Vertical magnetic field aided droplet-impact- magnetohydrodynamics of ferrofluids

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.