Several authors have noted bi-modal bubble size distributions (BSDs) in flotation systems at low frother or inorganic salt concentrations. The origin appears to be related to bubble-bubble interactions. The present work examines interactions among bubbles produced at a capillary using high-speed imaging. The study provides visual evidence of coalescence-related and wake-related mechanisms creating fine bubbles and bi-modal distributions. Four coalescence mechanisms are identified: coalescence-induced break-up, droplet formation and collision, liquid jet formation and collision, liquid jet disruption to droplets and collision; and two wake-related events: distortion and break-up of trailing bubble, and premature detachment. Comparing to the fine/coarse mode ratio in flotation systems (ca. 1/10) the possible relevant mechanisms are suggested. Knowing that frothers and certain inorganic salts act to retard coalescence, the origin of the bi-modal BSD is argued to be coalescence-related. (C) 2012 Elsevier Ltd. All rights reserved,