  The study reports the aspects of postimpact hydrodynamics of ferrofluid
droplets on superhydrophobic SH surfaces in the presence of a horizontal
magnetic field. A wide gamut of dynamics was observed by varying the impact
Weber number We, the Hartmann number Ha and the magnetic field strength
manifested through the magnetic Bond number Bom. For a fixed We 60, we observed
that at moderately low Bom 300, droplet rebound off the SH surface is
suppressed. The noted We is chosen to observe various impact outcomes and to
reveal the consequent ferrohydrodynamic mechanisms. We also show that
ferrohydrodynamic interactions leads to asymmetric spreading, and the droplet
spreads preferentially in a direction orthogonal to the magnetic field lines.
We show analytically that during the retraction regime, the kinetic energy of
the droplet is distributed unequally in the transverse and longitudinal
directions due to the Lorentz force. This ultimately leads to suppression of
droplet rebound. We study the role of Bom at fixed We 60, and observed that the
liquid lamella becomes unstable at the onset of retraction phase, through
nucleation of holes, their proliferation and rupture after reaching a critical
thickness only on SH surfaces, but is absent on hydrophilic surfaces. We
propose an analytical model to predict the onset of instability at a critical
Bom. The analytical model shows that the critical Bom is a function of the
impact We, and the critical Bom decreases with increasing We. We illustrate a
phase map encompassing all the post impact ferrohydrodynamic phenomena on SH
surfaces for a wide range of We and Bom.
