Experimental and Numerical Investigation of Isothermal Aperiodic Bubbling in Shear-Thinning Liquids

Bubble formation dynamics play a critical role in many industrial processes, including aeration systems, bubble column reactors, bioreactors, and wastewater treatment, where bubble size and formation frequency directly influence mass transfer performance. In this study, a combined experimental and computational investigation is conducted on air injection into aqueous hydroxyethyl cellulose (HEC) solutions. A wide range of gas flow rates, polymer concentrations, and capillary diameters are examined. Bubble size and formation frequency are measured using high-speed imaging to characterize several unique regimes spanning from single-bubble ejection to complex multiple-bubble formation. To isolate shear-thinning effects, comparative experiments are performed using a corresponding Newtonian fluid (glycerol) with the same zero-shear viscosity. Complementary CFD simulations are employed to assess the relative influence of shear-thinning viscosity and dynamic surface tension. Results show that surface tension dominates bubble formation at low flow rates in the single-bubble regime, while viscous effects become increasingly important at higher flow rates. Despite the surfactant-like behavior of polymer solutions, dynamic surface tension has a limited impact on bubble evolution. In the multiple-bubble regime, complex behaviors such as bubble doubling, chaining, and paired coalescence emerge, driven primarily by localized shear-thinning near the bubble interface. The work emphasizes that localized rheological variations, rather than time-dependent surface tension, are the dominant factor in controlling bubble evolution, particularly in complex multiple-bubble regimes, and highlights the necessity of accounting for non-Newtonian effects when predicting and controlling bubble dynamics in practical applications.