60th Annual Report on Research 2015

The objective of this project is to research the effects of interface-bound particle presence on the contact dynamics of bubbles in confining flow conduits, aiming to demonstrate particle-coated bubbles as a potential technology for preventing flow blockage during petroleum recovery. In particular, the focus is on how incorporating the bubble surface with electric charges prevents droplet contact with the conduit walls.

During the second year of this ACS PRF award, we focused our research efforts on numerical modeling of charged bubble deformation and thin film drainage in confining conduit walls. The fluid surrounding the bubble is an aqueous solution containing symmetric electrolytes of various concentrations. The channel wall is defined with a zeta-potential while the bubble interface is defined with various surface charge densities and polarities. Governing equations of ion transport, electrostatics, and laminar hydrodynamics are solved interactively and iteratively in the COMSOL Multiphysics environment with the Finite Element Method. The numerical solutions take advantage of 2D axisymmetry and transverse symmetry of the bubble and the flow channel. A free triangular mesh with special refinements in the vicinity of the bubble interface and the channel wall is adopted to accurately capture the film drainage in the lubrication layer, the bubble deformation, and the electrostatic interactions in the electric double layer (EDL). A mesh sensitivity study indicated that a distribution of elements on the order of 1 nm near the boundaries is needed to provide a sufficient resolution to capture the electrostatic effects of the EDL. A dynamic mesh is implemented to track the interface as it approaches the channel wall and enables increasing resolution of the gap domain as the lubrication film drains. Results of each simulation are analyzed in terms of interfacial deformation and whether the bubble interface makes an eventual contact with the fluid conduit wall.

Our results indicate that electrostatic interactions can be used to aid or counter surface tension-induced bubble contact with the confining channel. The time scale to contact depends on the surface charge densities as well as the ionic concentration in the continuous phase solution. In the case of repulsive electrostatic interactions, it is possible to reach a state where the lubrication layer surrounding the bubble is maintained, and thus can be taken advantage of to prevent channel flow blockage. Furthermore, even when the bubble interface and the flow channel have the same charge polarity, contact can be avoided under a specific range of interfacial charge density. Still, the method of charging the bubble interface, whether through adsorption of charged surfactants or nanoparticles, is expected to create additional surface tension effects and electrostatic interactions among themselves and lead to changes in interface morphology. Investigation of these aspects is currently underway.

In this year the ACS PRF award supported Mr. Jonathan Hui, who completed his Master’s Thesis on computationally modeling charged droplet deformation and thin film drainage in a confining conduit, and is currently a PhD degree candidate continuing this work under the guidance of the PI. Mr. Hui will present our findings at the 68^th American Physical Society Division of Fluid Dynamics conference held in Boston, MA.