Magnetophoresis induced coalescence of equal-sized oil droplets in paramagnetic aqueous carrier phase*

The coalescence of liquid droplets in an immiscible liquid carrier phase is a crucial step in industrial solvent extraction processes. More spontaneous and efficient coalescence leads to improved demulsification and higher yield. Coalescence reduces surface energy and is thus thermodynamically favorable. However, various repulsive forces can hinder the process, resulting in a non-zero residence time for coalescence. As droplets come into close contact, film drainage occurs until it reaches a critical thickness where van der Waals forces dominate, forming a liquid bridge between droplets. In this study, we investigate the coalescence of two identical millimeter-sized oil droplets dispersed in a paramagnetic Mn(II) solution under a high magnetic field gradient. The induced Kelvin force causes droplet magnetophoresis, bringing the droplets into close contact. Using high-speed imaging and particle image velocimetry, we analyze the flow field in the carrier phase before and during coalescence and statistically evaluate the residence time of coalescence. To mimic the repulsive forces encountered during the demulsification process, we introduce controlled amounts of surfactants (DeTAB, SDS) into the aqueous carrier. This method allows us to tune the repulsive Gibbs-Marangoni and electrostatic forces by varying the ionic strengths and surface concentrations, providing some insights into this complicated system.