Quantitative Study on Fast Kinetics of Hydrodynamic Liquid Absorption of CO ₂ Microbubbles Enabled by Microfluidic Visualization and Computational Fluid Dynamics Method

Liquid absorption is one of the mainstream approaches to carbon capture in response to the global warming crisis. Its performance is fundamentally subjected to the interphase mass transfer whose kinetics are determined and regulated by various factors, including absorbent types, surface-to-volume ratios, transfer distance, etc. Here, we report on a quantitative study of the fast kinetics of hydrodynamic liquid absorption of CO₂ microbubbles. In a microfluidic platform, three types of aqueous absorbents, namely, isophorone diamine (IPDA), monoethanolamine (MEA), and potassium hydroxide (KOH), are utilized as a continuous fluidic phase for absorbing gaseous CO₂ dispersed into flow-focused channels, and a train of microbubbles is formed. The flowing bubbles are visualized using an inverted microscope equipped with a high-speed camera. Their velocity and morphology are then quantitatively analyzed. The results show that the size of the CO₂ bubbles decreases rapidly within 100 ms since the contact with the absorbent and is approaching equilibrium at a constant minimum size. Overall, the volume shrinkage and the translated mass loss can be up to 99% within 200 ms. Absorption efficiency (mm/s), defined as the volume change of a gas bubble with unit cubic meter per unit surface area and per unit time, is positively correlated with the volume fraction (vol %) of the absorbent in solution. Yet as the latter reaches 20%, no differences in the absorption efficiency among the absorbents are observed. Computational fluid dynamics analysis reveals that flow vortices are constantly formed in both the bubbles and the liquid solution slug. The local convection of the fluid elements near the gas-liquid interface dominates over diffusion in the mass transfer of the gas molecules (Peclet number: 25,000-42,500). Intensified convection-diffusion processes near the interface strongly contribute to enhancing the dissolution of CO₂. Our findings are firm evidence that engineered mini bubbles flowing in liquid solution can drastically expedite the interphase mass transfer as well as the associated chemical processes.