Directional motion of charged particles near filter membrane

Abstract

Membrane has many applications in the fields of filtration and separation, but due to the attraction or repulsion exerted by the membrane, the particles will experience a directional motion. As a result, two totally opposite effects, i.e. particle enrichment and exclusion zone, take place in the vicinity of the membrane, and the underlying reason is still not clear. In this work, colloidal particles with negative surface charge are used as a model substance, with the advantages of monitoring the particle concentration in a real time and <i>in situ</i> way, to investigate the influence of cellulose membrane on the movement of particles. The experimental results show that the particles are enriched in the vicinity of the membrane. The diffusiophoresis effect originating from tiny number of ions released by the film is the main reason of the directional movement of the charged particles. Based on the two mechanisms of diffusiophoresis and diffusion, we construct a model and make relevant numerical calculation, and the numerical results are qualitatively consistent with the experimental results. Moreover, in addition to the longitudinal motion of the particles towards the filter membrane, diffusio-osmotic flow and particles lateral diffusion also result in the migration of particles towards the container wall, and further increasing the particle number near the wall.