Determination of Time-Evolving interfacial tension and ionic surfactant adsorption kinetics in microfluidic droplet formation process

Surfactant adsorption plays an important role in microfluidics, which can be investigated by the dynamic evolution of interfacial tension. A differential pressure-based method is proposed to understand the basic laws of time-evolving interfacial tension and adsorption kinetics of ionic surfactants during the microfluidic droplet formation processes. Instantaneous flow rates and flow resistances are precisely analyzed based on auto-recognized microscopic images and the waveform of differential pressure, and the interfacial tension is determined from the Young-Laplace equation with a time resolution of 1/60 s. The concentration of surfactant at the liquid–liquid interface is obtained according to the Frumkin adsorption model, leading to an in-depth understanding of the adsorption rates and the apparent mass transfer rates of surfactants. The surfactant adsorption is demonstrated to obey the kinetic-controlled adsorption mechanism and therefore a measurement method for the adsorption rate constant is created to understand the kinetic characteristics of small-molecule ionic surfactants.