Surfactant-laden drops in wall-bounded turbulence

The dynamics of large surfactant-laden drops in a turbulent channel flow is investigated using direct numerical simulations. Such a multiphase system is here described using a modified Phase Field Method (PFM), which accounts for both the interface and surfactant dynamics. The phase field variable is a marker function that defines the local concentration of each phase; it is uniform in the bulk of the phases and undergoes a smooth transition across the interface. The transport of both phase field and surfactant concentration is described by two Cahn–Hilliard (CH) equations. Their contribution is included in the Navier–Stokes (NS) equations via an interfacial term. This coupled NS–CH system describes the complex dynamics of surfactant-laden droplets in turbulence. A Fourier-Chebychev pseudo-spectral method is adopted to discretize the coupled system of equations in a closed channel geometry. The presence of surfactant at the interface strongly affects the interfacial dynamics: surfactant locally reduces surface tension (with respect to a clean interface) and can induce Marangoni stresses. We examine the effect of surfactant loading and strength on the dynamics and coalescence/breakup rates of a swarm of large drops in a turbulent channel flow.