CO$_2$-driven diffusiophoresis: motion of charged particles near CO$_2$ dissolving boundaries*

We present experimental and theoretical investigations for CO$_2$-driven diffusiophoresis of charged particles. An aqueous suspension of charged particles initially in contact with a CO$_2$ dissolving interface shows directional migration either up or down the concentration gradient, depending on the particle charges. We use a cylindrical CO$_2$ bubble in a circular Hele-Shaw cell to study the behavior of polystyrene particles. By experiments and numerical model calculations considering multicomponent gas dissolution, we prove the diffusiophoretic accumulation and exclusion of particles near the bubble interface. Then using two geometrical conditions with moving and fixed boundaries, we show diffusiophoresis of bacterial cells driven by dissolution of CO$_2$. \textit{Vibrio Cholerae}, a Gram-negative bacteria, has a negative surface charge and thus migrates away from a CO$_2$ source. Using PIV we show behaviors of wild type cells and a mutant lacking flagella, suggesting that CO$_2$-driven diffusiophoresis may prevent biofilm formation by reducing the population of cells approaching an interface.