Retarding spreading of surfactant drops on solid surfaces: Interplay between the Marangoni effect and capillary flows

Early time spreading of a water drop on a hydrophilic surface is characterized by the wetted radius which grows linearly in time. We report the unexpected result that the initial spread of surfactant-laden drops is impeded by Marangoni stresses, leading to a large increase in total spreading time. The nonuniform distribution of surfactants at the interface generates Marangoni stresses before the drop-solid contact suppresses film drainage and droplet expansion. Our experiments show that, remarkably, surfactants delay the initial fast motion of the three-phase contact lines.