A comprehensive study of droplet formation in a capillary embedded step T-junction: From squeezing to jetting

Abstract

The understanding of microdroplet formation characteristics in unconfined T-junction microchannels is relatively weak. Accordingly, an unconfined step T-junction microdevice was specially designed and fabricated to systematically investigate the droplet formation mechanism. A new flow named pseudo-squeezing flow is defined. A liquid–liquid flow pattern map including the pseudo-squeezing, dripping, and jetting flow in the unconfined T-junction is plotted, which shows that a much higher capillary number is needed to realize the flow regime transformation when compared with the confined T-junction. The unconfined T-junction could provide a much wider dripping flow regime, and generate monodispersed smaller droplets. Besides, the droplet diameter decreases with the continuous phase flow rate in the pseudo-squeezing and dripping flow but suddenly increases when the flow regime transfers from dripping to jetting. Importantly, the very short liquid plug can be easily formed in this unconfined T-junction. Finally, because of the same formation mechanism, the droplet size in pseudo-squeezing and dripping flow can be perfectly scaled with a same dimensionless model. This work could be the first comprehensive study of the droplet formation in three flow regimes in the unconfined T-junction.