Evolution of the optical transmission of a ferrofluid after magnetic field commutation is analyzed by means of an approach based on the so-called mixture laws: expressions which predict the effective permittivity of heterogeneous media as a function of their constituents' permittivities, their proportions and the way they are arranged. In particular, this work is based on a law proposed by Sihvola and Kong for the effective permittivity of a host substance with ellipsoidal inclusions. Ferrofluids are peculiar examples of this kind of media: with the solvent as host, the inclusions are nanoparticle agglomerates whose shapes become modified by magnetic field exposure. In this work, experimental optical transmission of a ferrofluid is compared with predictions based on Sihvola&Kong's law. A remarkable coincidence is obtained both in absence of magnetic field, without using any fitting parameter, and in presence of magnetic field, employing the inclusions' average ellipticity as fitting parameter. The results obtained for time dependent optical transmission of a ferrofluid after magnetic field switch on or off allow one to estimate how the average shape of the agglomerates evolves along time. On the other hand, mixture laws are proven to be an interesting alternative to scattering concepts to model the optical transmission changes experienced by ferrofluids once they are exposed to magnetic fields.

A numerical method to simulate the ferrofluid particle distribution evolution is presented. Also, the optical transmission of the distributions obtained is calculated by two numerical methods. The first one consists on a numerical propagation of an electromagnetic wave through the sample. The second one analyzes the aggregates' mean length to obtain the optical transmission through a mixture law. As an illustration of the possibilities of the method developed, it is applied to analyze how ferrofluid optical transmission changes after magnetic field application depend on intrinsic properties of the colloid such as its nanoparticle concentration and surfactant repulsion represented by means of the final distances between consecutive particles forming chains. Changes in the attenuation factor of these samples show the trends expected from the Literature.

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A method to calculate the optical transmission evolution of a ferrofluid after exposure to an external magnetic field is proposed. In the first part of this work, a simulation program is employed to simulate the nanoparticle rearrangement for different particle concentrations and different magnetic field intensities. In the second part, the simulated particle distributions' optical response is determined by means of a mixture law, which avoids typical huge computational times associated to this stage of the calculation. Experimental and simulated results with both parallel and perpendicular magnetic field orientations with respect to the incident light are compared. A significant correlation is obtained, which proves the usefulness of the proposed method in order to obtain the optical transmission evolution of a ferrofluid.