Surface tension in engineering and geophysical flows: from nonlinear waves to droplet interactions

Surface tension governs interfacial fluid dynamics in engineering applications and geophysical flows. Computing the surface tension force is challenging, as it requires a precise description of the fluid interface. The first part of this talk will be devoted to the role of surface tension in physical processes involving nonlinear wave dynamics. I will introduce a theoretical and numerical study of weakly nonlinear interactions of gravity–capillary waves in two-layer fluid systems, showing how surface tension can influence the strength of nonresonant interactions and give rise to a new type of resonant interaction. I will then present multiphase direct numerical simulations of strongly nonlinear breaking waves and quantify the bioluminescent light intensity emitted by oceanic microorganisms. The critical role of air bubbles in both the breaking-wave hydrodynamics and the bioluminescence process will be discussed.

In the second part of this talk, I will highlight recent progress on the interaction between raindrops and a deep pool, focusing on how this interaction influences the dynamics of secondary droplet formation. Our simulations are validated against established experimental and numerical results for single-raindrop impacts. The scaling of secondary droplets produced by direct raindrop–pool interactions will be explained through a theoretical framework analogous to that of bubbles in turbulent flows. From the simulations of two-raindrop cases, we identify a critical influence of the distance between the raindrops on the flow physics, which profoundly affects the secondary droplet statistics at later stages.