Emulsification in a microfluidic flow-focusing device: Effect of the dispersed phase viscosity

Abstract Flow regimes of two immiscible liquids are experimentally investigated in a rectangular microchannel using a water–oil system. Oil-water viscosity ratios are varied from 0.2 to 75, accompanied by varied oil–water velocity ratios from 10−2 to 102 and varied capillary numbers from 10−3 to 10−5. Four flow regimes are distinguished, including threading, jetting, dripping and squeezing regimes, based on the continuous phase’s capillary number (Ca) and the dispersed phase’s Weber number (We). An exponential correlation exists between the viscosity ratio at which the flow regime transits and the velocity ratio. Specifically, the viscosity ratio is found to have a larger effect on the slug length in the squeezing regime than the droplet size in the dripping regime. Therefore, two expressions are proposed to separately scale emulsions in squeezing and dripping regimes: one for prediction of droplet dimension in the dripping regime based on the continuous phase’s oil–water velocity ratio and Ca, and the other one for prediction of slug lengths in the squeezing regime, which involves the viscosity ratio in addition to abovementioned two parameters. Both scaling expressions are validated by available data in the existing literature.