Surfactant based control of droplet size in step emulsifiers*

In the step emulsification method, the sharp change in capillary pressure due to sudden release of fluidic confinement in a microchannel trigger's droplet pinch-off. Unlike most other microfluidic methods, it does not rely on the shear force due to the flow of the continuous phase to break off the droplet. This makes it a robust approach to generate highly monodisperse droplets across a wide range of fluid flow conditions and fluctuations in the microfluidic device. However, the downside of this remarkable stability is the lack of handles to tune the droplet size once the device geometry is fixed during fabrication. We explore the choice of surfactants as a potential route to control droplet sizes in step emulsifiers without changing the device geometry. Studying water droplet generation in a silicon-on-glass microfluidic device, we see that different surfactants reduce the water-oil interfacial tension to different values to tune the droplet size. Furthermore, while multiple surfactants can yield the same interfacial tension, the wetting of channel walls varies vastly. This enables droplet formation in one case while preventing its break-up in the other. Our study shows that surfactants can tune interfacial tension and channel wetting to control droplet size in step emulsifiers.