Motion of nanoparticles near rising and dissolving microbubbles

This paper presents a computational study of the motion of inertia-less particles in the vicinity of freely rising bubbles of carbon dioxide that dissolve into the suspension. The purpose is to assess the extent of particle motion due to diffusiophoresis that is caused by ion concentration gradients, in this case protons and bicarbonate ions. Calculations are reported for spherical bubbles with diameters from 10 to 100 μm. Particle trajectories are computed for bubbles with slip and no slip surfaces, which (respectively) represent the extremes of a high purity system and a contaminated system in which surfactants fully immobilise the surface. The extent of particle motion towards or away from the bubbles (depending on the sign of the diffusiophoretic mobility) is quantified. Particle motion due to removal of dissolved carbon dioxide from a suspension into air bubbles is also considered. In the cases where particles are attracted to the bubbles, some particle trajectories reach the surface of the bubble. The results suggest that this mechanism could therefore be exploited to extend the use of flotation to separate smaller particles.