Chain-like structures in polydisperse ferrofluids

An influence of polydispersity of ferrofluids on structure of internal chain-like aggregates is studied theoretically. For maximal simplification of analysis we consider a bidisperse model of ferrofluid consisting of relatively big and small particles. In accordance with the composition of many real ferrofluids, we suppose that the energy of magnetic interaction between big particles strongly exceeds thermal energy kT of the system; the energy of interaction between big and small particles is about kT; between two small particles it is significantly smaller than kT. This gives us ground to consider the model of chains, consisting of only big particles, surrounded by the gas of small ones. Both magnetic and steric interactions of the chains with this gas are taken into account. The analysis shows that the magnetic interaction decreases the length of the chains, whereas steric interaction increases it. The final influence of the gas of small particles on the chain is a result of competition of these interactions. Our estimates show that under the increase of hydrodynamical volume concentration of the small particles from zero till 30% the chains can vary several-fold in mean size.