Abstract
The ferrofluid droplets on hydrophobic surfaces are of interest in biomedicine, sensors, and oil-water separation. Ferro-fluid droplet impact on a hydrophobic surface under a magnetic influence is investigated. The effect of ferro-particle concentrations on the droplet behavior over the impacted surface is explored and the mechanisms involving droplet spreading, retraction, and break-off in the rebounding cycles are examined. The findings demonstrate that the ferrofluid droplet spreading factor reduces about 16–20% as the external magnetic field is applied. Increasing ferro-particle concentration to 0.05% (wt%) decreases the spreading factor significantly. Similarly, the reduction of restitution coefficient of the ferrofluid droplet is 16% for the ferro-particle concentration of 0.005 (%wt) while it is 71% for the concentration of 0.05 (%wt) during the external magnetic influence. The magnetic force created (∼2.28 × 10−5 N) remains higher than the capillary force (∼2.69 × 10−6 N), which results in the pinning of the ferro-particles on the hydrophobic surface. The droplet breaks off, giving rise to a newborn droplet during the rebounding because of the momentum dissipation. The interfacial force between the ferro-particles and the droplet fluid (∼1.47 × 10−4 N) is higher than the magnetic force, resulting in a small size newborn droplet with high concentrations of ferro-particles and pinning on the hydrophobic surface.