Anomalous diffusion in microchannel under magnetic field

We have performed experiments to characterize the diffusion of an aqueous ferrofluid in water submitted to a magnetic field. Experiments were carried out in a microfluidic device to take advantage of the low Reynolds number flow conditions at the microscale. We have measured the concentration profile across the microchannel, which defines a characteristic length of the diffusion zone. This characteristic length varies as the square-root of the distance from the entrance of the channel divided by the mean velocity, which evidences a diffusive regime. However the application of a magnetic field is shown to inhibit the diffusion, with an increasing efficiency as the field intensity increases. We propose an explanation of this effect based on the anisotropy of the diffusion coefficient due to the magnetic field. This hypothesis is corroborated by numerical simulations. (C) 2010 Published by Elsevier Ltd