Active droplet generation in flow-focusing microfluidics under influence of transverse magnetic field

Magnetic fields can be used to control the process of magnetic fluid dispersion in microfluidics. Here we demonstrate a method for generating magnetic fluid droplets of a given size and shape in a microfluidic chip with a flow-focusing configuration under the influence of a transverse magnetic field. The results of the study show that a magnetic field can have a significant impact on ferrofluid dispersion. At low volumetric flow rates of the continuous phase, magnetic force plays a primary role in ferrofluid dispersion due to disturbances on the magnetic fluid surface. Furthermore, the droplet diameter decreases with increasing magnetic field D_d ∼ H^-1/3. At high volumetric flow rates of the continuous phase, shear stress plays a primary role in ferrofluid dispersion. In this case, the droplet diameter depends on the continuous phase velocity according to the law D_d ∼ u^-1. The magnetic field and the volumetric flow rate of the continuous phase affect not only the liquid dispersion process but also the coalescence and deformation of droplets. The pressure gradient in the channel and the magnetic field contribute to droplet deformation. The thickness of the deformed droplet decreases with increasing volumetric flow rate according to the law l ∼ Q^-1. The non-uniformity of the velocity field distribution over the channel thickness is responsible for droplet coalescence. The magnetic field, on the contrary, prevents droplet coalescence. These findings open up new possibilities for generating soft magnetic robots of a given size and shape.