Influence of Surface-Active Agents on Wave-Crest Energy Dynamics and Air Entrainment in Breaking Waves

Surface-active agents present at the ocean surface originate from distinct natural sources, such as algal blooms, dissolved organic matter, and atmospheric pollutants. They play a crucial role in modifying the dynamics of breaking waves. We employ high-fidelity direct numerical simulations (DNS) to investigate the interaction of spilling and plunging waves with insoluble surfactants. Given the rapid nature of breaking, we treat the surfactants as insoluble. In the DNS, the initial wave profile follows a third-order Stokes wave, whose steepness is varied to generate spilling or plunging breakers. The current numerical method can be extended to focused wave packet simulations to study the influence of surfactants in more realistic, non-monochromatic wave breaking scenarios. We investigate the effects of surfactants on the evolution of wave crest and the subsurface air entrainment. Clean and contaminated cases are compared using post-processing tools that identify and track crest kinematics and bubble trajectories. Using these tools, we quantify relevant bubble metrics, including bubble count, size distribution, entrainment onset, residence time, and vertical transport. It is found that surfactants accumulate near the crest and influence capillaries downstream. We study changes in local curvature and vorticity near the crest under the influence of surfactant. An energy-based framework is applied to assess the energetic thresholds responsible for the onset of crest instability.