Effects of insoluble surfactants on breaking waves: regular and spilling regimes

We investigate the influence of insoluble surfactants on the spatio-temporal evolution of breaking waves, focusing on both regular and spilling regimes. Three-dimensional direct numerical simulations are conducted using an interface-tracking/level-set method that incorporates surfactant-induced Marangoni stresses. The simulations reveal that surfactant gradients, through Marangoni stresses, markedly alter the wave dynamics. While regular breakers exhibit only minor modifications in the presence of surfactants, increasing surfactant-induced Marangoni stresses in spilling breakers leads to pronounced changes in the crest evolution, vorticity generation, and even a transition towards plunging-like behavior. To quantify these effects, we also extend circulation-based theoretical frameworks to account for surfactant contributions. This work demonstrates the crucial role that surfactants play in the dynamics of breaking waves, revealing that their impact is primarily driven by Marangoni stresses rather than surface tension reduction.