Dynamic deformation of an air bubble when pressed against a solid surface

Interactions of air bubbles with solids are important in many practical areas including bubble nucleation, erosion, cavitation and flotation processes for separation of mineral ores, plastics and paper recycling streams. Air bubbles interact with solids via equilibrium surface forces (e.g. electrostatic and van der Waals) and hydrodynamic forces. These forces deform the bubble shape in interesting ways, including a hydrodynamic ‘dimpling’ effect that significantly slows the rate of approach of the bubble and solid surfaces. We present the results of experiments in which a solid mica surface was driven toward an air bubble in pure water using a modified Surface Forces Apparatus. An optical interference technique was used to provide high resolution measurements (< 2 nm) of the water film thickness and shape when the bubble comes very close to the solid. These measurements are compared to some earlier work using a mercury drop (J.N. Connor and R.G. Horn, Faraday Discussions 2003, 123, 193-206). We find that, as with the mercury drop, the film shape depends on the rate of approach and the duration of the drive after the onset of deformation. However, unlike the mercury drop case where the end result is a flat film, the long-time film between the air bubble and mica is thinner at the apex becoming thicker in the radial direction.