Breakup of a Viscoelastic Droplet in Co-Rotating Non-Twin Screw Channels

Breakup of a viscoelastic fluid droplet immersed in another viscoelastic fluid matrix is very complicated when complex motions are introduced by a co-rotating twin screw extruder (TSE). Disturbances resulting from medium frequencies and large amplitudes repeat, and a constant shear rate cannot be maintained. A dual-speed non-twin screw extruder (NTSE) was developed to further enhance the above disturbance where asymmetry was introduced into the two screw elements with a rotation speed ratio of 2. A visualization extruder was employed to examine the breakup process and dimensional variations when a viscoelastic droplet was injected into another viscoelastic fluid matrix in the channels of the TSE or NTSE. Red oil-based ink was prepared as a tracer, while a carboxymethyl cellulose sodium solution was prepared as the matrix fluid. The droplet's evolution along the screw channels was recorded by four CCD cameras to reveal how dispersion occurred. A moving film was designed to carry the mixer out of the exit, where the mixing pattern and distributions of the tracer droplets were counted to characterize the final mixing results. Final mixing was quantified by droplet area, long-axis length, and short-axis length, as well as their center-to-center spacing variances. The visualization results showed that the TSE and NTSE shared a similar breakup mechanism for which a capillary instability mechanism of tiny fluid threads in the core regions far away from the intermeshing zones and necking actions through interscrew gaps were responsible. The asymmetry of the NTSE resulted in such a highly efficient breakup that only the first several revolutions could be tracked clearly to show how periodic behavior occurred. The statistical results showed that NTSE offered an average droplet area ranging from 0.02 mm2 to 1.6 mm2, an average long-axis length ranging from 0.20 mm to 3.29 mm, and an average short-axis size ranging from 0.17 mm to 1.04 mm.