Surfactant-laden micro-scale droplet coalescence in Bancroft-breaking systems

The dispersed droplets in liquid-liquid droplet emulsions are often stabilized by the surfactant molecules adsorbed onto the droplet interfaces, which reduces the interfacial tension and generally inhibits droplet coalescence. Other factors, such as viscous stress and Marangoni stress, will also have major impacts on droplet stability. In this paper, systematic droplet coalescence experiments will be presented using a microfluidic Stokes trap geometry, as a function of viscosity ratio between droplet and continuous phase as well as surfactant concentration. The results show that more coalescence can be observed for systems with a lower viscosity ratio (smaller than 1), while few coalescences are observed for a higher viscosity ratio (larger than 1), as expected. More surprisingly, a non-monotonic trend of film drainage time as a function of surfactant concentration is observed for both Triton X100 and Glucopon 225DK. The film drainage time first increases and then decreases followed by plateaus with the increase of surfactant concentration, until a critical concentration value is reached. To better understand the non-monotonic relationship between film drainage times and surfactant concentration, two Marangoni numbers defined based on (1) the ratio of Marangoni time scale with surfactant diffusion timescale, and (2) the ratio of Marangoni stress with interfacial tension, are examined. The results from both Marangoni scaling agree with the experimental observations.