CO 2 Microbubbles in Silicone Oil (Part II: Henry's Constant and Anomalous Diffusion)

This work demonstrates the utility of microfluidic devices for characterizing diffusion mechanisms. We determined Henry's constant and characterized the diffusion process of gaseous CO in silicone oil. Using microfluidic techniques, we analyzed the evolution of the CO bubble size in a solvent flowing through a microchannel system. The reduction in bubble size due to the mass transfer of gaseous CO into the solvent fluid primarily affects their length. A microfluidic device was used to produce bubbles, consisting of a pressure-driven injection system for the gas and a flow-driven system for the liquid. Additionally, an optical device was coupled for tracking and studying the bubbles in the microchannels, enabling us to study their spatial and temporal evolution using image analysis. From this study, we found two diffusion regimes. The first is a superdiffusive process for short times. In this regime, due to the high concentration gradient values at the gas-liquid interface, we observed a higher rate of carbon dioxide transfer to the silicone oil. At longer times, we see that the gas transfer rate significantly decreases compared to the previous regime, leading to a subdiffusive process. In this latter regime, it was found that if we increase the gas pressure, the system approaches a normal diffusive process that coincides with previously conducted studies by other researchers. It is suggested that the subdiffusion could be due to the high degree of confinement of the bubbles within the microchannel, similar to what occurs in porous media, the high viscosity of the fluid, and the low gas pressure used in the tests. The microfluidic device proved to be a very efficient method for determining the diffusion process and Henry's constant in this case. Its easy fabrication and low cost make this type of device appropriate for substance characterization.