Flow patterns and slug scaling of liquid-liquid flow in square microchannels

Abstract

Liquid-liquid flow regimes in three square microchannels were visualized simultaneously both at the cross-shaped junction and in the microchannel “far” from the junction in order to reveal flow regime evolutions along the microchannel. At the inlet junction, three major flow regimes including tubing/threading, dripping and jetting were mapped using the aqueous capillary number versus the organic Weber number. Correspondingly, in the main microchannel, annular, slug and droplet flow patterns were mapped using two dimensionless numbers (Weber number times Ohnesorge number) of both phases. Both dripping and jetting regimes at the inlet junction can evolve into either slug or droplet flows in the main microchannel. Besides, it was realized that as the organic flow rate increases, the transitional aqueous flow rate at the slug-droplet transition firstly increases, then decreases and then increases again. The droplet formation mechanism has transited from dripping to jetting, which causes the slug-droplet transition to occur at a much lower aqueous flow rate. Moreover, a scaling relation for the slug size in dripping was developed, which can predict the slug length for five different liquid-liquid systems. It applies for liquid-liquid microfluidic devices with a cross-shaped inlet junction in the dripping regime, for slug sizes longer than 1.5 times the channel depth. The slug velocity has been correlated as functions of the capillary number Caj (µcj/γ) by using the continuous phase viscosity and the bulk velocity.