Influence of continuous phase viscosity on drop breakup mechanisms in a miniature cubic mixer at moderate Reynolds numbers

• Drop breakup studied in a 10 mL cubic unbaffled mixer with viscous organic phase. • Higher continuous phase viscosity increases droplets smaller than 100 micrometres. • Higher viscosity shifts breakup mechanism from eddy collision to wall elongation. • Mechanism shift and viscosity rise increase production of satellite drops. This experimental study focuses on the drop breakup in a 10 mL, cubic and unbaffled mixer, used in nuclear fuel reprocessing research, and involving operations in which the continuous phase is organic and viscous. In solvent extraction processes, drop size is a key parameter as it governs the interfacial area and provides insight into the presence of small droplets responsible for phase entrainment at the settler outlet. For this purpose, two experimental investigations were conducted in water-in-organic dispersions: the first aimed at measuring Drop Size Distributions (DSDs), and the second at studying single drop breakup. Five oils with viscosities ranging from 2 to 10 mPa s were used as the continuous phase, leading to Reynolds numbers between 200 and 1,100. The dispersion and drop breakage were captured using an out-of-flow high-speed camera, and the DSDs were determined with a previously published BYG-drop deep-learning algorithm. DSDs were measured under varying mixing times, observation areas, continuous phase viscosities, and impeller agitation rates. The results show that increasing the continuous phase viscosity leads to a change in the distribution shape and significantly increases the number of small diameter droplets. This observation was subsequently confirmed through single drop breakup experiments. It was found that increasing the continuous phase viscosity modifies the breakup mechanism, switching from drop-eddy interactions to breakup induced by elongational shear along the wall. The latter promotes drop stretching and the formation of a long, thin filament, resulting in an increased number of satellite droplets.