Direct numerical simulations of surfactants in bubbly tri-periodic turbulent flows

Turbulent bubbly flows are common in nature, industry and even our daily life; e.g. breaking waves, water treatment facilities and laundromats. A common element to these applications is the presence of surface-active agents, also known as surfactants. Surfactants have profound effects on these flows: by lowering the surface tension, they modify deformability of the interface and introduce Marangoni stresses. 

To investigate the mechanisms behind these effects, we perform direct numerical simulations of bubbles in sustained homogeneous isotropic turbulent flows, (Taylor Reynolds number~250). We model the bubbles and carrier phase using the volume-of-fluid method, and the surfactant as a third phase that is transported with the interface. We adopt a modified equation of state to account for the reduced surface tension due to surfactant. This allows us to investigate how the concentration of surfactant affects the equilibrium size distribution of bubbles, and how surfactant distributes as the bubbles breakup and coalesce.

We also analyse how the surfactant-laden bubbles affect the turbulent kinetic energy spectrum of the flow; we perform a scale-by scale analysis of each term in the Navier-Stokes equation to find how surfactant laden bubbles are disrupting Kolmogorov’s energy cascade.