Controlled Coating and Characterization of Interfaces to Produce Particle Stabilized Bubbles

Surface active particles have the ability to adsorb and rigidify an interface. They are particularly useful in stabilizing bulk foams and emulsions, where the entrained droplets and bubbles are coated with particles that inhibit or slow coalescence. Being able to control the coating process for individual bubbles would allow for the controlled production of particle coated bubbles, or capsules, that could be used to construct stabilized foams with tunable properties. This thesis performs a systematic study of the coating of air bubbles using surface active particles. Specifically, it examines a millifluidic coating process that uses silica – CTAB particle surfactant complexes. Using this colloidal system only requires changes to the particle or surfactant concentration to vary surface activity. The goal of the work is to understand the coating process that occurs and identify parameters that affect the final capsule shape.

Bubbles flowing in suspensions of silica – CTAB complexes exhibit two distinct behaviors compared to bubbles in either silica suspensions or CTAB solutions individually. First, particle coated bubbles exhibit fluid film thickening at the trailing end of the bubble. Second, the trailing end of the bubble can become deformed and crumpled under certain conditions. Both effects result from the adsorption of silica – CTAB complexes to the interface. The complexes form a rigid layer on the surface of the bubble, changing the boundary condition for fluid flow in the film regions and the material properties of the interface.

When producing capsules, production and formulation parameters are varied to identify controlling factors that determine the final capsule shape. Non-spherical capsules are generally observed after flowing through a capillary tube filled with silica – CTAB complexes if the tail cap is not deformed. The capsule aspect ratio increases with bubble length, up to a critical length where deformations to the tail cap are observed. The aspect ratio decreases and plateaus for bubbles longer than the critical length. The critical length, and thus the aspect ratio, vary the most with particle and glycerol concentration in suspension.

This thesis also produces a new apparatus, a beamline tensiometer, to simultaneously probe interfacial mechanics and particle structure and motion on an interface. Small Angle X-ray Scattering (SAXS) and X-Ray Photon Correlation Spectroscopy (XPCS) are performed on a particle coated bubble while measuring the surface tension in situ. The device has the unique capability of being able to wash the interface and introduce flocculating agents by exchanging the fluid surrounding the bubble. The silica – CTAB particle – surfactant system is used to demonstrate the functionality of the apparatus. The measurements enabled by the beamline tensiometer will help to further expand the understanding of the relationship between the interfacial mechanics and the responsible particle structure and motion on the interface. This insight can advance the ability to engineer the mechanical properties of foams and porous materials.