A study of bubble–particle interaction using atomic force microscopy

Interaction between solid particles and air bubbles is central to froth flotation. Measurement of such interaction forces has only recently been possible with the invention of the atomic force microscopy (AFM). In this paper, the AFM colloidal 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. Strong repulsive forces due to the hydrodynamic interaction were established and quantified for both hydrophobic and hydrophilic particles, and bubbles in deionised water and 1 mM KCl aqueous solutions. No surfactants were used. In the case of hydrophobic spheres, strong attraction between the surfaces, in addition to the repulsive hydrodynamic force, was observed, leading to the rupture of the intervening water film due to submicroscopic bubbles and the attachment of the particle to the air bubble at relatively large separation distances, which were of the order of 500–2000 nm. 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. An analysis of the AFM data was carried out to obtain the interaction force and relative separation distance. Theoretical hydrodynamic force calculation 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 and surface forces.