Effect of surfactant-laden droplets on turbulent flow topology

In this work we investigate flow topology modifications produced by a swarm of large surfactant-laden droplets released in a turbulent channel flow. Droplets have the same density and viscosity of the carrier fluid, so only surface tension effects are considered. We run one single-phase flow simulation at Re-tau = rho u(tau)h / mu = 300 and ten droplet-laden simulations at the same Re-tau with a constant volume fraction Phi similar or equal to 5.4%. For each simula tion, we vary the Weber number We (ratio between inertial and surface tension forces) and the elasticity number beta(s) (parameter that quantifies the surface tension reduction). We use direct numerical simulations of turbulence coupled with a phase-field method to investigate the role of capillary forces (normal to the interface) and Marangoni forces (tangential to the interface) on turbulence (inside and outside the droplets). As expected, due to the low volume fraction of droplets, we observe minor modifications in the macroscopic flow statistics. However, we observe major modifications of the vorticity at the interface and important changes in the local flow topology. We highlight the role of Marangoni forces in promoting an elongational type of flow in the dispersed phase and at the interface. We provide detailed statistical quantification of these local changes as a function of the Weber number and elasticity number, which may be useful for simplified models.