Abstract:
In this present article, we have investigated the magnetohydrodynamics of ferrofluid droplets impacting on
different wettability surfaces, in the presence of a vertical magnetic field. The spreading dynamics was studied
for a wide spectrum of magnetic Bond number (Bom), Hartmann number (Ha) and Weber number (We). In
absence of any magnetic field, the droplets exhibited secondary droplet pinch-off during rebound. In the presence
of magnetic field, the field modulated Rayleigh-Plateau instability delays the droplet pinch off as Bom increases.
For a fixed We, the rebound of the droplet was suppressed on a superhydrophobic (SH) surface for varying the
magnetic field strength (manifested through Bom). An analytical model based on the principle of conservation of
energy was formulated to explain the magnetic field modulated droplet pinch-off. We have also investigated the
influence of Bom on the temporal spreading dynamics of different Ha ferrofluid droplets impacting on hydrophilic
and SH surfaces. With an increase in Bom, the magneto-visco-capillarity of high Ha droplets inhibits the capillary
waves and motion of the contact line after impingement onto hydrophilic surface, compared to low Ha ferrofluid
drops. Instead, the droplet rim fragments into secondary droplets during retraction event, leading to the onset of
rim instability for low Ha ferrofluid drops with an increase in the magnetic field strength. We have also proposed
a theoretical formulation based on energy conservation principle to predict the experimentally measured
maximum spreading diameters under influence of magnetic field. The findings may hold significance in ferrofluid
based droplet microfluidics systems with magnetic control or actuation.
