  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 same density and viscosity of the carrier fluid, so that 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 equal to $\Phi \simeq5.4\%$.
For each simulation, 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.
