Particle capture and trapping by large deformable drops in turbulent channel flow*

We investigate numerically the capture of neutrally-buoyant, sub-Kolmogorov particles at the interface of large deformable drops in turbulent flow and the subsequent evolution of particle preferential distribution on the drop surface. Direct numerical simulation of turbulence, phase field modeling of the drop interface and Lagrangian particle tracking are used. Excluded-volume interactions, obtained by enforcing particle collisions, are included to prevent unphysical clustering of the trapped particles. Our results show that particles can be captured only if driven towards the drop surface by jet-like turbulent fluid motions. This process is similar to particle deposition at a solid wall and the resulting capture rate can be predicted via a simple mechanistic model that assumes a proportionality between the mass flux of captured particles and the mean concentration of particles that remain afloat in the bulk of the carrier phase. Once captured by the interfacial forces, particles tend to disperse on the surface, and collect into long-term trapping regions where the average surface velocity divergence sampled by the particles is zero. These regions surround the negative-divergence compression regions where particles are driven by the interfacial stresses and determine a much higher surface coverage compared to the case in which excluded-volume interactions are neglected. Such preferential accumulation is a direct consequence of the excluded-volume interactions among neighbouring particles. These regions correlate well with portions of the interface characterized by higher-than-mean curvature, indicating that modifications of the surface tension induced by the presence of tiny particles, and hence of drop deformability, will be stronger in the highly-convex regions of the interface.