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

•• 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.