Droplet wetting dynamics on nanostructured surfaces induced by magnetic field

The wetting transition of magnetic droplets from Cassie-Baxter to Wenzel states caused by an external magnetic field is investigated experimentally in the present work. We fabricated square-pillared surfaces of varying dimensions using photolithography. Droplet wetting experiments were performed for various configurations of the pillared substrates. The surface roughness was altered by varying the size of the square pillar, the gap between two pillars and the height of the pillars. The droplet deposited on the substrate experiences deformation and a wetting state transition upon the introduction of an external magnetic field. The externally applied magnetic field deforms the droplet shape and the pinning force due to pillars resists the deformation. Hence, the magnitudes of input magnetic energy required to break this energy-barrier is crucial in the understanding of the transition behaviour. In the present study an attempt has been made to obtain a regime map for roughness ratio vs magnetic Bond number for different volumes of ferrofluid droplet in which the deformation of ferrofluid droplet becomes irreversible and it undergoes Cassie-Baxter state to Wenzel state transition. Theoretical modelling of the droplet spreading dynamics in presence of an external magnetic field can better comprehend the wetting transition from energy minimization point of view.