The optimization of separation processes, like liquid-liquid extraction, and improvement of separation apparatus design start most importantly with a good understanding of the hydrodynamic coupling between the two phases and the mass transfer across the interface. This task becomes challenging for complex polydisperse systems. Population of droplets or bubbles is often considered in these processes. Given the complexity of a global study of the system, the study of mass transfer at the scale of a single droplet moving in the surrounding immiscible phase is of prime importance. This study represents a major step towards derivation of reliable mass transfer resistance correlations.