Collision of a small rising bubble with a large falling particle

The attachment of bubbles onto a collecting surface plays a critical role in flotation which is utilised for the separation of mineral ores, coal or plastic materials. While mineral flotation deals with fine particles and larger bubbles, this work is focused on the opposite case of an interaction of a single rising bubble (D-b < I mm) with a larger spherical particle, which falls down through a stagnant liquid. The collision is studied theoretically and experimentally. The theoretical model, based on an analysis of forces acting on the bubble, leads to a differential equation for the bubble motion. Both the mobile and immobile bubble surfaces are considered. The experimental bubble trajectory and velocity evolution are in good agreement with the theoretical model. The horizontal deflection of the bubble trajectory caused by the particle motion is dependent on the ratio of bubble terminal velocity and particle settling velocity. The influence of buoyancy, interception and inertial mechanisms on the collision efficiency is also examined. It is concluded that the buoyancy is the most significant mechanism for the interaction of small bubbles with large particles. (C) 2013 Elsevier B.V. All rights reserved.