Characterization of single bubble-particle interactions for microplastic removal and flotation applications

The interaction between solid particles and air bubbles in liquids holds both academic and practical importance, as it occurs naturally and plays a key role in various industrial applications such as microplastic removal, mineral flotation, and wastewater treatment. To enhance our understanding of this fundamental process for environmental and engineering purposes, this study experimentally investigates its most basic form; a single rising bubble colliding with a single falling particle. The particles, made of plastic materials with densities close to or slightly above water, represent common pollutants found in aquatic environments, with sizes ranging from 1 mm to 7 mm and shapes varying from regular spherical to irregular forms. By using different nozzle types to control bubble size, bubbles are generated with Reynolds numbers between 50 and 2,500. When these particles settle under gravity and encounter the buoyant upward movement of bubbles, two primary interactions, collision and adhesion, are observed, depending on respective properties of the bubble and particle, such as size, shape, and velocity, which are further developed into governing dimensionless parameters. This study identifies the hydrodynamic conditions that govern these interactions and constructs an interaction map based on experimental data, offering valuable guidance for optimizing bubble-assisted removal systems, improving flotation-based separation processes, and informing predictive models for particle behavior in both natural and engineered aquatic systems.