Breakup of viscoelastic droplets in an asymmetric twin-screw extruder

This study introduces an asymmetric twin-screw extruder (ATSE) featuring an internal baffle was inserted in one of the screw channels to enhance dispersive mixing. A visualization experimental prototype was developed, utilizing a moving transparent thin film to collect tracer droplets for further analysis. An aqueous solution of sodium carboxymethyl cellulose (CMC-Na) served as the matrix fluid, while a red oil-based ink acted as the tracer. The experimental results revealed three primary droplet breakup modes: (1) large-scale breakup driven by strong shear in the screw-barrel gap; (2) formation of drag tails followed by droplet breakup due to asymmetric perturbations; and (3) small-scale breakup primarily attributed to capillary instability. Importantly, the baffle height significantly influenced droplet breakup behavior. Although a smaller baffle height created strong shear, it reduced the opportunities for large droplets to penetrate the gap between the screws and the barrel, resulting in larger final droplet sizes. In addition, an increased viscosity ratio typically makes droplet breakup more challenging. However, in ATSE, the unique characteristics of elongation flow field and prolonged residence time due to 'backflow' actively facilitate droplet breakup.