(227o) Breakup Dynamic of Viscoelastic Thread for Droplet Formation in a Flow-Focusing Device

Breakup Dynamic of Viscoelastic Thread for Droplet Formation
in a Flow-Focusing Device

Wei Du^a, Xiqun Gao^b, Taotao Fu^a, Chunying Zhu^a, Youguang Ma^a*

^a State Key Laboratory of Chemical Engineering, Collaborative
Innovation Center of Chemical science and Engineering, School of Chemical
Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China

* Corresponding author: ygma@tju.edu.cn

^b Yifang Industry Corporation, Liaohua Petrochemical Fiber
Company, Liaoyang
111003, P. R. China

Abstract£º

The breakup dynamic of viscoelastic
thread for droplet formation was investigated by a high-speed digital camera in
a flow-focusing device with square cross-section of 400°Á400µm.
Dilute polyethylene oxide (PEO) - glycerol aqueous solution was used as the
dispersed phase, while mineral oil with 4% (wt) surfactant sorbitanlauric acid
ester (Span 20) was used as the continuous phase. Viscoelastic
droplets were generated at the intersections of microfluidic device (Fig. 1).

The flow regime could be
categorized into two patterns based on a critical droplet length: (i) dripping
with satellites, (ii) dripping without satellites. In the regime with satellites
droplets, the pinch-off of the viscoelastic thread of the dispersed phase would take
place at both ends or at either end of the thread. While in the regime without
satellite droplets, the rupture always occurs firstly at the upstream end of
the thread.

For both flow patterns,
the breakup processes of thread could be divided into two stages by a critical 'pinch
time' t_p: 'pre-stretch'
stage (t < t_p) and the 'elastocapillary
balance' stage (t > t_p). In the 'pre-stretch' stage,
the variation of the minimum width of the dispersed thread with the remaining
time (t_p ¨C t)
could be scaled as a power¨Claw relationship: w_m/w_c°Ø(t_p
¨C t)^¦Á, here ¦Á=0.36±0.03.
In the 'elastocapillary balance' stage, it would exhibit an exponentially relationship
between the minimum width of the thread and time: w_m/w_c°Øexp(-t/3¦Ë).
In which the effective relaxation time ¦Ë extracted from the exponentially
evolution of the thread is larger than the estimated Zimm relaxation time for
the viscoelastic fluid.

Whether the satellite
droplets could be generated or not depends primarily on the initial aspect
ratio of the thread at the beginning of the 'elastocapillary balance' stage and
afterward the evolution dynamics of the thread. For
the regime with satellite droplets, the thread would firstly undergo a rapid
stretching stage and then a linearly slow stretching process. For the regime without
satellite droplets, it would undergo a rapid stretching stage at first and then
a self-similar "elastic drainage" process with a constant length.

This study on breakup
dynamic of viscoelastic thread for droplet formation in a flow-focusing device
is conducive to the fully understanding of the rupture process, it would provide
a perfect way to gain insight into the viscoelastic droplet formation with
monodispersity and at same time provide the foundation for further theoretical
and experimental investigations on the droplet formation and breakup processes
for soft matter synthesis and design in microfluidic devices.

Keywords£ºmicrofluidic,
viscoelastic, interface, pinch-off, satellite droplet.

Fig. 1 (a) Schematic diagram of the microfluidic device. All the
cross-sections of these microchannels are 400 µm (height) ´ 400 µm (width). Unit: mm. (b) 30w PEO/ glycerol/Water droplets are generated at the
intersections of microfluidic
device (dashed line box in Fig. 1 (a)).