Abstract
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•• A laser-based phase-locking system is developed and integrated with a Micro Particle Image Velocimetry system enabling quantitative measurement of the internal velocity of moving droplets.•• Effects of surfactant size on the flow field of moving droplets are investigated by choosing two surfactants, Sodium Dodecyl Sulphate (SDS) and Tween 20, which have a large difference in adsorption timesale.•• Effects of surfactant concentration (zero to 10X) on the internal flow field of moving droplets are evaluated and discussed.•• Effects of droplet shape (slug and spherical) and droplet generation regimes on the internal flow field of moving droplets with surfactant are investigated and discussed.•• It is revealed that achievement of complete remobilisation in the internal flow depends on surfactant type, concentration, and droplet operating regime.
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.