The role of surfactants on the shape of confined droplets moving in circular microchannel

Droplet microfluidics is widespread in many chemical and biological applications where each droplet can be considered as a single, independent reactor unaffected by the presence of channel walls. This compartmentalization is facilitated by the addition of surfactants to increase the emulsion stability. However, the presence of surfactants is expected to strongly affect the dynamics and shape of flowing droplets. We report a systematic experimental study of the curvature of the front and the rear menisci of confined droplets flowing in a circular channel, with and without surfactants. In detail, the role played by surfactants on the droplet shape is investigated by dispersing them either in the droplet or in the continuous phases. The curvatures are evaluated by varying droplet speed, interfacial tension, and surfactant concentration. The curvature of the droplet front is found to scale with the capillary number (Ca) regardless of the presence or absence of surfactants. Differently, the curvature of the rear meniscus strongly depends on the surfactant concentration and whether surfactants are dispersed in the droplet or continuous phases. The surfactant accumulation at concentrations higher than the critical micelle concentration leads to an unexpected increase in the curvature in the former case and to droplet breakup in the latter.