Direct quantitative measurements of the particle trajectories on the surface of an air bubble were obtained using a CCD high-speed video camera system. Rupture of the intervening water film between the hydrophobic particle and air bubble surfaces with the

Study of interaction forces between solid particles and air bubbles is a key to understanding a range of technologically important phenomena, including the flotation separation of particles. Measurement of such interaction forces has only recently been made possible with the introduction of the atomic force microscopy (AFM). In this paper, the AFM probe technique was used to measure hydrodynamic interaction forces between a solid sphere attached to an AFM cantilever and an air bubble placed on an AFM piezoelectric stage at different approach speeds. Interaction forces before the interfacial water film rupture, as well as hydrophobicity of the particle can be established. In the case of hydrophobic spheres, strong attraction between the surfaces, leading to the rupture of the intervening water film and the attachment of the particle to the air bubble was observed. In the case of hydrophilic spheres, the rupture of the intervening water film and the attachment of the particle to the air bubble did not take place. Strong repulsive forces due to the hydrodynamic interaction are quantified. Theoretical hydrodynamic force shows agreement with experimental data for larger separation distances. Deviations at shorter distances are related to the deformation of air–water interface due to the particle approach, as well as intermolecular and surface forces.