Diffusion and thermodiffusion studies in ferrofluids with a new two-dimensional forced Rayleigh-scattering technique

In this paper, we present a new simple two-dimensional forced Rayleigh-scattering (FRS) experimental set-up for determination of the nanoparticle-diffusion coefficient (D-M) and the Soret coefficient (S-T) in colloids. For this purpose, we give a two-timescale model for the evolutions of temperature and colloid concentration (similar to that given for a former one-dimensional FRS method) and a complete description of the signals diffracted by a squared-lattice grating. Both transport properties in ferrofluids (magnetic colloids) determined with this new set-up are in good agreement with those found with samples already studied using the one-dimensional technique. This work is completed by studying new samples. Experimental results we obtained confirm and make clearer the following: (i) the strong Soret effect in ferrofluids has a nanoparticle origin and (ii) furthermore, this origin lies in the immediate surroundings of the nanoparticles (ionic or surfacted coating and dispersion liquid).