Droplet transport through micro-orifices is central to numerous biomedical, chemical, and industrial microfluidic applications where confinement and rheology jointly dictate performance. While many studies have examined Newtonian systems, the dynamics of non-Newtonian droplets under geometric confinement remains less understood. In particular, the Carreau–Yasuda model offers a realistic description of shear-thinning fluids, such as polymeric or bio-relevant solutions, across wide shear-rate ranges. This study employs a three-dimensional axisymmetric level set framework in COMSOL Multiphysics to investigate the passage of droplets through orifices of varying geometry, focusing on how the position of the narrowest section influences velocity distribution, viscosity fields, and pressure evolution. The continuous phase is modeled as sodium carboxymethyl cellulose solutions at multiple concentrations, while the dispersed phase is Newtonian. Model validation against established dripping and jetting regimes, as well as analytical velocity profiles, confirms the robustness of the numerical approach. Results reveal that both rheology and orifice shape critically modulate droplet behavior: stronger shear-thinning intensifies pressure gradients, accelerates deformation, and thins lubrication films, whereas geometric positioning of the constriction governs the timing and persistence of vorticity generation and internal recirculation. Specifically, orifices narrowed at the outlet sustain downstream mixing and delayed shear peaks, while entrance-narrowed orifices induce early, transient shear layers and localized circulation. These findings provide new mechanistic insights into the coupling between non-Newtonian rheology and micro-orifice geometry, with implications for droplet-based microreactors, controlled encapsulation, and lab-on-a-chip platforms.
Kootenaei et al. (2026) studied this question.