Diffusiophoretic Enhancement of Mass Transfer by Nanofluids

Observations of an enhanced mass transfer in nanofluids have led to several propositions for the underlying cause, but none of them have been clearly established. Here, we reproduce the enhancement phenomenon within a glass capillary containing fluorescein di-sodium dye solution on one side and alumina nanoparticle suspension on the other, avoiding convective interferences present earlier. The enhancement is explained by the counter-convective motion of the dye solution in response to the diffusiophoretic motion of the nanoparticles towards a higher concentration of the dye. The velocity of the dye front agrees with the theoretical estimate obtained from the diffusiophoretic velocity of alumina nanoparticles in a gradient of fluorescein di-sodium electrolyte solution. With a suitably chosen nanofluid, it should now be possible to effect the enhancement (or suppression) of mass transfer of a given solute.