(268f) Enhanced Mass Transfer in Colloidal Systems Is Due to Diffusiophoresis

Nanofluids (colloidal suspensions) are known to enhance the apparent diffusivity of a solute---a phenomenon known as enhanced mass transfer similar to the enhancement of heat transfer (or thermal conductivity). But there are situations where the mere presence of colloidal particles do not show any appreciable change in the apparent diffusivity. For long the underlying mechanism has not been understood, and these contrasting behaviours have not been explained. We show that the enhancement is explained by the counter-convective motion of the solute solution in response to the diffusiophoretic motion of the colloidal particle towards a higher concentration of the of the solute with which it has an attractive interaction. By performing experiments in a controlled environment, we not only show the enhancement but also validate our hypothesis by independent measurements of the diffusiophoretic velocity. We also argue, within the same framework, why in certain cases there is no significant enhancement.