Abstract
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The present paper reports actuation of sessile ferrofluid droplets over a hydrophobic substrate in the presence of a time-dependent magnetic field generated by an electromagnet. The internal hydrodynamics of the ferrofluid droplet in the presence of magnetic field are measured using both bright field visualization and micro-particle image velocimetry (μ-PIV) techniques. During ON cycle of the magnetic field, bright field visualizations show the migration of nanoparticles towards the contact line near the vicinity of the electromagnet resulting in aggregation of nanoparticles inside the droplet. Similarly, aggregated nanoparticles at the contact line from the ON cycle are observed to disperse from the cluster of nanoparticles during the OFF cycle of the magnetic field. Both migration and dispersion of nanoparticles result in bulk motion inside the ferrofluid droplet during the ON and OFF cycle of the magnetic field. Velocity measurements from μ-PIV technique successfully validate the qualitative measurements of flow field from bright field visualization technique. A critical frequency is observed for the applied magnetic field above which negligible dispersion of nanoparticles resulted inside the ferrofluid droplet during the OFF cycle of the magnetic field.