Shear-induced tail breakup of droplets (bubbles) flowing in a straight microfluidic channel

• The interface instability of droplet (bubble) in a microchannel was investigated.
• The formation mechanism and influencing factors of the tip stream were studied.
• The critical droplet size was proposed to predict the appearance of the tip stream.

Dispersed droplets and bubbles were generated in a microfluidic flow-focusing device to investigate the morphological developments of the droplets (bubbles) flowing in the downstream straight micro-channel by a high speed camera. The present study is mainly aimed at the influences of both the dispersed droplet (bubble) size and two-phase average flow velocity on the formation of tip stream in the rear of a droplet (bubble). It was found that the particle deformation increased with the droplet (bubble) length or the capillary number. There exists a critical droplet (bubble) length dependent on capillary number, beyond which the tip stream ejecting tiny daughter droplets will take place. The generated tiny droplets are usually three orders of magnitude smaller than primary droplet. This study provides the possibility to avoid a high polydispersity index in microchannels.

Four kinds of shape were observed by changing the flow rates ratio from φ=0.27–1.15 with constant Ca=0.55: (1) droplet with blunt butt, (2) droplet with concave butt, (3) droplet with a tail stream which ejects transiently small daughter droplets, (4)droplet with two tail streams which eject stably small daughter droplets.Figure optionsDownload high-quality image (210 K)Download as PowerPoint slide