Dissolution Arrest and Stability of Particle-Covered Bubbles
Original entry: Tom Kodger, APPHY 226, Spring 2009
M. Abkarian, A.B. Subramaniam, S.Kim, R.J. Larsen, S.Yan, H.A. Stone. PRL 99, 188301 (2007);
Armored bubbles, surface evolver, Laplace overpressure
Experiments show that bubbles covered with monodisperse polystyrene particles, with particle to bubble radius ratios of about 0.1, evolve to form faceted polyhedral shapes that are stable to dissolution in ir-saturated water. We perform SURFACE EVOLVER simulations and find that the faceted particle-covered bubble represents a local minimum of energy. At the faceted state, the Laplace overpressure vanishes, which together with the positive slope of the bubble pressure-volume curve, ensures phase stability. The repulsive interactions between the particles cause a reduction of the curvature of the gas-liquid interface, which is the mechanism that arrests dissolution and stabilizes the bubbles.
Capillarity In Action
A spherical gas bubble within a liquid, saturated with the entrained gas, has a positive Laplace pressure and therefore is unstable. If this 'overpressure' can be mitigated, the stability of bubble can be lengthened several orders of magnitude. The authors shot that the stability of armored bubbles (bubbles with particles at the interface), which adopt various nonspherical and irregular shapes can be understood in terms of the local geometry of the liquid-gas interface as characterized by the mean and Gaussian curvatures at the scale of individual particles.
For armored bubbles with 'a/R ~ 0.1', where 'a' is the stabilizing particle diameter and 'R' is the bubble radius, the authors observe facets with a fivefold disclination at the intersections typically unoccupied by a particle (Fig. 1).
To further investigate, surface evolver simulations are performed. Where the surface tension of the particles is 30 times that of the bubble, a contact angle of 40degrees is applied, and a simple exponential repulsion is applied due to the anionic nature of the particles, which is justified due to limited large scale surface rearrangements near the jammed state, as verified experimentally. The volume of one particles, Vp per bubble volume, V, is incrementally simulated and the equillibrium particle positions measured. (Fig. 2).
Where buckling occurs, the asphericity of the particle stabilized bubble shows distinct transitions. The pressure difference is also calculated and found to be 0 at a distinct Vp/V value of ~130. At this point, the mean curvature, <H> is 0, the bubble overpressure become mitigated and its stability increased dramatically.