On nonequilibrium shrinkage of supercritical CO₂ droplets in a water-carrier microflow

We report an experimental study on the hydrodynamic shrinkage of supercritical carbon dioxide (scCO₂) microdroplets during a nonequilibrium process. After scCO₂ microdroplets are generated by water shearing upon a scCO₂ flow in a micro T-junction, they are further visualized and characterized at the midpoint and the ending point of a straight rectangular microchannel (width × depth × length: 150 μm × 100 μm × 1.5 mm). The measured decreases in droplet size by 8%-36% indicate and simply quantify the droplet shrinkage which results from the interphase mass transfer between the droplet and the neighboring water. Using a mathematical model, the shrinkage of scCO₂ droplets is characterized by solvent-side mass transfer coefficients (k_s: 1.5 × 10⁻⁴-7.5 × 10⁻⁴ m/s) and the Sherwood number (Sh: 7-37). In general, k_s here is two orders of magnitude larger than that of hydrostatic liquid CO₂ droplets in water. The magnitude of Sh numbers highlights the stronger effect of local convections than that of diffusion in the interphase mass transfer. Our results, as reported here, have essential implications for scCO₂-based chemical extractions and carbon storage in deep geoformations.