  Magnetic fields and magnetic materials have promising microfluidic
applications. For example, magnetic micro-convection can enhance mixing
considerably. However, previous studies have not explained increased effective
diffusion during this phenomenon. Here we show that enhanced interface smearing
comes from a gravity induced convective motion within a thin microfluidic
channel, caused by a small density difference between miscible magnetic and
non-magnetic fluids. This motion resembles diffusive behavior and can be
described with an effective diffusion coefficient. We explain this with a
theoretical model, based on a dimensionless gravitational Rayleigh number, and
verify it by numerical simulations and experiments with different cell
thicknesses. Results indicate the applicability and limitations for
microfluidic applications of other colloidal systems. Residual magnetic
micro-convection follows earlier predictions.
