  Droplet breakup is an important phenomenon in the field of microfluidics to
generate daughter droplets. In this work, a novel breakup regime in the widely
studied T-junction geometry is reported, where the pinch-off occurs laterally
in the two outlet channels, leading to the formation of three daughter
droplets, rather than at the center of the junction for conventional
T-junctions which leads to two daughter droplets. It is demonstrated that this
new mechanism is driven by surface tension, and a design rule for the
T-junction geometry is proposed. A model for low values of the capillary number
$Ca$ is developed to predict the formation and growth of an underlying carrier
fluid pocket that accounts for this lateral breakup mechanism. At higher values
of $Ca$, the conventional regime of central breakup becomes dominant again. The
competition between the new and the conventional regime is explored.
Altogether, this novel droplet formation method at T-junction provides the
functionality of alternating droplet size and composition, which can be
important for the design of new microfluidic tools.
