Particle trapping in merging flow junctions by fluid-solute-colloid-boundary interactions

Merging of different streams in channel junctions represents a common mixing process that occurs in systems ranging from soda fountains and bathtub faucets to chemical plants and microfluidic devices. Here, we report a spontaneous trapping of colloidal particles in a merging flow junction when the merging streams have a salinity contrast. We show that the particle trapping is a consequence of nonequilibrium interactions between the particles, solutes, channel, and the freestream flow. A delicate balance of transport processes results in a stable near-wall vortex that traps the particles. We use three-dimensional particle visualization and numerical simulations to provide a rigorous understanding of the observed phenomenon. Such a trapping mechanism is unique from the well-known inertial trapping enabled by vortex breakdown [Proc. Natl. Acad. Sci. USA 111, 4770 (2014)PNASA60027-842410.1073/pnas.1321585111], or the solute-mediated trapping enabled by diffusiophoresis [Phys. Rev. X 7, 041038 (2017)2160-330810.1103/PhysRevX.7.041038], as the current trapping is facilitated by both the solute and the inertial effects, suggesting a new mechanism for particle trapping in flow networks.