Diffusiophoretic field flow fractionation in the presence of a pH gradient*

Diffusiophoresis is the spontaneous motion of particles under concentration gradients of solutes. When there are electrolyte concentration gradients formed across a channel flow, particles with nonzero surface potential can undergo diffusiophoresis and accumulate toward side walls. The direction of motion depends on the mobilities of particles and the system demonstrates diffusiophoretic-driven field flow fractionation. In this study, we show a channel flow configuration where there is initially a steady-state stream of a particle suspension at a high pH. Then, we impose a concentration gradient of ions due to CO₂ dissolution from one side wall and, consequently, particles undergo diffusiophoresis. We study experimentally and theoretically the diffusion of multiple ions in the system and the pH-dependent diffusiophoretic behaviors of the particles.