Experimental investigation of on-demand ferrofluid droplet generation in microfluidics using a Pulse-Width Modulation magnetic field with proposed correlation

Micro-magnetofluidics offers a promising tool to regulate the drop formation process with versatile applications in engineering and biomedicine. In the present study, on-demand ferrofluid drop generation at a T-junction is investigated utilizing a magnetic pulse. Also, a novel method for ferrofluid droplet formation is introduced using a non-uniform Pulse-Width Modulation (PWM) magnetic field. A novel mechanism of drop generation named "beating regime" was seen for the first time in which the ferrofluid moves back and forth before the breakup. The effect of the magnetic induction, continuous phase flow rate, duty cycle, and applied frequency on the generation frequency and drop diameter was investigated under the PWM magnetic field and compared with those under the DC magnetic field. The results showed that greater values of drop diameter and generation frequency are obtained either when the magnetic induction and/or duty cycle increases or when the applied frequency decreases. The regime maps of ferrofluid droplet formation were presented for different magnetic Bond and Capillary numbers under both PWM and DC magnetic fields and compared with each other. Finally, a correlation was presented to estimate the dimensionless drop diameter on the basis of four nondimensionalized parameters, showing a 7.2 % average relative error.