(142al) Slow Growth of the Rayleigh-Plateau Instability in Aqueous Two Phase Systems

Microfluidic uses of aqueous two-phase systems (ATPS) are limited due to the difficulty in making monodisperse droplets from aqueous polymer solutions that are immiscible. This paper teaches how to generate such water-in-water microdroplets from a thread based on a study of the Rayleigh-Plateau instability. Careful vibration-free experiments with controlled actuation of the flow allowed direct measurement of the growth rate of this instability. In our experiments, we used the well-known aqueous two phase system of dextran and polyethylene glycol solutions. We report growth rates of the order of 1/s for this system, which is more than an order of magnitude slower than the growth rates in an analogous experiment with two immiscible Newtonian fluids with viscosities and interfacial tension that closely matched the ATPS experiment. Viscoelastic effects and adhesion to the walls were excluded as explanations for the observed behavior. The results are remarkable, because all current theory suggests that such dilute polymer solutions should break up faster, not slower, than the analogous Newtonian case.