  Control of fluid dynamics at the micrometer scale is essential to emulsion
science and materials design, which is ubiquitous in everyday life and is
frequently encountered in industrial applications. Most studies on multiphase
flow focus on oil-water systems with substantial interfacial tension. Advances
in microfluidics have enabled the study of multiphase flow with more complex
dynamics. Here, we show that the evolution of the interface in a jet surrounded
by a co-flowing continuous phase with an ultra-low interfacial tension presents
new opportunities to the control of flow morphologies. The introduction of a
harmonic perturbation to the dispersed phase leads to the formation of
interfaces with unique shapes. The periodic structures can be tuned by
controlling the fluid flow rates and the input perturbation; this demonstrates
the importance of the inertial effects in flow control at ultra-low interfacial
tension. Our work provides new insights into microfluidic flows at ultra-low
interfacial tension and their potential applications.
