Particle Separation through Diverging Nanochannels via Diffusiophoresis and Diffusioosmosis

Abstract

Colloid separation from dilute solutions is of significant relevance in a multitude of technological applications. Here we use numerical simulations to investigate a recently proposed particle separation technique based on diffusiophoresis. Diffusiophoresis refers to the chemotactic migration of particles induced by local chemical gradients that allows for efficient particle manipulation. It can act in a cooperating or competing manner with respect to fluid advection, and is usually accompanied by diffusioosmosis that refers to the bulk flow adjacent to a solid surface. We consider a microfluidic setup with cocurrent flow in two horizontal channels connected by vertical diverging nanochannels, where the top flow contains suspended colloidal particles and solute at a low concentration and the bottom flow contains solute at a higher concentration. We analyze the fluid, solute, and particle dynamics to show that the particle motion is driven into the diverging nanochannels under the combined influence of fluid advection and diffusiophoresis, the latter arising due to the imposed solute gradient. Selective separation and characterization of the particles based on their size and surface charge can be achieved, whose efficiency can be improved by controlling the diffusiophoretic mobility of the particle, solute P\'eclet number, and imposed solute gradient. Diffusioosmosis can enhance or inhibit the diffusiophoretic separation, depending on the surface charge of the solid surfaces.