Convection rolls and 3D particle dynamics in merging solute streams

Microparticles migrate in response to gradients in solute concentration through diffusiophoresis and diffusioosmosis. Merging parallel streams of fluid with distinct solute concentrations is a common strategy for producing a concentration gradient in microfluidic devices; the gradient is then (approximately) perpendicular to the flow and results in the transverse migration of particles. This is particularly useful in separation and filtration processes, as it results in regions of particle accrual and depletion in continuous flows. This has been examined in several classic papers on diffusiophoresis, but previous works do not comment on particle dynamics near walls of nonzero surface charge, which we find to be distinct from the dynamics in the bulk. We show, through experiments and simulations, that diffusioosmotic flow along certain boundaries can result in significant particle focusing in near-wall regions. The "convection rolls," which draw plain polystyrene particles away from regions of high solute concentration close to walls, are apparent in the inlet region near a glass surface but vanish when the surface is plated with gold (a conductor). This could have implications for the fabrication of microfluidic devices for applications of diffusiophoresis.