Diffusion and thermodiffusion studies in ferrofluids with a new
two-dimensional forced Rayleigh-scattering technique
G.
Demouchy, A. Mezulis, A. Bee, D. Talbot, J. C. Bacri and A. Bourdon
In this paper, we present a new simple two-dimensional forced Rayleigh-scattering
(FRS) experimental set-up for determination of the nanoparticle-diffusion
coefficient (DM) and the Soret coefficient (ST ) 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).