Impingement of ferrofluid droplets on superamphiphobic surfaces under magnetic fields

Ferrofluids are novel functional materials that have extensively been exploited in fluid-based management and control systems, and thus underlying the dynamic interactions between ferrofluid and solid under various conditions is rather essential. Herein, we report an experimental investigation on the impingement of millimeter-sized ferrofluid droplets on superamphiphobic surfaces. Comparing to droplet impact under no magnetic field, the employment of a non-uniform vertical magnetic field does not change the types of impact phenomena but alters their transitional boundaries and affects droplet dynamic behaviors. At low We, the magnetic force couples with hydrodynamic forces to slightly enlarge the upper threshold for complete rebound, while the enhanced droplet-surface adhesion prolongs the contact time and decreases the restitution coefficient of bouncing droplets; at intermediate We, the promoting effect of the magnetic field on partial rebound was also identified due to the strong droplet-surface adhesion and the additional magnetic force; at high We, a decrease in the damping coefficient and spring constant of the post-impact droplet oscillations emerges if a magnetic field is applied, which is attributed to the volume and shape effects and well explained by simple scaling analyses. We also demonstrate that upward jets are still stimulated in ferrofluid droplet impacts, but they follow scaling laws distinct from simple liquids. Regardless of whether a magnetic field is applied, ferrofluid droplet spreading is mainly dominated by capillary and inertial forces, and such a conclusion can only be drawn when the characteristic length scale for droplet inertia is correctly chosen for data analyses.