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

Abstract

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 at low volumetric flow rates of the continuous phase, the magnetic field plays the main role in ferrofluid dispersion. With increasing magnetic field strength, the droplet diameter decreases according to the law Dd~H^(-1⁄3). With increasing continuous phase velocity, shear stresses begin to play the main 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 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.