Forced oscillation of ferrofluid marbles under an alternating magnetic field

The study of oscillating liquid marbles, droplets with an outer layer made of hydrophobic nanoparticles, has gained significant attention from researchers due to their applications in digital microfluidic systems and their rich underlying physics. This study investigates the forced oscillation of ferrofluid marbles (FMs) and different oscillation regimes by applying an alternating magnetic force to these marbles. Initially, the influence of frequency and duty cycle of the magnetic field, volume of FM, and electromagnet current on the deformation of marble is studied. It is also observed that in conditions with significant marble deformation, the marble jumps off the surface. The minimum electric current required for marble to jump off the surface is also investigated under different circumstances. Regarding the shape of these oscillating marbles, three regimes, namely roundish, peanut, and trilobe are observed, and the conditions under which each of these regimes occurs are studied. In weak magnetic fields, liquid marble oscillation patterns initially exhibit a roundish regime. Upon reaching a critical magnetic field, influenced by factors such as marble volume, excitation frequency, and magnetic field duty cycle, a transition occurs, shifting the oscillation patterns to peanut or trilobe regimes. Additionally, it is observed for the first time that under specific conditions of alternating magnetic field, the liquid marbles oscillate on a horizontal plane when a vertical magnetic field is applied. These horizontal oscillations occur only within a limited range of the electromagnet current or excitation frequency.