Microfluidic flow focusing devices have been used to synthesize micrometer-scale emulsion droplets by exploiting the presence of added surfactant in one or both liquid phases. We have previously identified the thread formation mode of drop breakup, which occurs in a particular range of surfactant concentrations and flow rates. Here, a thin thread is drawn between two primary droplets. As the thread elongates, it disintegrates into a stream of tiny droplets, whose sizes depend on the final thread diameter and the physical properties of the liquids. In the present work, we investigate the role of surfactant desorption kinetics on the evolution of the thread length and on the region of phase space in which thread formation occurs.

We consider a homologous series of three nonionic C_nE₈ (n = 10, 12 and 14) surfactants dissolved in the dispersed phase liquid. These molecules have similar adsorption kinetics and diffusivities but very different desorption rates. We observe that the ultimate length of the thread before it disintegrates into a stream of tiny droplets depends strongly on the desorption kinetics. Using high-speed video microscopy, we observe that the thinnest threads are formed in the presence of the surfactant with the longest alkane tail (C₁₄E₈). To quantify the effects of these surfactants on the thread formation process, we measure the thread length as a function of dimensionless time and determine the appropriate dimensionalization for the problem. We also construct phase diagrams to indicate the ranges of surfactant concentration in which thread formation occurs as a function of dimensionless parameters. Our results suggest that the thread formation process can be optimized to form ever smaller droplets through the fine tuning of the timescales for the convection, diffusion, adsorption, and desorption of the surfactants.