Investigation on the oil bridge between air bubble and glass surface in water: Formation, deformation, mechanical force and implications for froth flotation

In froth flotation, the insoluble oil as a liquid bridge between an air bubble and a solid surface in water is a common and important interface phenomenon. This paper investigated the impact of the oil bridge between an air bubble and a glass plane from aspects of mechanical forces acting on complex interfaces. An air bubble was pulling towards an oil droplet anchored on a flat glass slide under water compulsively. In this process, the highspeed camera was applied to observe the deformation of the interfaces and the mechanical forces were measured using a micro-force sensor. It was found that the air-water-oil (A-W-O) three-phase contact line (TPL) can form, expand and become stable rapidly within 85.0 ms when the bubble is attached to the oil droplet. The expansion of the A-W-O TPL provides an extra adhesion force to the air bubble. The oil formed an oil bridge connecting the bubble and the solid surface which shortened the attachment distance and extended the detachment distance. The mechanical force resulting from the oil bridge acting on the air bubble is equivalent to an air bubble directly adhering to a hydrophobically modified surface. The capillary force and pressure force were calculated and were found to be the major components of the mechanical force in the detaching process. This mechanical force was directly influenced by the interfacial tension vectors, the interface curvatures, and the TPL radius. The deformation on the interface, especially the A-W interface, is significant to the detachment process and related mechanical forces.