Droplet breakup with in situ marangoni-induced elongational flow*

Breaking unconfined droplets in an elongational flow has been investigated both theoretically
and experimentally. However, it is extremely difficult to generate a saddle point within
the droplet as, in these former studies, the elongational flow is generated in the external phase
surrounding the droplet. In this work, we experimentally investigate the breakup of a
confined droplet in a microfluidic channel using in situ elongational flow generated by
symmetric thermocapillary pumping. Mineral oil droplets with lengths ranging from 800-1600
μm are squeezed into a rectangular PDMS channel (cross section: 200μm x 30 μm)
with an aqueous solution containing SDS as the outer phase. A local chromium heating resistor
of width 50 μm is located at the bottom of the channel. The surface tension between the two working
liquids increases with temperature such that switching on the heating resistors at the center of
the droplet generates a Marangoni flow at the droplet interface from the tips towards the
equator of the droplet. The interfacial flow thus generates the pumping of the outer phase towards the equator, leading to the droplet’s breaking. Interestingly, while the droplet is 3D in a non-axisymmetric geometry, our simple 2D model seems to capture the main features of droplet breakup.