Abstract
Dissolution and dissociation of CO₂ in an aqueous phase induce diffusiophoretic motion of charged particles. Such phenomenon can be applied to maintaining a surface free of bacteria, by migrating the nearby cells away from a CO₂ source. Dissolution and dissociation of CO₂ in an aqueous phase induce diffusiophoretic motion of suspended particles with a nonzero surface charge. We report CO₂-driven diffusiophoresis of colloidal particles and bacterial cells in a circular Hele-Shaw geometry. Combining experiments and model calculations, we identify the characteristic length and time scales of CO₂-driven diffusiophoresis in relation to system dimensions and CO₂ diffusivity. The motion of colloidal particles driven by a CO₂ gradient is characterized by measuring the average velocities of particles as a function of distance from the CO₂ sources. In the same geometrical configurations, we demonstrate that the directional migration of wild-type V. cholerae and a mutant lacking flagella, as well as S. aureus and P. aeruginosa, near a dissolving CO₂ source is diffusiophoresis, not chemotaxis. Such a directional response of the cells to CO₂ (or an ion) concentration gradient shows that diffusiophoresis of bacteria is achieved independent of cell shape, motility and the Gram stain (cell surface structure). Long-time experiments suggest potential applications for bacterial diffusiophoresis to cleaning systems or anti-biofouling surfaces, by reducing the population of the cells near CO₂ sources.
