Bubble Rise Dynamics in Complex Fluids

de Azin Padash

Formation of gas bubbles in complex fluids and their subsequent rise due to buoyancy is a very important fundamental phenomenon both in nature and industry. Bubble size and bubble velocity are critical parameters which govern the interfacial transport phenomena and play an important role in gas-solid contact. These characteristics affect the operating parameters as well as the design of equipment in industrial applications. Non-Newtonian, Shear-thickening fluids have been studied extensively due to their immense potential for commercial use in shock absorbing and force damping applications, such as liquid body armor, sports and personal protection. Furthermore, a better understanding of shear-thickening fluid is pertinent to industrial processing for enhancing flow, preventing the breakage or clogging of mixing equipment, and preventing clogging in narrow orifices. Despite their significance, many aspects of the flow of these non-Newtonian fluids remain poorly understood. In the first part of this dissertation, we study the dynamics of rising bubbles in three dimensional fluidized beds using computational fluid dynamics-discrete element method (CFD-DEM) to shed light on the physics underpinning phenomena uncovered previously using magnetic resonance imaging (MRI). We were able to understand the underlying mechanism behind the anomalous collapse of a bubble in side-by-side injection as well as an alternating asynchronous pinch-off pattern due to jet interaction in a fluidized bed by looking into the gas streamlines and the drag force on the particles. In the second part of this dissertation, we study dynamics of rising bubbles in Newtonian fluids and non-Newtonian cornstarch-water suspensions experimentally using optical imaging. We were able to identify that Capillary number (Ca) is a key dimensionless parameter governing the regimes of interacting jets in water. We also observed a periodic coalescence of bubbles at the same points in space in cornstarch-water suspensions and attributed this behavior to leading bubbles entering a shear thickening regime. Further, we identified the key dimensionless parameters for wobbling behavior of single bubbles in cornstarch suspensions to be Bond (Bo), and Reynolds (Re) number, regardless of the bubble being in a Newtonian or a shear-thinning regime. We believe our findings can be applied in industry to optimize the mass transport and liquid mixing for a range of applications.