Drops spreading on fluid surfaces: Transition from Laplace to Marangoni regime

We show the occurrence of two distinguished classical regimes of wetting, namely, Laplace and solutal Marangoni, during the spreading of oil drops on a surfactant-laden aqueous phase in a single surfactant-oil-water system. The spreading kinetics is found to follow a power-law behavior not only in the Laplace and Marangoni regimes, but also in the transition regime. Our experimental findings are corroborated with the scaling laws. The results demonstrate that increasing the surfactant concentration across the critical micelle concentration is instrumental to obtain the Laplace to Marangoni transition. Moreover, this transition does not depend on surfactant chemistry; instead, it depends on the adsorption/desorption kinetics of surfactant molecules to/from the interfaces that are created or annihilated during drop spreading.