Microfluidic investigation of dynamic surfactant rearrangement at water-oil interfaces during single droplet formation*

Dynamic surfactant behaviour is of paramount importance in industrial and research application. Microfluidic droplet formation is a dynamic process that involves the use of surfactants for the stabilisation of the produced droplets. It has been shown that during droplet formation surfactant molecules rearrange on the interface creating concentration gradients that lead to surface tension gradients that affect droplet formation. We use a microfluidic setting to study the surfactant rearrangement during droplet formation. We have shown that surfactants with smaller molecular weights, therefore larger diffusion coefficients, are more capable in coping with the short time scales of droplet formation present in microfluidics. This has been done by examining the velocities along the interface moments before detachment. Interfaces where surfactant is present display velocities smaller than surfactant free interfaces. Furthermore, the number of satellite droplets produced appears to be increasing with increasing surfactant concentration, suggesting additional effects, even at concentrations above the critical micellar concentration.