Speed up bubbling in a tapered co-flow geometry

• A tapered co-flow microchannel is proposed to generate monodisperse bubbles/droplets.
• The wetting mode and the shearing mode are distinguished to form Taylor bubble train.
• The bubble/droplet sizes and the bubbling frequency are found in perfect exponential proportion to the penetration length of central needle.
• The flow patterns are decided by the gas/liquid flow rates, the bubble length is piecewise linear function of the gas/liquid flow rate ratio.

A numerical study of massive bubble generation by single-nozzle injection into a co-flowing tapered geometry is reported. Focusing on single-bubble generation in the dynamic and Taylor flow regime, the effects of flow rates and nozzle injection length on bubble/droplet sizes and bubbling frequencies are investigated. The geometry confinement limits bubble’s radial expansion, forces the bubble to extrude in both axial directions and blocks the incoming fluxes, and makes effect of “front-end stretching and rear-end squeezing”, what is a combination with “stretching” under tapering enhanced co-flow shear and “squeezing” by upstream phase holdup. When bubbles detach, different processes of periodical breakup, including shearing mode and wetting mode, are distinguished to be dependent on gas and liquid flow rates. For the periodical breakup mode, it is found that, with the increase of nozzle injection length, the resulted bubbling frequency increases exponentially with the nozzle injection length, while the sizes of bubble/droplet decrease exponentially in reverse. Therefore, the tapering acceleration of flow enhances the “stretching and squeezing” effect, and generates bubbles in smaller size and with higher frequency. Meanwhile, the pattern diagram is dominated by gas/liquid flow rates and is almost not affected by the nozzle injection length, which functionally decouples the roles of flow rates and the nozzle injection length between flowing modes and bubble size/bubbling frequency controls. Also, it is found that the bubble length is piecewise linear function of the gas/liquid flow rate ratio for both the wetting mode and the shearing mode. All these characteristics of tapered configuration promote the monodispersity and maneuverability of bubbling in co-flowing microdevices.