Particle capture by drops in turbulent flow

Abstract: We examine the process of particle capture by large deformable drops in turbulent channel flow. We simulate the solid-liquid-liquid three-phase flow with an Eulerian-Lagrangian method based on Direct Numerical Simulation of turbulence coupled with a Phase Field Model, to capture the interface dynamics, and Lagrangian tracking of small (sub-Kolmogorov) particles. Drops have same density and viscosity of the carrier liquid, and neutrally-buoyant solid particles are one-way coupled with the other phases. Our results show that particles are transported towards the interface by jet-like turbulent motions and, once close enough, are captured by interfacial forces in regions of positive surface velocity divergence. These regions appear to be well correlated with high-enstrophy flow topologies that contribute to enstrophy production via vortex compression or stretching. Examining the turbulent mechanisms that bring particles to the interface, we have been able to derive a simple mechanistic model for particle capture. The model can predict the overall capture efficiency and is based on a single turbulent transport equation in which the only parameter scales with the turbulent kinetic energy of the fluid measured in the vicinity of the drop interface. The model is valid in the limit of non-interacting particles and its predictions agree remarkably well with numerical results.