Review of the role of surfactant dynamics in drop microfluidics

Surfactants are employed in microfluidic systems not just for drop stabilisation, but also to study local phe-nomena in industrial processes. On the scale of a single drop, these include foaming, emulsification and stability of foams and emulsions using statistically significant ensembles of bubbles or drops respectively. In addition, surfactants are often a part of a formulation in microfluidic drop reactors. In all these applications, surfactant dynamics play a crucial role and need to be accounted for. In this review, the effect of surfactant dynamics is considered on the level of standard microfluidic operations: drop formation, movement in channels and coa-lescence, but also on a more general level, considering the mechanisms controlling surfactant adsorption on time -and length-scales characteristic of microfluidics. Some examples of relevant calculations are provided. The ad-vantages and challenges of the use of microfluidics to measure dynamic interfacial tension at short time-scales are discussed.