Microfluidic channels are widely used to produce droplets with controlled size, important in emulsification, inkjet printing and chemical analysis. Surfactants are often added to vary interfacial properties and control drop size. Equilibrium values of interfacial tension are commonly used to correlate the results. The droplet formation process, however, depends on the dynamic values of the interfacial tension, which are linked to the surfactant diffusion and absorption kinetics to the interface. Microfluidics can, in fact, be used to obtain changes in interfacial tension over time.<br/> <br/>The dynamic interfacial tension of different surfactants was studied during drop formation in a microfluidic channel. A low viscosity silicone oil (4.6 mPa s) was used as the continuous phase and a mixture of 48% w/w water and 52% w/w glycerol was the dispersed phase. Three surfactants, an anionic (SDS), a cationic (DTAB) and a non-ionic (TX100) with different critical micelle concentrations (CMC) were added in the dispersed aqueous phase, at a wide range of concentrations below and above CMC. The drop formation process was investigated with high speed imaging and the results were used to obtain dynamic interfacial tension. <br/> <br/>Five patterns of drop formation were identified, namely squeezing, dripping, jetting, threading and tip streaming/jetting, whose boundaries were affected by the surfactant type and concentration. The dynamic interfacial tension values in the millisecond scale were calculated for all surfactants using the drop formation times in the dripping and jetting regimes and a correlation of drop size with capillary number. Criteria were proposed for the drop formation times to ensure that equilibrium interfacial tension is reached, and does not affect drop formation. In addition, by using the dynamic interfacial tension values instead of the equilibrium ones, it was found that the transition boundaries between the dripping and the jetting regimes collapsed for all surfactants.

Dynamic interfacial tension during drop formation in microchannels with different surfactants

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