An experimental investigation into filament formation during drop generation by pseudo-plastic viscoelastic biofluid at T-junction generators

The advancements in droplet microfluidics have made it an eminent technology for high-throughput biomedical applications involving biofluids such as blood, saliva, tear film, and synovial fluid, all characterized by their non-Newtonian/viscoelastic properties. However, most existing models for the design and optimization of droplet microfluidic systems are primarily based on Newtonian fluids. Therefore, studies that systematically explore the dynamics underlying biofluid droplet formation and manipulation are critically important, both fundamentally and practically. This study uses artificial tear films to experimentally investigate the effects of operational parameters such as geometric conditions (width ratio Λ = wdwc and aspect ratio h* = hwc), fluid properties (viscosity ratio η = μdμc), and flow rate ratios (ϕ = QdQc) on filament formation during droplet generation in a T-junction geometry. All the experiments were conducted at a constant capillary number of the continuous phase (Ca = μcQcwchγ). The results indicate that filament growth and rupture characteristics are determined by the predominance of two distinct regions: pre-stretch, governed by the interplay between inertial and capillary forces, and elastocapillarity, regulated by elastic and capillary forces. Specifically, filament rupture is influenced by inertia–capillary effects at low flow rates and viscosity ratios. In contrast, the domination of elastic effects is more pronounced at higher flow rates and viscosity ratios. Additionally, reductions in the width (Λ) and aspect (h*) ratio when Λ≥0.5 and h*≥0.4 increase the elastic effects within the filaments while inertial effects dominate for Λ<0.5 and h*<0.4. The relaxation times (λ) are obtained from the exponential thinning curve, with longer times observed for cases with high elastic effects. Finally, the filament collapses into satellite droplets, and their monodispersity increases with elasticity.