Stability and dynamics of bubbles with particle-laden interfaces
Solid particles can adsorb at fluid-fluid interfaces, much like molecular surfactants. Unlike molecular surfactants though, the adsorption of solid particles can be considered to be virtually irreversible, due to a large capillary energy associated with adsorption. As a result, solid particles can be used to stabilise drops and bubbles. Remarkably, particle-stabilised bubbles are found to be indefinitely stable, because the particle monolayer can arrest bubble dissolution, which is otherwise driven by the Laplace pressure.
In this talk I will present two phenomena observed in our laboratory when particle-stabilised bubbles are subjected to changes in pressure or temperature.
The bubbles can be driven into periodic compression-expansion using ultrasound waves, causing significant deformation and microstructural changes in the particle monolayer. We observe shape oscillations driven by a parametric instability, and migration of the particles to the antinodes. The compression of the interface causes particle expulsion, and we have uncovered different particle expulsion scenarios depending on the mode of bubble deformation, including highly directional patterns of particle release during shape oscillations. Complete removal of particle monolayers from bubbles can be achieved in under a millisecond.
In the second part of the talk, I will present results on the effect of changes in temperature on the lifetime of particle-stabilised microbubbles in water. We found that a decrease in temperature destabilises particle-coated microbubbles beyond dissolution arrest. A simple model describing the effect of the change in temperature on mass transfer suggests that the dominant mechanism of destabilization is the increased solubility of the gas in the liquid. We also found that dissolution of a particle-coated bubble can lead either to buckling of the coating, or to gradual expulsion of particles, depending on the particle-to-bubble size ratio.