Nanobubbles and the Nanobubble Bridging Capillary Force

Interactions between hydrophobic surfaces at short separation distances (at the nanometer scale) are very important in a number of industrial applications. For example, in the froth flotation mineral separation process it is the interaction between the hydrophobic particle and the bubble which is paramount in separating the valuable minerals from the gangue. A number of studies, most notably using the atomic force microscope (AFM) and the surface force apparatus (SFA) have found the existence of a long range hydrophobic attractive force between hydrophobic surfaces that cannot be explained by classical colloidal science theories. In many cases, this force is an artefact due to the accumulation of sub-microscopic bubbles, the so called nanobubbles, at the liquid-hydrophobic solid interface. Thus, what was thought to be a hydrophobic force was actually a capillary force resulting from the gaseous bridge formed from the coalescence of nanobubbles, that is, the nanobubble bridging capillary force (NBCF). It is the purpose of this thesis to provide further insight into the accumulation of soluble gases at the liquid-hydrophobic solid interface and the resulting NBCF. Specifically, this thesis studies these phenomena from a fundamental standpoint and additionally relates the findings to froth flotation mineral separation. A systematic method to measure the NBCF by controlling the size of the gaseous capillary bridge was devised in this thesis. Control of the capillary bridge was achieved by utilising the solvent-exchange method to accumulate nanobubbles at the surface, followed by surface scanning of the colloidal probe over the flat surface to harvest nanobubbles. Thus, the NBCF has been controlled to allow for greater success in modelling the interaction, understanding the geometric parameters of the bridge, observing changes in friction force due to nanobubbles and understanding the influence of ethanol on the force. An outcome of this thesis was the development of a capillary force model which describes the NBCF. The model considers a constant volume and constant contact angle assumption for a gaseous capillary bridge of toroidal geometry. The model was very successful in describing the NBCF at long separation distances (>20nm) for both the approach and retract interactions. The close fitting between the experimental data and the model allowed accurate determinations of the advancing and receding contact angles, bridge geometry and volume. The successful implementation of the capillary force model allowed a link between the bridge volume, and the resulting adhesion to the friction force between hydrophobic solid surfaces in water. Additionally, the model allowed the change from an attractive to a repulsive NBCF to be described by a change from a concave to convex bridge geometry. Thus, this thesis has added considerable knowledge to the fundamental aspects of nanobubbles and the NBCF. The final chapters of this thesis utilised the knowledge gained from the fundamental studies to understand the influence of nanobubbles on flotation. In the first study, the influence of NaCl concentration on the morphology of gaseous domains on a graphite surface is discussed in relation to the increased recovery of coal in saline water. In the second study, methanol treatment of a ZnS ore was found to increase the floatability due to slime removal and the artificial formation of nanobubbles