  Microparticles migrate in response to gradients in solute concentration
through diffusiophoresis and diffusioosmosis. Merging streams of fluid with
distinct solute concentrations is a common strategy for producing a steady
concentration gradient with continuous flow in microfluidic devices; the solute
concentration gradient and consequent diffusiophoresis are primarily normal to
the background flow. This is particularly useful in separation and filtration
processes, as it results in regions of particle accrual and depletion in
continuous flows. Such systems have been examined in several classic papers on
diffusiophoresis, with a focus on the particle dynamics far from boundaries. We
show, through experiments, simulations, and theory, that diffusioosmotic flow
along certain boundaries can result in significant changes in particle dynamics
and particle focusing in near-wall regions. The nonzero velocity at charged
surfaces draws solute and particles along the boundary until the flow
ultimately recirculates. These "convection rolls," which result in the spanwise
migration of polystyrene particles close to boundaries, are apparent near a
glass surface but vanish when the surface is coated with gold. The
three-dimensional nature of the dynamics could have implications for the design
of microfluidic devices: Channel materials can be selected to enhance or
suppress near-wall flows. Additionally, we demonstrate the importance of
considering solute concentration-dependent models for diffusiophoretic and
diffusioosmotic mobility in capturing the dynamics of particles, particularly
in regions of low solute concentration.
