In flotation, the separation of fine particles (≤10 µm) by bubbles is inefficient. Because of their low mass and inertia, fine particles follow the fluid streamlines around the bubble rather than colliding with it. However, particle-bubble interaction is enhanced as particle size decreases for ultrafine particles that undergo Brownian diffusion (≤1µm). A minimum in particle-bubble collection efficiency is expected at particle sizes where both the interception and Brownian diffusion mechanisms dominate the collision process. The aim of this study is to study the interaction of particles with bubbles as a function of particle size (0.1-7 µm) and surface hydrophobicity (modified by methylation). Single bubble flotation was used as a tool to study the interaction between silica particles and bubbles in non-turbulent conditions. Fluorescent dye was incorporated into the ultrafine particles and fluorescent spectroscopy was used to measure the particle concentration in both feed and concentrate due to its low detection limits compare with other techniques. The combined experimental results showed that particle-bubble collection efficiency decreases with increasing particle size up to 1-2 µm and then increases with increasing particle size. Also, the collection efficiency was found to increase with increasing surface hydrophobicity. These results were used to test several particle-bubble collision and attachment models.