Technical Papers and Presentations

Introduction

Oversized microbubbles traveling through confining conduits can create flow blockages when the lubrication layer surrounding them drains, requiring undesirable increases in pressure gradient for discharging channel contents. In increasingly demanding industrial applications, such as petroleum and natural gas extraction, these obstructions can be exponentially impactful when considering the number of fissures created in one well. Similarly, oversized bubble blockages can concern any pumping application such as biochemical analysis or medical device design and manufacturing.

An oversized bubble flowing through a conduit will have a thin-film of continuous phase fluid entrained around it acting as a lubrication layer during movement. As a bubble may encounter obstacles or stop, that thin film drains as surface tension becomes dominant in dictating bubble shape. With a draining thin-film due to surface tension, the bubble interface may be drawn into contact with the confining conduit. This paper studies the possibility of utilizing electrostatic forces to modify bubble morphology and identifies how a repulsive behavior can be formulated.

Model

In order to understand the basis of electrostatic influence on a charged bubble interface, a fundamental model was developed to determine a range of repulsive and attractive interactions due to electric potential. A spherical bubble of 50µm radius was modeled in a water filled cylindrical microchannel of 50.2µm radius. Varying ionic concentration in the water and varying electric potential applied to the channel walls and bubble interface, electrostatic force effects were quantified by observing bubble deformation.

Utilizing the Two Phase Flow, Moving Mesh interface in COMSOL Multiphysics^® simulation software, a 2D axisymmetric spherical air bubble was modeled in a water-filled cylindrical channel with planar symmetry along its cross section. The Electrostatics interface was used to apply an electric potential on the channel wall and bubble interface while grounding the channel outlet. The Transport of Dilute Species interface was coupled to the Two Phase Flow, Moving Mesh interface, to determine ion distribution in the water due to advection, and coupled with the Electrostatics interface, to determine ion distribution due to the electrostatic boundary conditions. A customized weak contribution was developed in the Two Phase Flow Moving Mesh interface, coupling the electric potential via Maxwell’s equations in an interfacial term of the Navier-Stokes equations.

Results and Conclusion

This model shows a clear ability to create repulsive and attractive scenarios between the bubble interface and channel wall. The combination of electric potential and ion concentration for a given geometry are significant factors of electrostatic influence on interfacial deformation. With larger electric potentials of like polarity, thicker thin films can be maintained, but this seems to exist only below a particular threshold of ion concentration. With larger standoff distances, lower concentrations are required for a far-field effect. The data gathered prescribes the ability to develop repulsive electrostatic environments and points to the capability of precluding bubble contact in confining conduits where greater potentials and more confined spaces could amplify repulsive behavior.