Droplet breakup at the entrance to a bypass channel in a microfluidic system

We experimentally and numerically investigate the breakup of a droplet at the entrance to a microfluidic bypass channel. The polydimethylsiloxane (PDMS) microchannels are manufactured using standard photolithography techniques and we employ a high-speed camera to visualize the interfacial dynamics of the droplets. Three-dimensional (3D) numerical simulations are carried out for conditions causing both droplet breakup and nonbreakup. The droplets are generated upstream of the bypass channel using a standard T junction and the subsequent interaction with the entrance to the bypass channel is studied. In particular, the effects of capillary number (Ca) and the relative width of the bypass channel with respect to the main channel (β) are examined. The regimes of breakup and nonbreakup are experimentally plotted on the Ca-β plane with the volume of the daughter droplets formed. The finger length in the bypass channel is also plotted on this regime map. There exists a critical value of Ca at constant β and a critical value of β at constant Ca at which droplet breakup occurs. Combining measurements with numerical simulations, we show that the droplet breakup occurs if the hydrodynamic pressure drops across the droplet and average shear stress overcomes the differential Laplace pressure. We also find that droplet breakup at the entrance to the bypass channel can exhibit squeezing and dripping regimes, which are equivalent to those occurring in droplet generation at a T junction.