Impact of ferrofluid droplets on hydrophobic surfaces with presence of magnetic fields

The motion of ferrofluid droplet on hydrophobic surface has attracted significant research interests due to its potential applications in biomedicine, sensor technologies, and oil-water separation processes. This study investigates ferrofluid droplet impact on a hydrophobic surface and analyzes how ferromagnetic particles in the droplet affect the interaction characteristics of droplet in terms of spreading, retraction, and the formation of secondary droplets during rebound cycles. The results reveal that introducing a magnetic field reduces the droplet spreading factor by approximately 20%. Additionally, increasing ferro particles concentrations to 0.05% (wt%) gives rise to a significant reduction in the spreading factor. The effect on the droplet rebound height is also noteworthy: for a lower particle concentration (0.005%, wt%), the restitution coefficient decreases by 15.5%. However, at high weight concentration (0.05%), this decrease is as much as 69%, which indicates a strong dependency on particle concentration. The analysis shows that the force generated under magnetic field, which is around 2.28 × 10^-5 N, is substantially greater than the capillary force, which is ~ 2.69 × 10^-6 N, leading to the attachment of droplets at magnet when the spacing from the magnet to droplet becomes small. During the rebound, the droplet experiences momentum dissipation, which results in the break-off of smaller "newborn" droplets. This phenomenon is primarily governed by the interfacial force (∼ 1.47 × 10^-4 N) due to the particles and the surrounding liquid that is larger than the magnetic force itself. The newborn droplets tend to have a higher concentration of particles and are prone to sticking on the magnet.