Effects of surfactant size and concentration on the internal flow fields of moving slug and Disk-like droplets via μ-PIV

Understanding the effects of surfactant on the internal flow field of moving droplets is of fundamental and practical importance. A laser-based phase-locking system was developed and coupled with a micro particle image velocimetry system enabling quantitative measurement of the internal droplet flow filed. Five concentrations (zero to 10X critical micelle concentration) of each of the two surfactants, Sodium Dodecyl Sulphate (SDS) and Tween 20, which present a large difference in adsorption timescale, were evaluated. Slug-and disk-shaped droplets were considered because of their different contact area with channel walls and thus the resulted friction. Squeezing and transition droplet formation regimes which normally yield monodispersed droplets were evaluated using SDS. Flow retardation was observed at low surfactant concentrations for slug droplets, which was primarily attributed to the induced Marangoni flow opposing the internal flow. Achievement of complete remobilisation in the internal flow depends on surfactant type, concentration, and droplet operating regime.(c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

Understanding the effects of surfactant on the internal flow field of moving droplets is important for both fundamental research and practical applications. This study developed a laser-based phase-locking system coupled with a micro particle image velocimetry system to quantitatively measure the internal droplet flow field. By evaluating two surfactants at different concentrations, it was found that the shape and flow of the droplets were influenced. At low concentrations, flow retardation occurred due to the induced Marangoni flow opposing the internal flow. The achievement of complete remobilisation in the internal flow depended on the type, concentration, and operating regime of the surfactant.