(289a) Breakup and Coalescence of Bubbles in Microfluidic T-Junctions

Breakup
and coalescence of bubbles in microfluidic T-junctions

Yining
Wu, Taotao Fu, Chunying Zhu, Youguang Ma*

State Key Laboratory of Chemical
Engineering, School of Chemical Engineering and Technology, Tianjin University,
Tianjin 300072,
P.R. China

* Corresponding author: ygma@tju.edu.cn

Abstract
This paper aims at the breakup and
coalescence of bubbles in microfluidic T-junctions by using a high-speed
digital camera. The microfluidic channels have uniform square cross-section
with 400µm wide and 400µm deep. The microfluidic device consists of the bubble
formation section, snakelike microchannel and a symmetric loop. Bubbles were
formed in a microfluidic flow-focusing junction and drove into the snakelike
microchannel. And then, the bubble moved towards the loop. In the
loop, the microchannel divides into two symmetric parts at the T-junction
divergence, and then the two channels reconvert to a single channel again at the
T-junction convergence.

Asymmetrical breakup of bubbles at the
microfluidic T-junction divergence stemming from the feedback effect of bubble
behaviours at the T-junction convergence in the symmetrical loop were studied.
The experiments were performed under gas/liquid flow rates ratio ranging from
0.084 to 4.333. Four bubble behaviours (bubble pair
asymmetrical collision, bubble pair staggered flow, single bubble
flow and dynamic transformation flow) were observed at the T-junction
convergence in different gas and liquid flow rates. The feedback effects of
asymmetrical collision and staggered flow of bubble pairs at T-junction
convergence on bubble behaviour at T-junction divergence were mainly
investigated. The result showed that the feedback effect is negligible at relatively
low flow rates when no collision of bubble pairs occurs. And the bubble pair
asymmetrical collision at T-junction convergence or the amplified effect of
structured blemish of microchannel at relatively high flow rates is primarily
responsible for asymmetric breakup of bubbles at T-junction divergence.

The bubble coalescence at the
microfluidic T-junction convergence in the
symmetrical loop was also studied. The symmetric loop was designed to fabricate
uniform bubble pairs in glycerol-water mixture at the T-junction divergence.
And the bubble pair head-on collision occurs at the downstream T-junction
convergence. Four primary responses of collision including point contact coalescence,
surface
contact coalescence, slipping contact coalescence and non-coalescence were distinguished at the T-junction
convergence by changing gas and liquid flow rates or the viscosity of the
liquid phase. The results indicated that with the increase of the capillary
number the responses of collision will transform from point contact coalescence
to non-coalescence and the coalescence efficiency of surface contact
coalescence and slipping contact coalescence will reduce.

This study gives quantitative data and
qualitative analysis of breakup and coalescence of bubbles at microfluidic T-junctions
in a microfluidic loop. The fully understanding about these phenomena would be
helpful to manipulate bubbles or droplets by designing complex geometries such
as a loop in microfluidic devices. Moreover, our experimental results suggest
that the three-dimensional numerical simulation is an urgent task for modelling
bubble/droplet behaviours in square cross-sectional microchannels with complex
geometries owing to the existence of gutters. Therefore, this paper is useful to
further experimental and numerical investigations on microfluidics and the
design of microfluidic devices for application in the future.

Keywords:
breakup, coalescence, bubble collision, microfluidics, microfluidic loop