Diffusioosmotic Reversal of Colloidal Focusing Direction in a Microfluidic T-Junction

Solute gradients next to an interface drive a diffusioosmotic flow, the origin of which lies in the intermolecular interactions between the solute and the interface. These flows on the surface of colloids introduce an effective slip velocity, driving their diffusiophoretic migration. In confined environments, the interplay between diffusiophoresis and diffusioosmosis governs the motion of colloids. Previous studies have indeed demonstrated the quantitative modulation of phoretic migration by the osmotic flows. Here, we show that diffusioosmotic flows can lead to qualitatively distinct outcomes, reversing the direction of colloidal focusing expected from diffusiophoresis alone. Using microfluidic experiments in a T-junction, numerical simulations, and theoretical modeling, we explain our observations to be due to an interplay between diffusiophoretic migration of colloids toward the walls and their entrainment in a diffusioosmotic vortex. We show this focusing to be persistent for a range of salt types, salt gradients, and flow rates, and establish a criterion for its emergence. Our work sheds light on how boundaries modulate the solute-mediated transport of colloids in confined environments and how the colloidal trajectories can be utilized to infer the surface properties.