Formation and breakup dynamics of ferrofluid drops

Drop-based hydromechanics has gained extensive attention, especially drop formation and breakup, due to the wide-ranging applications and rich physical phenomena. Magnetic field as an externally active means, displays enormous potential in manipulating drops. In this work, the formation and breakup dynamics of ferrofluid drops with an upward magnetic field (UM), downward magnetic field (DM) and no magnetic field (NM) were respectively investigated and compared. A high-speed camera was used to observe the morphological evolution of ferrofluid drops during the formation at a nozzle in air. The shape evolution of the initial drops, primary drops and satellite drops was exhaustively studied. It was found that the volume, shape as well as formation frequency of ferrofluid drops could be actively controlled by an applied magnetic field. Through analyzing quantitatively the minimum neck diameter and corresponding position, the breakup process reveals complex nonlinear dynamics and exhibits finite time singularities and self-similarity, regardless of the initial conditions. Furthermore, the primary pinch-off processes both in the axial and radial directions were deeply investigated and the results unveil that the axial elongation is much faster than the radial diminution. In a word, the gravity-compensating and gravity-superimposing magnetic fields exert a negligible effect on the pinch-off mechanism, but directly determine the location of primary pinch-off point. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.