Control of plug flow dynamics in microfluidic T-junction using pulsations of dispersed phase flow rate

Active control methods are commonly employed in droplet-based microfluidics. While passive methods of droplet generation face limitations, active methods, including flow rate pulsations provide an avenue for precise control. We investigate the influence of sinusoidal pulsations of dispersed phase flow rate on the plug flow dynamics in a T-junction microchannel. The study covers a range of fluid sets, including gas-liquid and liquid-liquid systems. In the absence of pulsations, distinct regimes of plug formation were identified, and plug length, velocity, and natural frequency of formation were determined. Introducing flow rate pulsations at frequencies proportional to the natural ones f = fpulse/fplug, we observed distinct plug length distributions, including double- or triple-mode, drop-on-demand mode, and multi-mode distribution. The influence of pulsations on plug length was particularly notable at low dimensionless frequencies (f < 1), especially for fluid combinations with higher viscosity ratios. The model was proposed to explain plug length distribution patterns under pulsatile conditions. Micro-PIV measurements provided insights into the velocity fields within plugs, revealing the stretching of streamlines without altering plug length. Our findings contribute to the understanding of active control methods in droplet-based microfluidics and suggest potential applications for precise control over microfluidic processes.