(601a) Coating and Crumpling of Armored Gas Filled Capsules

Bubbles flowing through the confined geometry of a fluid filled capillary tube are affected by the presence of surface active molecules or particles that modify the interfacial mechanics. Molecular surfactants adsorb to the bubble interface and lead to Marangoni stresses that result in changes to the bubble shape as the capillary number and bubble length vary. Surface active particles also lead to Marangoni stresses, but additionally impart significant interfacial elasticity. We investigate the case of bubble flow in a capillary filled with surface active particle-surfactant complexes. Cetyltrimethylammonium bromide (CTAB) surfactant adsorbs to the surface of silica nanoparticles in a glycerol-water suspension to form the surface active complexes. Bubbles are dispensed via a co-flow capillary apparatus and the initial bubble length is varied. Capillary number is varied by adjusting the bubble velocity and suspension viscosity through the glycerol concentration. Fluid film thickness is measured along the length of the bubble at several positions in the capillary. These measurements are compared with bubble flow through surfactant solutions and with predictions from a Bretherton-type model. The adsorbed particle-surfactant complexes form a rigid layer at the tail end of the bubble, changing the mechanical properties of the interface. The behavior of the bubble changes with the bubble length, and two critical bubble lengths are observed. Above the first critical length, two fluid film regions form: a thin film and a thickened film. The thickened film arises from the rigid particle layer on the interface. Above the second critical length, the bubble tail cap begins to crumple and collapse. The crumpling behavior occurs soon after the bubble is dispensed into the capillary, and the critical lengths vary with the bubble velocity. These results allow us to infer rheological and mechanical properties for the interface that are associated with the crumpling phenomenon.