Coflowing aqueous and oil-based ferrofluid streams exposed to a magnetic field

We report the transition between stream and droplet regimes in a coflow of an aqueous stream and oil-based ferrofluid. The transition between stream and droplet regimes is typically attained by controlling the capillary numbers (Ca) of the phases. Remarkably, we experimentally evidence a transition between the regimes by adjusting the exposure of the system to a magnetic field, with Ca fixed. We represent the various regimes: stable coflow, interface deformation, and droplet generation, in terms of the magnetic bond number (Bo_m) and the ratio of capillary numbers of the phases (Ca_r). The different regimes are a consequence of the interplay of the magnetic, viscous, and interfacial tension forces, represented by the two dimensionless numbers. We explain the regimes in terms of the magnetic pinch-off (τ_mp) and advection (τ_a) time scales: for τ_mp ≫ τ_a a stable coflow is observed, for τ_mp ∼ τ_a interface deformation is observed, and for τ_mp ≪ τ_a droplet breakup is observed. We study the interface deformation and droplet size from experiments and predict the same from theoretical scaling. We find the interface deformation increases and the droplet size decreases with increases in Bo_m and Ca_r. The present study may find applications in magnetic field-assisted on-demand droplet generation in microfluidics.