(225f) Non-Spherical Armored Bubbles: Response to Mechanical Stress and Surfactants

For over a century, colloidal particles have been used to stabilize emulsions and foams, though the detailed study of these systems has only been undertaken relatively recently. Here we report our observation that the fusion of spherical particle-covered bubbles and drops leads to a distinct mechanical regime that allows the support of non-spherical shapes even in the absence of inter-particle bonds. In this regime, the steric jamming of the particles on the closed interface supports the anisotropic interfacial stresses, resulting in a new class of dispersed interfacial solid. The O(1) particle-scale yield strain of the solid makes it resistant to uniform stresses, while non-uniform stresses result in essentially perfect plasticity, which allows fatigue-free transition between stable high aspect-ratio, convex, concave and topologically complex bubbles. Stable jammed structures can also be obtained through the natural dissolution of particle-covered bubbles. These stable non-spherical bubbles dissolve when exposed to surfactants, and exhibit distinct morphological, microstructural, and lifetime changes, which correlate with the concentration of surfactant employed. For low concentrations of surfactant, an armored bubble remains non-spherical while dissolving, whereas for concentrations close to and above the surfactant critical micellar concentration (cmc) the particle shell unjams and the bubble reverts to a spherical shape before dissolving. We propose a microstructural interpretation, supported by our experimental observations of particle dynamics on the bubble interface, that recognizes the role of interfacial jamming and stresses in particle-stabilization and surfactant mediated destabilization of armored bubbles.