Tip-mode microfluidics for highly viscous small droplets

Creating well-controlled high-viscosity small droplets is imperative for material fabrication and bio-printing, posing significant challenges. This research successfully addresses these challenges by employing a novel tip mode within a 3D Asymmetric Hydrodynamic Focusing (3D-AHF) system to produce droplets with a maximum viscosity of 300 mPa⋅s and a minimum size of 10 μm (1/5 feature size). In this newfound tip mode, the viscous dispersed liquid is partially pinned at the tube outlet, forming a stable tip markedly smaller than the tube outlet, facilitating the emission of small droplets. The weak dependency on dispersed liquid viscosity for tip formation and droplet generation is systematically examined through theoretical and experimental investigations. Developed theoretical models, elucidating the characteristics of this new tip mode and predicting droplet sizes, are experimentally validated. Demonstrations of using the 3D-AHF system as templates for fabricating elastic microspheres and highly sensitive microsphere-type sensors underscore its simplicity and efficacy for applications relying on small and viscous droplets.