Spatiotemporally-Resolved Dynamics of Dispersing Ferrofluid Aggregates

Ferrohydrodynamics research has been approached predominantly from either numerical or basic experimental techniques. However, to date, these experimental techniques have been limited to ultrasonic point measurements or shadowgraphs due to the opacity of the ferrofluids. As a result, the complete dynamics of many ferrohydrodynamics flows have remained unexplored. In this work, Time Resolved Digital Particle Image Velocimetry (TRDPIV) is employed to fully resolve the dynamic interaction of ferrofluid aggregates with bulk nonmagnetic fluids. This topic is hydrodynamically rich, where shearing between the aggregate and bulk flow develop into the Kelvin-Helmholtz instability. Ferrofluid aggregates are mixed with fluorescent particles in order to enable visualization of the internal flow structure of the aggregate and generate quantitative velocity measurements. The TRDPIV measurements are made in a 15 mm square channel where ferrofluid retained by a 0.5 Tesla permanent magnet is studied as it disperses. The effects of both steady and pulsatile flows are quantified, as are the impact of varying the magnetic field gradients. In both steady and pulsatile flows, a recirculation region is observed within the ferrofluid, driven by the shear layer between the bulk flow and aggregate interface. The interaction of the aggregate with the flow is also governed by the aggregate height relative to that of the test section. Higher, larger aggregates are less stable, and therefore, more likely to be dispersed by the bulk flow. As the aggregate diminishes in size, it is both more stable and is less subject to shearing forces from the flow. Flow pulsatility enriches the dynamics of the flow and generates complex flow structures resulting from interaction between the aggregate and bulk flow. This work is the first to explore the rich spatiotemporal behavior of dispersing ferrofluid aggregates interacting with steady and unsteady bulk flows.