Giant effective liquid-self diffusion in stagnant liquids by magnetic nanomixing

Many chemical engineering applications require tools to intensify processes in regions where Fickian molecular diffusion is the dominant mechanism, such as in boundary layers, microporous catalysts or microfluidics. We demonstrate in this study that spinning magnetic nanoparticles (MNPs) by means of rotating magnetic fields (RMFs) gives rise to an intriguing nanomotion mechanism capable of triggering giant diffusion in stagnant liquids and thus able to stimulate transport beyond the molecular diffusion barrier especially in stagnant liquids. To evidence this mechanism, we report original water self-diffusion coefficients measured in aqueous media containing very low concentrations of ferrite MNPs that can be rotated in uniform RMF. The self-diffusion coefficient of distilled water (i.e., D0 ≈ 3.5 × 10−9 m2/s) was enhanced up to 200 times by application of a rotating magnetic field in stagnant-liquid conditions. It was concluded that in absence of macroscopic convective flows, MNPs may prove to be efficient nanostirrers to enhance liquid transport properties at nanoscale. By delivering giant diffusion around them, rotating MNPs can constitute an appealing nanomixing process intensification tool.