Field-induced self-assembled ferrofluid aggregation in pulsatile flow

Ferrofluid aggregation and dispersion occurs at several length scales in pulsatile flow applications, e.g., in ferrofluidic pumps, valves, and biomedical applications such as magnetic drug targeting. Because of a yet limited understanding, ferrohydrodynamic investigations involving laboratory-scale studies in idealized geometries are of considerable use. We have injected a ferrofluid into a pulsatile host flow and produced field-induced dissolution (aggregation) using external magnets. A comparison is made with ferrofluid aggregation in a steady flow. Subsequently, the accumulation and dispersion of the ferrofluid aggregates in pulsatile flow are characterized by analyzing their size, mean position, and the flow frequency spectrum. The maximum aggregate size Amax, time to form it tmax, and the aggregate half-life thalf are found to scale according to the relations Amax∝Re−0.71, tmax∝Re−2.1, and thalf∝Re−2.2. While the experiments are conducted at a macroscopic length scale for useful experimental resolution, the results also enable an understanding of the micro- and mesoscale field-assisted self-assembly of magnetic nanoparticles.