Colloid Separation by CO2-Induced Diffusiophoresis.

We present a microfluidic crossflow separation of colloids enabled by the dissolution of CO2 gas in aqueous suspensions. The dissolved CO2 dissociates into H+ and HCO3- ions, which are efficient candidates for electrolytic diffusiophoresis, because of the fast diffusion of protons. By exposing CO2 gas to one side of a microfluidic flow channel, a crossflow gradient can be created, enabling the crossflow diffusiophoresis of suspended particles. We develop a simple two-dimensional model to describe the coupled transport dynamics that is due to the competition of advection and diffusiophoresis. Furthermore, we show that oil nanoemulsions can be effectively separated by utilizing highly charged particles as a carrier vehicle, which is otherwise difficult to achieve. These results demonstrate a portable, versatile method for separating particles in broad applications including oil extraction, drug delivery, and bioseparation.