(99a) Formation and Disruption of a Particle Coating on a Confined Bubble

When surface-active nanoparticles adsorb at fluid interfaces, they impart elasticity to the interface and provide stability against coalescence and deformation. Compared with molecular surfactants, nanoparticles diffuse slowly, desorb very slowly, and experience colloidal interactions. These characteristics allow the coverage and mechanical properties of particle-coated interfaces to be manipulated in a fluid flow. Here, we examine the coupling of hydrodynamics and particle adsorption for elongated bubbles translating along a capillary tube filled with a surface-active nanoparticle suspension. In this experiment, particle transport and adsorption timescales are comparable to bubble residence times, and the ratio can be manipulated by varying flow rate, tube length, and suspension viscosity. Upon formation of the bubble, particles are strongly convected to the trailing end of the bubble, resulting in a rigid trailing cap. The bubble exhibits a classical Bretherton-like film that is uniform in thickness for short bubbles. Longer bubbles exhibit a sharp transition from a thin film at the leading edge to a thick film at the trailing edge, characteristic of a Marangoni stress along the interface. At higher speeds, the trailing cap appears to crumple, shedding layers of nanoparticles. We relate these observations to the interfacial hydrodynamics and the development of elasticity on the bubble interface. These observations highlight the critical role of processing history on the development of particle-stabilized interfaces.