On demand manipulation of ferrofluid droplets by magnetic fields

•Facile and controllable manipulation of ferrofluid droplet size by a permanent magnet.•Droplet merging and generation of larger droplets after merging studied.•Controllable change in droplet diameter, droplet diameter increased by three times.•Effect of magnetic field intensity, carrier medium viscosity, initial droplet size, flow rates, and flow rate ratio on change in droplet size studied.•Droplet size change induced by the applied magnetic field is higher for a lower viscosity carrier medium.•Numerical simulations and experimental results show a similar trend. Magnetic droplets, consisting of magnetic nanoparticles in a carrier fluid, are of high interest due to applications such as remote and wireless control in a microfluidic environment. We investigated the influence of magnetic field on the control of ferrofluid droplet size in a nonmagnetic carrier fluid. Generation of larger droplets by a re-pumping mechanism was studied. The magnetic field leads to coalescence and mixing of the magnetic droplets. A significant response of the ferrofluid droplets to changes in flow rate ratio, the viscosity of the carrier medium and magnetic field intensity was observed. The droplet size can be increased by three times of its initial diameter by tuning the magnetic field intensity. Our modeling results show a similar trend to the experimental findings. Such control, mixing, and re-pumping of droplets is relevant to novel Lab-on-a-Chip applications.