Coalescence dynamics of ferrofluid droplets suspended in non-magnetic fluid medium

We experimentally reveal the coalescence dynamics of ferrofluid drops suspended on another fluid medium in the absence or presence of an external magnetic field. By tuning the initial droplet size, viscosity ratio between ferromagnetic droplet fluid medium and non-magnetic surrounding fluid medium, magnetic field intensity, we can alter the coalescence hydrodynamics of the droplets. High speed visualisation of the three step phenomena: droplet approach, film drainage and film rupture engendering coalescence reveal fascinating results. The influence of drop initial size, viscosity ratio between droplet fluid and surrounding medium fluid, and strength of applied magnetic field on the drop approach speed, droplet aspect ratio and drop fusion dynamics, are highlighted. The aforementioned parameters are elucidated in terms of non-dimensional numbers such as Weber number and magnetic bond number. The magnetohydrodynamic stresses generated due to the influence of magnetic field and the interfacial tension stress between drop phase and surrounding fluid phase governs the interface profile of droplets suspended in another medium with no fluid motion. The externally applied magnetic field deforms the droplet shape and the surface tension force resists the deformation. The drop deformation is dependent on applied magnetic field and in turn the magnetic field profile inside and around the droplet is dependent on the shape of the droplet. This two-way coupling of the interfacial drop shape and variation of magnetic field is crucial for analytical modelling of the hydrodynamics. The results obtained from the present study can be extended towards the understanding of several applications such as emulsion and oil recovery techniques, to name a few.