(19c) Diffusiophoresis in Microfluidic Dead-End Configuration: Influence of the Environment

Diffusiophoresis is the spontaneous motion of colloidal particles under a concentration gradient of solutes. For particles with nonzero surface charge suspended in electrolyte solutions, a combination of electrophoresis and chemiphoresis results in the net motion. The typical diffusiophoretic mobility of such particles is a function of the surface potential, and it can be approximated as a constant for an ideal limit of fixed zeta potential and a thin double layer. However, in common systems the effects of a finite double layer thickness, relative to the particle size, are significant in dilute electrolytes, whereas in concentrated electrolytes, charge screening could result in a decrease in the diffusiophoretic mobility. Also, the presence of multiple ions may influence the observed particle motion in a non-monotonic way due to the coupled ionic fluxes.

The straight dead-end pores allow experimental observations of diffusiophoretic particle transport under one-dimensional solute gradients. Due to the charged nature of the materials forming the microfluidic device (e.g. PDMS), a diffusioosmotic flow is generated along the concentration gradient, near the walls of the pores. Such slip velocity, in order to maintain the zero liquid flux condition inside the dead-end pores, induces fluid flow that affects the observations of the particle motion. Both the particle diffusiophoresis and the wall diffusioosmosis are influenced by the concentrations, ion types, and combinations of electrolytes.

We first present the electrokinetic aspects of the system using one-dimensional (1D) and two-dimensional (2D) distribution of polystyrene particles. Then we show how these effects are combined with the flow structure and the channel geometry, by unravelling the three-dimensional (3D) dispersion of the particles.