265604 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: email@example.com
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