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We used volume-of-fluid (VOF) method to perform three-dimensional numericalsimulations of droplet formation of Newtonian fluids in microfluidic T-junctiondevices. To evaluate the performance of the VOF method we examined the regimesof drop formation and determined droplet size as a function of systemparameters. Comparison of the simulation results with four sets of experimentaldata from the literature showed good agreement, validating the VOF method.Motivated by the lack of adequate studies investigating the influence ofviscosity ratio ({\lambda}) on the generated droplet size, we mapped thedependence of drop volume on capillary number (0.001 < Ca < 0.5) and viscosityratio (0.01 < {\lambda} < 15). We find that for all viscosity ratiosinvestigated, droplet size decreases with increase in capillary number.However, the reduction in droplet size with capillary number is stronger for{\lambda} < 1 than for {\lambda} > 1. In addition, we find that at a givencapillary number, the size of droplets does not vary appreciably when {\lambda}< 1, while it increases when {\lambda} > 1. We develop an analytical model forpredicting droplet size that includes a viscosity-dependent breakup time forthe dispersed phase. This improved model successfully predicts the effects ofviscosity ratio observed in simulations. Results from this study are useful forthe design of lab-on-chip technologies and manufacture of microfluidicemulsions, where there is a need to know how system parameters influencedroplet size.

Volume-of-fluid simulations in microfluidic T-junction devices: Influence of viscosity ratio on droplet size