Dynamics of non-Newtonian agarose gel droplet formation in two-phase microfluidic systems

Droplet-based microfluidics is a valuable tool in interdisciplinary research fields like cell biology and diagnostics. Newtonian fluids, like aqueous-based solutions, are commonly used for droplet generation. However, non-Newtonian fluids, e.g., hydrogels, are becoming increasingly popular as the dispersed phase. In this study, we investigate the dynamics of non-Newtonian ultra-low-gelling agarose droplet formation under different conditions to evaluate stability, with an aim to better understand the underlying physics of droplet formation. We varied the agarose gel concentration, temperature (40, 50, and 60 °C), and the flow rate ratio (ϕ) between the continuous and dispersed phase and observed droplet formation dynamics in the squeezing regime (capillary number, Cac < 0.015) in a T-junction under different flow conditions. We experimentally investigated the droplet size (LD/w) as a function of those four parameters and found that LD/w depends strongly on ϕ, the agarose concentration, and temperature (which affects the viscosity ratio, λ), but is only weakly dependent on Cac. We then confirmed our experimental findings with numerical simulations, which showed good agreement across all conditions. We numerically showed that the agarose droplet formation process consists of five stages, namely, filling, necking, pinching, threading, and breakup, where threading is an additional stage with a non-Newtonian dispersed phase. Finally, with numerical simulation, we concluded that threading length (lthread) is directly proportional to ϕ and has a complex relation with agarose concentration, and temperature.