Microbubbles play an important role in the adhesion between particle and bubble. In this paper, the bridging action of microbubbles on the adhesion between particle and bubble was studied at different pre-wetting times in the sodium oleate system. Many microbubbles formed on coal surface when the coal particles wetted by water. The coal-microbubble flocs were formed through the bridging action of microbubbles. The coal-microbubble flocs attached to the floating bubble increased the adhesion efficiency between bubble and particle because the process of coal-bubble adhesion was converted to the process of coal flocs-bubble adhesion. When the pre-wetting time increased, the number of microbubbles on coal surface decreased, resulting in the decrease of coal-microbubble flocs, which finally reduced the froth product yield of flotation. Additionally, an evaluation index (particle carrying index, PCI) was proposed to assess particle-bubble collection efficiency and the PCI values were positively correlated with the yield of froth products. (C) 2020 Elsevier B.V. All rights reserved.