Controlling lateral nanomixing and velocity profile of dilute ferrofluid capillary flows in uniform stationary, oscillating and rotating magnetic fields

Abstract

The influence of magnetic-field dependent viscosity (rotational viscosity) on molecular transport of species in dilute ferrofluids has been studied. For this purpose, a Taylor dispersion test in a capillary tube has been performed while suspended magnetic nanoparticles (MNPs) are subjected to both magnetic field and low Re shear flow field. Axial dispersion has been quantified from residence time distributions (RTDs) and tracer injection tests conducted in three distinct situations where the capillary is subjected to (a) uniform transverse rotating magnetic field (TRMF), (b) uniform transverse oscillating magnetic field (TOMF), and (c) uniform axial static magnetic field (ASMF). The various types of magnetic fields have been generated in a specially designed stator energized by three phase, AC and DC currents. Results obtained from the three cases are reported in terms of axial dispersion coefficients. For TRMF, an increase in lateral mixing is observed whereas no significant effect is detected for TOMF. In ASMF, the lateral mixing mechanism is retarded by magnetically locked MNPs. Both effects under TRMF and ASMF reach a plateau as MNP concentration in the liquid is increased. These findings highlight the effect of rotational viscosity on diffusion of other species hosted in dilute ferrofluids and point to attractive applications to engineering fields where transport phenomena are central. Analysis of RTD breakthrough times enabled laminar velocity profile in capillary flow to be reconstructed. It suggests that (magnetic field-free) parabolic velocity profiles evolve towards flattened and protruded shapes, respectively, in TRMF and ASMF. These results confirm that magnetically-excited MNPs may be considered as a potentially appealing tool to mediate molecular transport phenomena at the nanoscale such as in nano/microfluidic systems.