Magnetically controllable generation of ferrofluid droplets

This paper reports the manipulation of ferrofluid droplets by using a microfluidic flow-focusing device equipped with a magnetic tweezer. Besides the traditional flow rate controlling method, the magnetic field also can be applied to control the size of the droplets. Two major effects in magnetic manipulation process: magnetoviscous effect and magnetic drag effect, were studied. Under a fixed flow rate (CP = 1 mL/h, DP = 0.2 mL/h), the average sizes of ferrofluid droplets were tunable from 135 to 95 μm by varying the magnetic field from 0 to 60 mT. Moreover, square wave magnetic field can be used to periodically generate droplets with different sizes. These results are helpful to understand the generation mechanism of the ferrofluid droplet and supply a novel method for manipulating droplets with a predetermined size and distribution.