Droplet microfluidics offers a range of Lab-on-a-chip (LoC) applications. However, wireless and programmable manipulation of such droplets is a challenge. We address this challenge by experimental and modelling studies of uniform magnetic field induced merging of ferrofluid based droplets. Control of droplet velocity and merging was achieved through uniform magnetic field and flow rate ratio. Conditions for droplet merging with respect to droplet velocity were studied. Merging and mixing of colour dye + magnetite composite droplets was demonstrated. Our experimental and numerical results are in good agreement. These studies are useful for wireless and programmable droplet merging as well as mixing relevant to biosensing, bioassay, microfluidic-based synthesis, reaction kinetics, and magnetochemistry.

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Magnetic droplets on a microfluidic platform can act as micro-robots, providing wireless, remote, and programmable control. This field of droplet micro-magnetofluidics (DMMF) is useful for droplet merging, mixing and synthesis of Janus structures. Specifically, magnetic Janus particles (MJP) are useful for protein and DNA detection as well as magnetically controlled bioprinting. However, synthesis of MJP with control of the functional phases is a challenge. Hence, we developed a high flow rate, surfactant-free, wash-less method to synthesize MJP by integration of DMMF with hybrid magnetic fields. The effects of the flow rate, flow rate ratio, and hybrid magnetic field on the magnetic component of the Janus droplets and the MJP were investigated. It was found that the magnetization, particle size, and phase distribution inside MJP could be readily tuned by the flow rates and the magnetic field. The magnetic component in the MJP could be concentrated after mixing at flow rate ratio values less than 7.5 and flow rates less than 3 ml h. The experimental results and our simulations are in good agreement. The synthesized magnetic-fluorescent Janus particles were used for protein detection, with BSA as a model protein.

Spreading of a water based ferrofluid core, cladded by a diamagnetic fluid, in threestream micromixer channels was studied. This spreading, induced by an external magnetic field, is known as magnetofluidic spreading (MFS). MFS is useful for various novel applications where control of fluid-fluid interface is desired, such as micromixers or micro-chemical reactors. However, fundamental aspects of MFS are still unclear, and a model without correction factors is lacking. Hence, in this work, both experimental and numerical analyses were undertaken to study MFS. We show that MFS increased for higher applied magnetic fields, slower flow speed of both fluids, smaller flow rate of ferrofluid relative to cladding, and higher initial magnetic particle concentration. Spreading, mainly due to connective diffusion, was observed mostly near the channel walls. Our multi-physics model, which combines magnetic and fluidic analyses, showed, for the first time, excellent agreement between theory and experiment. These results can be useful for lab-on-a-chip devices. C 2015 AIP Publishing LLC. [http://dx.

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Magnetofluidic spreading (MFS) is a phenomenon in which a uniform magnetic field is used to induce spreading of a ferrofluid core cladded by diamagnetic fluidic streams in a three-stream channel. Applications of MFS include micromixing, cell sorting and novel microfluidic lab-on-a-chip design. However, the relative importance of the parameters which govern MFS is still unclear, leading to non-optimal control of MFS. Hence, in this work, the effect of various key parameters on MFS was experimentally and numerically studied. Our multi-physics model, which combines magnetic and fluidic analysis, showed excellent agreement between theory and experiment. It was found that spreading was mainly due to cross-sectional convection induced by magnetic forces, and can be enhanced by tuning various parameters. Smaller flow rate ratio, higher magnetic field, higher core stream or lower cladding stream dynamic viscosity, and larger magnetic particle size can increase MFS. These results can be used to tune magnetofluidic spreading in microchannels.

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