Generation of an O/W emulsion in a flow-focusing microchip: Importance of wetting conditions and of dynamic interfacial tension

To date, there is no information on the microfluidic emulsification of dibutyl adipate and n-butyl acetate in water. Since these solvents are very suitable for microencapsulation by interfacial polymerization, it is highly necessary to study the emulsification behavior of these solvents in microchannels. This work shows that the microfluidic emulsification of these solvents in water may fail to obtain stabilised flow regimes. This is due to droplet coalescence and wall wetting, even if a hydrophilic microchip is used. Hydrodynamic results show that squeezing and dripping regimes are especially affected because of the wall wetting by the dispersed phase. This difficulty can be circumvented by adding a surfactant (here Tween 80) into the aqueous phase. However, high surfactant concentrations (ten times the critical micelle concentration) should be used for the water-dibutyl adipate system. Indeed, comparison of flow maps for several surfactant concentrations seems to indicate that the dynamic interfacial tension is higher than the one expected (equilibrium), for surfactant concentrations lower than one hundred times the critical micelle concentration. The estimated diffusion time of Tween 80 is compared to the droplet formation time at different conditions. The choice of more appropriate dimensionless numbers to represent flow maps is also discussed. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.