various types of chain aggregates are considered. The probabilities of chain structure appearance are calculated, and the phase diagram, allowing to find the most probable structure with only the continuous particle size distribution known, is built. Our results demonstrate that in spite of a very weak dipole-dipole interaction between the small size fraction particles, their presence exerts a decisive influence on the ferrofluid microstructure. The chain shortening caused by the small particles sticking to the edges of chains formed by large particles is discovered theoretically. The latter effect is proved to exist by the recent computer simulations on bidisperse ferrofluid makeup. The application of the developed model to the description of magnetic birefringence phenomena in weak external magnetic fields shows a very good agreement with experimental data.